<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
"http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd">
<refentry id="re-drbdconf">
  <refentryinfo>
    <date>6 May 2011</date>

    <productname>DRBD</productname>

    <productnumber>8.4.0</productnumber>
  </refentryinfo>

  <refmeta>
    <refentrytitle>drbd.conf</refentrytitle>

    <manvolnum>5</manvolnum>

    <refmiscinfo class="manual">Configuration Files</refmiscinfo>
  </refmeta>

  <refnamediv>
    <refname>drbd.conf</refname>

    <refpurpose>Configuration file for DRBD's devices <indexterm significance="normal">
        <primary>drbd.conf</primary>
      </indexterm></refpurpose>
  </refnamediv>

  <refsect1>
    <title>Introduction</title>

    <para>The file <option>/etc/drbd.conf</option> is read by <option>drbdadm</option>.</para>

    <para>The file format was designed as to allow to have a verbatim copy of the file on both
    nodes of the cluster. It is highly recommended to do so in order to keep your configuration
    manageable. The file <option>/etc/drbd.conf</option> should be the same on both nodes of the
    cluster. Changes to <option>/etc/drbd.conf</option> do not apply immediately.</para>

    <para>By convention the main config contains two include statements. The first one includes
    the file <option>/etc/drbd.d/global_common.conf</option>, the second one all file with a
    <option>.res</option> suffix.</para>

    <para>
        <title>A small example.res file</title>

        <programlisting format="linespecific">resource r0 {
	net {
		protocol C;
		cram-hmac-alg sha1;
		shared-secret "FooFunFactory";
	}
	disk {
		resync-rate 10M;
	}
	on alice {
		volume 0 {
			device    minor 1;
			disk      /dev/sda7;
			meta-disk internal;
		}
		address   10.1.1.31:7789;
	}
	on bob {
		volume 0 {
			device    minor 1;
			disk      /dev/sda7;
			meta-disk internal;
		}
		address   10.1.1.32:7789;
	}
}</programlisting>
      In this example, there is a single DRBD resource (called r0) which uses protocol C
    for the connection between its devices. It contains a single volume which runs on host
    <replaceable>alice</replaceable> uses <replaceable>/dev/drbd1</replaceable> as devices for its
    application, and <replaceable>/dev/sda7</replaceable> as low-level storage for the data. The
    IP addresses are used to specify the networking interfaces to be used. An eventually running
    resync process should use about 10MByte/second of IO bandwidth. This resync-rate statement is
    valid for volume 0, but would also be valid for further volumes. In this example it assigns
    full 10MByte/second to each volume.</para>

    <para>There may be multiple resource sections in a single drbd.conf file. For more examples,
    please have a look at the
    <ulink url="http://www.drbd.org/users-guide/"><citetitle>DRBD User's Guide</citetitle></ulink>.</para>

  </refsect1>

  <refsect1>
    <title>File Format</title>

    <para>The file consists of sections and parameters. A section begins with a keyword, sometimes
    an additional name, and an opening brace (<quote>{</quote>). A section ends with a closing
    brace (<quote>}</quote>. The braces enclose the parameters.</para>

    <para>section [name] { parameter value; [...] }</para>

    <para>A parameter starts with the identifier of the parameter followed by whitespace. Every
    subsequent character is considered as part of the parameter's value. A special case are
    Boolean parameters which consist only of the identifier. Parameters are terminated by a
    semicolon (<quote>;</quote>).</para>

    <para>Some parameter values have default units which might be overruled by K, M or G. These
    units are defined in the usual way (K = 2^10 = 1024, M = 1024 K, G = 1024 M).</para>

    <para>Comments may be placed into the configuration file and must begin with a hash sign
    (<quote>#</quote>). Subsequent characters are ignored until the end of the line.</para>

    <refsect2>
      <title>Sections</title>

      <variablelist>
        <varlistentry>
          <term xml:id="skip"><option>skip</option></term>

          <listitem>
            <para><indexterm significance="normal">
                <primary>drbd.conf</primary>

                <secondary>skip</secondary>
              </indexterm> Comments out chunks of text, even spanning more than one line.
            Characters between the keyword <option>skip</option> and the opening brace
            (<quote>{</quote>) are ignored. Everything enclosed by the braces is skipped. This
            comes in handy, if you just want to comment out some '<option>resource [name]
            {...}</option>' section: just precede it with '<option>skip</option>'.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term xml:id="global"><option>global</option></term>

          <listitem>
            <para><indexterm significance="normal">
                <primary>drbd.conf</primary>

                <secondary>global</secondary>
              </indexterm> Configures some global parameters. Currently only
            <option>minor-count</option>, <option>dialog-refresh</option>,
            <option>disable-ip-verification</option> and <option>usage-count</option> are allowed
            here. You may only have one global section, preferably as the first section.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term xml:id="common"><option>common</option></term>

          <listitem>
            <para><indexterm significance="normal">
                <primary>drbd.conf</primary>

                <secondary>common</secondary>
              </indexterm> All resources inherit the options set in this section. The common
            section might have a <option>startup</option>, a <option>options</option>, a
            <option>handlers</option>, a <option>net</option> and a <option>disk</option>
            section.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term xml:id="resource"><option>resource <replaceable>name</replaceable></option></term>

          <listitem>
            <para><indexterm significance="normal">
                <primary>drbd.conf</primary>

                <secondary>resource</secondary>
              </indexterm> Configures a DRBD resource. Each resource section needs to have two (or
            more) <option>on <replaceable>host</replaceable></option> sections and may have a
            <option>startup</option>, a <option>options</option>, a <option>handlers</option>, a
            <option>net</option> and a <option>disk</option> section. It might contain
            <option>volume</option>s sections.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term xml:id="on"><option>on <replaceable>host-name</replaceable></option></term>

          <listitem>
            <para><indexterm significance="normal">
                <primary>drbd.conf</primary>

                <secondary>on</secondary>
              </indexterm> Carries the necessary configuration parameters for a DRBD device of the
            enclosing resource. <replaceable>host-name</replaceable> is mandatory and must match
            the Linux host name (uname -n) of one of the nodes. You may list more than one host
            name here, in case you want to use the same parameters on several hosts (you'd have to
            move the IP around usually). Or you may list more than two such sections.
            <programlisting format="linespecific">	resource r1 {
		protocol C;
		device minor 1;
		meta-disk internal;

		on alice bob {
			address 10.2.2.100:7801;
			disk /dev/mapper/some-san;
		}
		on charlie {
			address 10.2.2.101:7801;
			disk /dev/mapper/other-san;
		}
		on daisy {
			address 10.2.2.103:7801;
			disk /dev/mapper/other-san-as-seen-from-daisy;
		}
	}
	</programlisting>See also the <option>floating</option> section keyword. Required statements in
            this section: <option>address</option> and <option>volume</option>. Note for backward
            compatibility and convenience it is valid to embed the statements of a single volume
            directly into the host section.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term xml:id="volume"><option>volume <replaceable>vnr</replaceable></option></term>

          <listitem>
            <para><indexterm significance="normal">
                <primary>drbd.conf</primary>

                <secondary>volume</secondary>
              </indexterm> Defines a volume within a connection. The minor numbers of a replicated
            volume might be different on different hosts, the volume number
            (<replaceable>vnr</replaceable>) is what groups them together. Required parameters in
            this section: <option>device</option>, <option>disk</option>,
            <option>meta-disk</option>.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term xml:id="stacked-on-top-of"><option>stacked-on-top-of <replaceable>resource</replaceable></option></term>

          <listitem>
            <para><indexterm significance="normal">
                <primary>drbd.conf</primary>

                <secondary>stacked-on-top-of</secondary>
              </indexterm> For a stacked DRBD setup (3 or 4 nodes), a
            <option>stacked-on-top-of</option> is used instead of an <option>on</option> section.
            Required parameters in this section: <option>device</option> and
            <option>address</option>.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term xml:id="floating"><option>floating <replaceable>AF addr:port</replaceable></option></term>

          <listitem>
            <para><indexterm significance="normal">
                <primary>drbd.conf</primary>

                <secondary>on</secondary>
              </indexterm> Carries the necessary configuration parameters for a DRBD device of the
            enclosing resource. This section is very similar to the <option>on</option> section.
            The difference to the <option>on</option> section is that the matching of the host
            sections to machines is done by the IP-address instead of the node name. Required
            parameters in this section: <option>device</option>, <option>disk</option>,
            <option>meta-disk</option>, all of which
            <emphasis>may</emphasis> be inherited from the resource section, in which case you may
            shorten this section down to just the address identifier. <programlisting
            format="linespecific">	resource r2 {
		protocol C;
		device minor 2;
		disk      /dev/sda7;
		meta-disk internal;

		# short form, device, disk and meta-disk inherited
		floating 10.1.1.31:7802;

		# longer form, only device inherited
		floating 10.1.1.32:7802 {
			disk /dev/sdb;
			meta-disk /dev/sdc8;
		}
	}
	</programlisting></para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term xml:id="s-disk"><option>disk</option></term>

          <listitem>
            <para><indexterm significance="normal">
                <primary>drbd.conf</primary>

                <secondary>disk</secondary>
              </indexterm> This section is used to fine tune DRBD's properties in respect to the
            low level storage. Please refer to <citerefentry>
                <refentrytitle>drbdsetup</refentrytitle>

                <manvolnum>8</manvolnum>
              </citerefentry> for detailed description of the parameters. Optional parameters:
            <option>on-io-error</option>, <option>size</option>, <option>fencing</option>,
            <option>disk-barrier</option>, <option>disk-flushes</option>,
            <option>disk-drain</option>, <option>md-flushes</option>,
            <option>max-bio-bvecs</option>, <option>resync-rate</option>,
            <option>resync-after</option>, <option>al-extents</option>, <option>al-updates</option>,
            <option>c-plan-ahead</option>, <option>c-fill-target</option>,
            <option>c-delay-target</option>, <option>c-max-rate</option>,
            <option>c-min-rate</option>, <option>disk-timeout</option>,
	    <option>discard-zeroes-if-aligned</option>,
	    <option>rs-discard-granularity</option>,
	    <option>read-balancing</option>.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term xml:id="net"><option>net</option></term>

          <listitem>
            <para><indexterm significance="normal">
                <primary>drbd.conf</primary>

                <secondary>net</secondary>
              </indexterm> This section is used to fine tune DRBD's properties. Please refer to
            <citerefentry>
                <refentrytitle>drbdsetup</refentrytitle>

                <manvolnum>8</manvolnum>
              </citerefentry> for a detailed description of this section's parameters. Optional
            parameters: <option>protocol</option>, <option>sndbuf-size</option>,
            <option>rcvbuf-size</option>, <option>timeout</option>, <option>connect-int</option>,
            <option>ping-int</option>, <option>ping-timeout</option>,
            <option>max-buffers</option>, <option>max-epoch-size</option>,
            <option>ko-count</option>, <option>allow-two-primaries</option>,
            <option>cram-hmac-alg</option>, <option>shared-secret</option>,
            <option>after-sb-0pri</option>, <option>after-sb-1pri</option>,
            <option>after-sb-2pri</option>, <option>data-integrity-alg</option>,
            <option>no-tcp-cork</option>, <option>on-congestion</option>,
            <option>congestion-fill</option>, <option>congestion-extents</option>,
            <option>verify-alg</option>, <option>use-rle</option>,
            <option>csums-alg</option>,
            <option>socket-check-timeout</option>.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term xml:id="startup"><option>startup</option></term>

          <listitem>
            <para><indexterm significance="normal">
                <primary>drbd.conf</primary>

                <secondary>startup</secondary>
              </indexterm> This section is used to fine tune DRBD's properties. Please refer to
            <citerefentry>
                <refentrytitle>drbdsetup</refentrytitle>

                <manvolnum>8</manvolnum>
              </citerefentry> for a detailed description of this section's parameters. Optional
            parameters: <option>wfc-timeout</option>, <option>degr-wfc-timeout</option>,
            <option>outdated-wfc-timeout</option>, <option>wait-after-sb</option>,
            <option>stacked-timeouts</option> and <option>become-primary-on</option>.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term xml:id="options"><option>options</option></term>

          <listitem>
            <para><indexterm significance="normal">
                <primary>drbd.conf</primary>

                <secondary>options</secondary>
              </indexterm> This section is used to fine tune the behaviour of the resource object.
            Please refer to <citerefentry>
                <refentrytitle>drbdsetup</refentrytitle>

                <manvolnum>8</manvolnum>
              </citerefentry> for a detailed description of this section's parameters. Optional
            parameters: <option>cpu-mask</option>, and
            <option>on-no-data-accessible</option>.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term xml:id="handlers"><option>handlers</option></term>

          <listitem>
            <para><indexterm significance="normal">
                <primary>drbd.conf</primary>

                <secondary>handlers</secondary>
              </indexterm> In this section you can define handlers (executables) that are started
            by the DRBD system in response to certain events. Optional parameters:
            <option>pri-on-incon-degr</option>, <option>pri-lost-after-sb</option>,
            <option>pri-lost</option>, <option>fence-peer</option> (formerly oudate-peer),
            <option>local-io-error</option>, <option>initial-split-brain</option>,
            <option>split-brain</option>, <option>before-resync-target</option>,
            <option>after-resync-target</option>.</para>

            <para>The interface is done via environment variables:<itemizedlist>
                <listitem>
                  <para><option>DRBD_RESOURCE</option> is the name of the resource</para>
                </listitem>

                <listitem>
                  <para><option>DRBD_MINOR</option> is the minor number of the DRBD device, in
                  decimal.</para>
                </listitem>

                <listitem>
                  <para><option>DRBD_CONF</option> is the path to the primary configuration file;
                  if you split your configuration into multiple files (e.g. in
                  <option>/etc/drbd.conf.d/</option>), this will not be helpful.</para>
                </listitem>

                <listitem>
                  <para><option>DRBD_PEER_AF</option> , <option>DRBD_PEER_ADDRESS</option> ,
                  <option>DRBD_PEERS</option> are the address family (e.g. <option>ipv6</option>),
                  the peer's address and hostnames.</para>
                </listitem>
              </itemizedlist> <option>DRBD_PEER</option> is deprecated.</para>

            <para>Please note that not all of these might be set for all handlers, and that some
            values might not be useable for a <option>floating</option> definition.</para>
          </listitem>
        </varlistentry>
      </variablelist>
    </refsect2>

    <refsect2>
      <title>Parameters</title>

      <variablelist>
        <varlistentry>
          <term xml:id="minor-count"><option>minor-count <replaceable>count</replaceable></option></term>

          <listitem>
            <para><indexterm significance="normal">
                <primary>drbd.conf</primary>

                <secondary>minor-count</secondary>
              </indexterm><replaceable>count</replaceable> may be a number from 1 to 1048575.</para>

            <para><replaceable>Minor-count</replaceable> is a sizing hint for DRBD. It helps to
            right-size various memory pools. It should be set in the in the same order of
            magnitude than the actual number of minors you use. Per default the module loads with
            11 more resources than you have currently in your config but at least 32.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term xml:id="dialog-refresh"><option>dialog-refresh <replaceable>time</replaceable></option></term>

          <listitem>
            <para><indexterm significance="normal">
                <primary>drbd.conf</primary>

                <secondary>dialog-refresh</secondary>
              </indexterm><replaceable>time</replaceable> may be 0 or a positive number.</para>

            <para>The user dialog redraws the second count every <replaceable>time</replaceable>
            seconds (or does no redraws if <replaceable>time</replaceable> is 0). The default
            value is 1.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term xml:id="disable-ip-verification"><option>disable-ip-verification</option></term>

          <listitem>
            <indexterm significance="normal">
              <primary>drbd.conf</primary>

              <secondary>disable-ip-verification</secondary>
            </indexterm>

            <para>Use <replaceable>disable-ip-verification</replaceable> if, for some obscure
            reasons, drbdadm can/might not use <option>ip</option> or <option>ifconfig</option> to
            do a sanity check for the IP address. You can disable the IP verification with this
            option.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term xml:id="udev-always-use-vnr"><option>udev-always-use-vnr</option></term>

          <listitem>
            <indexterm significance="normal">
              <primary>drbd.conf</primary>

              <secondary>udev-always-use-vnr</secondary>
            </indexterm>

      <para>When udev asks drbdadm for a list of device related symlinks,
      drbdadm would suggest symlinks with differing naming conventions,
      depending on whether the resource has explicit
      <literal>volume VNR { }</literal> definitions,
      or only one single volume with the implicit volume number 0:
      <programlisting><![CDATA[
# implicit single volume without "volume 0 {}" block
DEVICE=drbd<minor>
SYMLINK_BY_RES=drbd/by-res/<resource-name>

# explicit volume definition: volume VNR { }
DEVICE=drbd<minor>
SYMLINK_BY_RES=drbd/by-res/<resource-name>/VNR
]]></programlisting>
	</para>

	<para>If you define this parameter in the global section,
	drbdadm will always add the <literal>.../VNR</literal> part,
	and will not care for whether the volume definition was implicit or explicit.
	</para>

	<para>For legacy backward compatibility, this is off by default,
	but we do recommend to enable it.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term xml:id="usage-count"><option>usage-count <replaceable>val</replaceable></option></term>

          <listitem>
            <indexterm significance="normal">
              <primary>drbd.conf</primary>

              <secondary>usage-count</secondary>
            </indexterm>

            <para>Please participate in
            <ulink url="http://usage.drbd.org"><citetitle>DRBD's online usage counter</citetitle></ulink>.
             The most convenient way to do so is to set
            this option to <option>yes</option>. Valid options are: <option>yes</option>,
            <option>no</option> and <option>ask</option>.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term xml:id="protocol"><option>protocol <replaceable>prot-id</replaceable></option></term>

          <listitem>
            <indexterm significance="normal">
              <primary>drbd.conf</primary>

              <secondary>protocol</secondary>
            </indexterm>

            <para>On the TCP/IP link the specified <replaceable>protocol</replaceable> is used.
            Valid protocol specifiers are A, B, and C.</para>

            <para>Protocol A: write IO is reported as completed, if it has reached local disk and
            local TCP send buffer.</para>

            <para>Protocol B: write IO is reported as completed, if it has reached local disk and
            remote buffer cache.</para>

            <para>Protocol C: write IO is reported as completed, if it has reached both local and
            remote disk.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term xml:id="device"><option>device <replaceable>name</replaceable> minor
          <replaceable>nr</replaceable></option></term>

          <listitem>
            <para><indexterm significance="normal">
                <primary>drbd.conf</primary>

                <secondary>device</secondary>
              </indexterm> The name of the block device node of the resource being described. You
            must use this device with your application (file system) and you must not use the low
            level block device which is specified with the <option>disk</option> parameter.</para>

            <para>One can ether omit the <replaceable>name</replaceable> or <option>minor</option>
            and the <replaceable>minor number</replaceable>. If you omit the
            <replaceable>name</replaceable> a default of /dev/drbd<replaceable>minor</replaceable>
            will be used.</para>

            <para>Udev will create additional symlinks in /dev/drbd/by-res and
            /dev/drbd/by-disk.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term xml:id="disk"><option>disk <replaceable>name</replaceable></option></term>

          <listitem>
            <para><indexterm significance="normal">
                <primary>drbd.conf</primary>

                <secondary>disk</secondary>
              </indexterm> DRBD uses this block device to actually store and retrieve the data.
            Never access such a device while DRBD is running on top of it. This also holds true
            for <citerefentry>
                <refentrytitle>dumpe2fs</refentrytitle>

                <manvolnum>8</manvolnum>
              </citerefentry> and similar commands.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term xml:id="address"><option>address <replaceable>AF addr:port</replaceable></option></term>

          <listitem>
            <para><indexterm significance="normal">
                <primary>drbd.conf</primary>

                <secondary>address</secondary>
              </indexterm> A resource needs one <replaceable>IP</replaceable> address per device,
            which is used to wait for incoming connections from the partner device respectively to
            reach the partner device. <replaceable>AF</replaceable> must be one of
            <option>ipv4</option>, <option>ipv6</option>, <option>ssocks</option> or
            <option>sdp</option> (for compatibility reasons <option>sci</option> is an alias for
            <option>ssocks</option>). It may be omited for IPv4 addresses. The actual IPv6 address
            that follows the <option>ipv6</option> keyword must be placed inside brackets:
            <literal moreinfo="none">ipv6 [fd01:2345:6789:abcd::1]:7800</literal>.</para>

            <para>Each DRBD resource needs a TCP <replaceable>port</replaceable> which is used to
            connect to the node's partner device. Two different DRBD resources may not use the
            same <replaceable>addr:port</replaceable> combination on the same node.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term xml:id="meta-disk"><option>meta-disk internal</option></term>

          <term><option>meta-disk <replaceable>device</replaceable></option></term>

	  <term><option>meta-disk <replaceable>device</replaceable> [<replaceable>index</replaceable>]</option></term>

          <listitem>
            <para><indexterm significance="normal">
                <primary>drbd.conf</primary>

                <secondary>meta-disk</secondary>
              </indexterm> Internal means that the last part of the backing device is used to
	      store the meta-data.  The size of the meta-data is computed based on the size of the
	      device.</para>

            <para>When a <replaceable>device</replaceable> is specified, either with or without an
	    <replaceable>index</replaceable>, DRBD stores the meta-data on this device.  Without
	    <replaceable>index</replaceable>, the size of the meta-data is determined by the size
	    of the data device. This is usually used with LVM, which allows to have many variable
	    sized block devices. The meta-data size is 36kB + Backing-Storage-size / 32k, rounded up
	    to the next 4kb boundary.  (Rule of the thumb: 32kByte per 1GByte of storage, rounded up
	    to the next MB.)</para>

	    <para>When an <replaceable>index</replaceable> is specified, each index number refers to
	    a fixed slot of meta-data of 128 MB, which allows a maximum data size of 4 TiB. This way,
	    multiple DBRD devices can share the same meta-data device.  For example, if /dev/sde6[0]
	    and /dev/sde6[1] are used, /dev/sde6 must be at least 256 MB big. Because of the hard size
	    limit, use of meta-disk indexes is discouraged.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term xml:id="on-io-error"><option>on-io-error <replaceable>handler</replaceable></option></term>

          <listitem>
            <para><indexterm significance="normal">
                <primary>drbd.conf</primary>

                <secondary>on-io-error</secondary>
              </indexterm><replaceable>handler</replaceable> is taken, if the lower level device
            reports io-errors to the upper layers.</para>

            <para><replaceable>handler</replaceable> may be <option>pass_on</option>,
            <option>call-local-io-error</option> or <option>detach.</option></para>

            <para><option>pass_on</option>: The node downgrades the disk status to inconsistent, marks the
            erroneous block as inconsistent in the bitmap and retries the IO on the remote node.</para>

            <para><option>call-local-io-error</option>: Call the handler script
            <option>local-io-error</option>.</para>

            <para><option>detach</option>: The node drops its low level device, and continues in
            diskless mode.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term xml:id="fencing"><option>fencing <replaceable>fencing_policy</replaceable></option></term>

          <listitem>
            <para><indexterm significance="normal">
                <primary>drbd.conf</primary>

                <secondary>fencing</secondary>
              </indexterm> By <option>fencing</option> we understand preventive measures to avoid
            situations where both nodes are primary and disconnected (AKA split brain).</para>

            <para>Valid fencing policies are:</para>

            <variablelist>
              <varlistentry>
                <term xml:id="dont-care"><option>dont-care</option></term>

                <listitem>
                  <para>This is the default policy. No fencing actions are taken.</para>
                </listitem>
              </varlistentry>

              <varlistentry>
                <term xml:id="resource-only"><option>resource-only</option></term>

                <listitem>
                  <para>If a node becomes a disconnected primary, it tries to fence the peer's
                  disk. This is done by calling the <option>fence-peer</option> handler. The
                  handler is supposed to reach the other node over alternative communication paths
                  and call '<option>drbdadm outdate res</option>' there.</para>
                </listitem>
              </varlistentry>

              <varlistentry>
                <term xml:id="resource-and-stonith"><option>resource-and-stonith</option></term>

                <listitem>
                  <para>If a node becomes a disconnected primary, it freezes all its IO operations
                  and calls its fence-peer handler. The fence-peer handler is supposed to reach
                  the peer over alternative communication paths and call 'drbdadm outdate res'
                  there. In case it cannot reach the peer it should stonith the peer. IO is
                  resumed as soon as the situation is resolved. In case your handler fails, you
                  can resume IO with the <option>resume-io</option> command.</para>
                </listitem>
              </varlistentry>
            </variablelist>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term xml:id="disk-barrier"><option>disk-barrier</option></term>

          <term xml:id="disk-flushes"><option>disk-flushes</option></term>

          <term xml:id="disk-drain"><option>disk-drain</option></term>

          <listitem>
            <indexterm significance="normal">
              <primary>drbd.conf</primary>
              <secondary>disk-barrier</secondary>
            </indexterm>

            <indexterm significance="normal">
              <primary>drbd.conf</primary>

              <secondary>disk-flushes</secondary>
            </indexterm>

            <indexterm significance="normal">
              <primary>drbd.conf</primary>
              <secondary>disk-drain</secondary>
            </indexterm>

            <para>DRBD has four implementations to express write-after-write dependencies to its
            backing storage device. DRBD will use the first method that is supported by the
            backing storage device and that is not disabled. By default the <emphasis>flush</emphasis> 
	    method is used.</para>

	    <para>Since drbd-8.4.2 <option>disk-barrier</option> is disabled by default
	    because since linux-2.6.36 (or 2.6.32 RHEL6) there is no reliable way to determine if queuing
	    of IO-barriers works. <emphasis>Dangerous</emphasis> only enable if you are
	    told so by one that knows for sure.</para>

            <para>When selecting the method you should not only base your decision on the
            measurable performance. In case your backing storage device has a volatile write cache
            (plain disks, RAID of plain disks) you should use one of the first two. In case your
            backing storage device has battery-backed write cache you may go with option 3.
	    Option 4 (disable everything, use "none") <emphasis>is dangerous</emphasis>
	    on most IO stacks, may result in write-reordering, and if so,
	    can theoretically be the reason for data corruption, or disturb
	    the DRBD protocol, causing spurious disconnect/reconnect cycles.
	    <emphasis>Do not use</emphasis> <option>no-disk-drain</option>.</para>

            <para>Unfortunately device mapper (LVM) might not support barriers.</para>

            <para>The letter after "wo:" in /proc/drbd indicates with method is currently in use
            for a device: <option>b</option>, <option>f</option>, <option>d</option>,
            <option>n</option>. The implementations are:</para>

            <variablelist>
              <varlistentry>
                <term>barrier</term>

                <listitem>
                  <para>The first requires that the driver of the backing storage device support
                  barriers (called 'tagged command queuing' in SCSI and 'native command queuing'
                  in SATA speak). The use of this method can be enabled by setting the
                  <option>disk-barrier</option> options to <option>yes</option>.</para>
                </listitem>
              </varlistentry>

              <varlistentry>
                <term>flush</term>

                <listitem>
                  <para>The second requires that the backing device support disk flushes (called
                  'force unit access' in the drive vendors speak). The use of this method can be
                  disabled setting <option>disk-flushes</option> to <option>no</option>.</para>
                </listitem>
              </varlistentry>

              <varlistentry>
                <term>drain</term>

                <listitem>
                  <para>The third method is simply to let write requests drain before write
                  requests of a new reordering domain are issued. This was the only implementation
                  before 8.0.9.</para>
                </listitem>
              </varlistentry>

              <varlistentry>
                <term>none</term>

                <listitem>
                  <para>The fourth method is to not express write-after-write dependencies to
		  the backing store at all, by also specifying <option>no-disk-drain</option>.
		  This <emphasis>is dangerous</emphasis>
		  on most IO stacks, may result in write-reordering, and if so,
		  can theoretically be the reason for data corruption, or disturb
		  the DRBD protocol, causing spurious disconnect/reconnect cycles.
		  <emphasis>Do not use</emphasis> <option>no-disk-drain</option>.</para>
                </listitem>
              </varlistentry>
            </variablelist>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term xml:id="md-flushes"><option>md-flushes</option></term>

          <listitem>
            <indexterm significance="normal">
              <primary>drbd.conf</primary>

              <secondary>md-flushes</secondary>
            </indexterm>

            <para>Disables the use of disk flushes and barrier BIOs when accessing the meta data
            device. See the notes on <option>disk-flushes</option>.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term xml:id="max-bio-bvecs"><option>max-bio-bvecs</option></term>

          <listitem>
            <indexterm significance="normal">
              <primary>drbd.conf</primary>

              <secondary>max-bio-bvecs</secondary>
            </indexterm>

            <para>In some special circumstances the device mapper stack manages to pass BIOs to
            DRBD that violate the constraints that are set forth by DRBD's merge_bvec() function
            and which have more than one bvec. A known example is: phys-disk -&gt; DRBD -&gt; LVM
            -&gt; Xen -&gt; misaligned partition (63) -&gt; DomU FS. Then you might see "bio would
            need to, but cannot, be split:" in the Dom0's kernel log.</para>

            <para>The best workaround is to proper align the partition within the VM (E.g. start
            it at sector 1024). This costs 480 KiB of storage. Unfortunately the default of most
            Linux partitioning tools is to start the first partition at an odd number (63).
            Therefore most distribution's install helpers for virtual linux machines will end up
            with misaligned partitions. The second best workaround is to limit DRBD's max bvecs
            per BIO (= <option>max-bio-bvecs</option>) to 1, but that might cost
            performance.</para>

            <para>The default value of <option>max-bio-bvecs</option> is 0, which means that there
            is no user imposed limitation.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>
            <option>disk-timeout</option>
          </term>
          <listitem>
            <indexterm significance="normal">
              <primary>drbd.conf</primary>
              <secondary>disk-timeout</secondary>
            </indexterm>
      <para>If the lower-level device on which a DRBD device stores its data does
      not finish an I/O request within the defined
      <option>disk-timeout</option>, DRBD treats this as a failure. The
      lower-level device is detached, and the device's disk state advances to
      Diskless.  If DRBD is connected to one or more peers, the failed request
      is passed on to one of them.</para>

      <para>This option is <emphasis>dangerous and may lead to kernel panic!</emphasis></para>

      <para>"Aborting" requests, or force-detaching the disk, is intended for
      completely blocked/hung local backing devices which do no longer
      complete requests at all, not even do error completions.  In this
      situation, usually a hard-reset and failover is the only way out.</para>

      <para>By "aborting", basically faking a local error-completion,
      we allow for a more graceful swichover by cleanly migrating services.
      Still the affected node has to be rebooted "soon".</para>
      <para>By completing these requests, we allow the upper layers to re-use
      the associated data pages.</para>

      <para>If later the local backing device "recovers", and now DMAs some data
      from disk into the original request pages, in the best case it will
      just put random data into unused pages; but typically it will corrupt
      meanwhile completely unrelated data, causing all sorts of damage.</para>

      <para>Which means delayed successful completion,
      especially for READ requests, is a reason to panic().
      We assume that a delayed *error* completion is OK,
      though we still will complain noisily about it.</para>
      <para>The default value of
      <option>disk-timeout</option> is 0, which stands for an infinite timeout.
      Timeouts are specified in units of 0.1 seconds. This option is available
      since DRBD 8.3.12.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term xml:id="discard-zeroes-if-aligned"><option>discard-zeroes-if-aligned <group choice="req" rep="norepeat">
	     <arg choice="plain" rep="norepeat">yes</arg>
	     <arg choice="plain" rep="norepeat">no</arg>
	  </group></option></term>
          <listitem>
	    <para>
            <indexterm significance="normal">
              <primary>drbd.conf</primary>
              <secondary>discard-zeroes-if-aligned</secondary>
            </indexterm>
	There are several aspects to discard/trim/unmap support on linux
	block devices.  Even if discard is supported in general, it may fail
	silently, or may partially ignore discard requests.  Devices also
	announce whether reading from unmapped blocks returns defined data
	(usually zeroes), or undefined data (possibly old data, possibly
	garbage).
	</para><para>
	If on different nodes, DRBD is backed by devices with differing discard
	characteristics, discards may lead to data divergence (old data or
	garbage left over on one backend, zeroes due to unmapped areas on the
	other backend). Online verify would now potentially report tons of
	spurious differences.  While probably harmless for most use cases
	(fstrim on a file system), DRBD cannot have that.
	</para><para>
	To play safe, we have to disable discard support, if our local backend
	(on a Primary) does not support "discard_zeroes_data=true".  We also have to
	translate discards to explicit zero-out on the receiving side, unless
	the receiving side (Secondary) supports "discard_zeroes_data=true",
	thereby allocating areas what were supposed to be unmapped.
	</para><para>
	There are some devices (notably the LVM/DM thin provisioning) that are
	capable of discard, but announce discard_zeroes_data=false.  In the case of
	DM-thin, discards aligned to the chunk size will be unmapped, and
	reading from unmapped sectors will return zeroes. However, unaligned
	partial head or tail areas of discard requests will be silently ignored.
	</para><para>
	If we now add a helper to explicitly zero-out these unaligned partial
	areas, while passing on the discard of the aligned full chunks, we
	effectively achieve discard_zeroes_data=true on such devices.
	</para><para>
	Setting <option>discard-zeroes-if-aligned</option> to <option>yes</option>
	will allow DRBD to use discards, and to announce discard_zeroes_data=true,
	even on backends that announce discard_zeroes_data=false.
	</para><para>
	Setting <option>discard-zeroes-if-aligned</option> to <option>no</option>
	will cause DRBD to always fall-back to zero-out on the receiving side,
	and to not even announce discard capabilities on the Primary,
	if the respective backend announces discard_zeroes_data=false.
	</para><para>
	We used to ignore the discard_zeroes_data setting completely.  To not
	break established and expected behaviour, and suddenly cause fstrim on
	thin-provisioned LVs to run out-of-space instead of freeing up space,
	the default value is <option>yes</option>.
	</para><para>
	This option is available since 8.4.7.
	</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term><option>--disable-write-same <group choice="req" rep="norepeat">
          <arg choice="plain" rep="norepeat">yes</arg>
          <arg choice="plain" rep="norepeat">no</arg>
          </group></option></term>
          <listitem>
            <para>
              <indexterm significance="normal">
                <primary>drbd.conf</primary>
                <secondary>disable-write-same</secondary>
              </indexterm>
              Some disks announce WRITE_SAME support to the kernel but fail with
              an I/O error upon actually receiving such a request. This mostly
              happens when using virtualized disks -- notably, this behavior has
              been observed with VMware's virtual disks.
              </para><para>
              When <option>disable-write-same</option> is set to <option>yes</option>,
              WRITE_SAME detection is manually overriden and support is disabled.
              </para><para>
              The default value of <option>disable-write-same</option> is
              <option>no</option>. This option is available since 8.4.7.
            </para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>
            <option>read-balancing <replaceable>method</replaceable></option>
          </term>
          <listitem>
            <indexterm significance="normal">
              <primary>drbd.conf</primary>
              <secondary>read-balancing</secondary>
            </indexterm>
	    <para>
	      The supported <replaceable>methods</replaceable> for load balancing of
	      read requests are <option>prefer-local</option>, <option>prefer-remote</option>,
	      <option>round-robin</option>, <option>least-pending</option>,
	      <option>when-congested-remote</option>, <option>32K-striping</option>,
	      <option>64K-striping</option>, <option>128K-striping</option>,
	      <option>256K-striping</option>, <option>512K-striping</option>
	      and <option>1M-striping</option>.</para>
              <para> The default value of <option>read-balancing</option>
              is <option>prefer-local</option>.
	      This option is available since 8.4.1.
	      </para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term xml:id="rs-discard-granularity">
            <option>rs-discard-granularity <replaceable>byte</replaceable></option>
          </term>
          <listitem>
            <indexterm significance="normal">
              <primary>drbd.conf</primary>
              <secondary>rs-discard-granularity</secondary>
            </indexterm>
	    <para>
	      When <option>rs-discard-granularity</option> is set to a non zero, positive
	      value then DRBD tries to do a resync operation in requests of this size.
	      In case such a block contains only zero bytes on the sync source node,
	      the sync target node will issue a discard/trim/unmap command for
	      the area.</para>
	      <para>The value is constrained by the discard granularity of the backing
	      block device. In case <option>rs-discard-granularity</option> is not a
	      multiplier of the discard granularity of the backing block device DRBD
	      rounds it up. The feature only gets active if the backing block device
	      reads back zeroes after a discard command.</para>
              <para> The default value of <option>rs-discard-granularity</option>
              is 0. This option is available since 8.4.7.
	      </para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term xml:id="sndbuf-size"><option>sndbuf-size <replaceable>size</replaceable></option></term>

          <listitem>
            <para><indexterm significance="normal">
                <primary>drbd.conf</primary>

                <secondary>sndbuf-size</secondary>
              </indexterm><replaceable>size</replaceable> is the size of the TCP socket send
            buffer. The default value is 0, i.e. autotune. You can specify smaller or larger
            values. Larger values are appropriate for reasonable write throughput with protocol A
            over high latency networks. Values below 32K do not make sense. Since 8.0.13 resp.
            8.2.7, setting the <replaceable>size</replaceable> value to 0 means that the kernel
            should autotune this.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term xml:id="rcvbuf-size"><option>rcvbuf-size <replaceable>size</replaceable></option></term>

          <listitem>
            <para><indexterm significance="normal">
                <primary>drbd.conf</primary>

                <secondary>rcvbuf-size</secondary>
              </indexterm><replaceable>size</replaceable> is the size of the TCP socket receive
            buffer. The default value is 0, i.e. autotune. You can specify smaller or larger
            values. Usually this should be left at its default. Setting the
            <replaceable>size</replaceable> value to 0 means that the kernel should autotune
            this.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term xml:id="timeout"><option>timeout <replaceable>time</replaceable></option></term>

          <listitem>
            <para><indexterm significance="normal">
                <primary>drbd.conf</primary>

                <secondary>timeout</secondary>
              </indexterm> If the partner node fails to send an expected response packet within
            <replaceable>time</replaceable> tenths of a second, the partner node is considered
            dead and therefore the TCP/IP connection is abandoned. This must be lower than
            <replaceable>connect-int</replaceable> and <replaceable>ping-int</replaceable>. The
            default value is 60 = 6 seconds, the unit 0.1 seconds.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term xml:id="connect-int"><option>connect-int <replaceable>time</replaceable></option></term>

          <listitem>
            <para><indexterm significance="normal">
                <primary>drbd.conf</primary>

                <secondary>connect-int</secondary>
              </indexterm> In case it is not possible to connect to the remote DRBD device
            immediately, DRBD keeps on trying to connect. With this option you can set the time
            between two retries. The default value is 10 seconds, the unit is 1 second.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term xml:id="ping-int"><option>ping-int <replaceable>time</replaceable></option></term>

          <listitem>
            <para><indexterm significance="normal">
                <primary>drbd.conf</primary>

                <secondary>ping-int</secondary>
              </indexterm> If the TCP/IP connection linking a DRBD device pair is idle for more
            than <replaceable>time</replaceable> seconds, DRBD will generate a keep-alive packet
            to check if its partner is still alive. The default is 10 seconds, the unit is 1
            second.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term xml:id="ping-timeout"><option>ping-timeout <replaceable>time</replaceable></option></term>

          <listitem>
            <para><indexterm significance="normal">
                <primary>drbd.conf</primary>

                <secondary>ping-timeout</secondary>
              </indexterm> The time the peer has time to answer to a keep-alive packet. In case
            the peer's reply is not received within this time period, it is considered as dead.
            The default value is 500ms, the default unit are tenths of a second.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term xml:id="max-buffers"><option>max-buffers <replaceable>number</replaceable></option></term>

          <listitem>
            <para><indexterm significance="normal">
                <primary>drbd.conf</primary>

                <secondary>max-buffers</secondary>
              </indexterm>
  Limits the memory usage per DRBD minor device on the receiving side,
  or for internal buffers during resync or online-verify.
  Unit is PAGE_SIZE, which is 4 KiB on most systems.
  The minimum possible setting is hard coded to 32 (=128 KiB).
  These buffers are used to hold data blocks while they are written to/read from disk.
  To avoid possible distributed deadlocks on congestion, this setting is used
  as a throttle threshold rather than a hard limit.  Once more than max-buffers
  pages are in use, further allocation from this pool is throttled.
  You want to increase max-buffers if you cannot saturate the IO backend on the
  receiving side.
  </para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term xml:id="ko-count"><option>ko-count <replaceable>number</replaceable></option></term>

          <listitem>
            <para><indexterm significance="normal">
                <primary>drbd.conf</primary>

                <secondary>ko-count</secondary>
              </indexterm> In case the secondary node fails to complete a single write request for
            <replaceable>count</replaceable> times the <replaceable>timeout</replaceable>, it is
            expelled from the cluster. (I.e. the primary node will kill and restart the connection.)
	    To disable this feature, you should explicitly set it to 0; defaults may change between versions.
	    </para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term xml:id="max-epoch-size"><option>max-epoch-size <replaceable>number</replaceable></option></term>

          <listitem>
            <para><indexterm significance="normal">
                <primary>drbd.conf</primary>

                <secondary>max-epoch-size</secondary>
              </indexterm> The highest number of data blocks between two write barriers. If you
            set this smaller than 10, you might decrease your performance.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term xml:id="allow-two-primaries"><option>allow-two-primaries</option></term>

          <listitem>
            <para><indexterm significance="normal">
                <primary>drbd.conf</primary>

                <secondary>allow-two-primaries</secondary>
              </indexterm> With this option set you may assign the primary role to both nodes. You
            only should use this option if you use a shared storage file system on top of DRBD. At
            the time of writing the only ones are: OCFS2 and GFS. If you use this option with any
            other file system, you are going to crash your nodes and to corrupt your data!</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term xml:id="unplug-watermark"><option>unplug-watermark <replaceable>number</replaceable></option></term>

          <listitem>
            <indexterm significance="normal">
              <primary>drbd.conf</primary>

              <secondary>unplug-watermark</secondary>
            </indexterm>
            <para>
      This setting has no effect with recent kernels that use explicit on-stack
      plugging (upstream Linux kernel 2.6.39, distributions may have backported).
            </para>
            <para>When the number of pending write requests on the standby (secondary) node
            exceeds the <option>unplug-watermark</option>, we trigger the request processing of
            our backing storage device. Some storage controllers deliver better performance with
            small values, others deliver best performance when the value is set to the same value
            as max-buffers, yet others don't feel much effect at all.
            Minimum 16, default 128, maximum 131072.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term xml:id="cram-hmac-alg"><option>cram-hmac-alg</option></term>

          <listitem>
            <para><indexterm significance="normal">
                <primary>drbd.conf</primary>

                <secondary>cram-hmac-alg</secondary>
              </indexterm> You need to specify the HMAC algorithm to enable peer authentication at
            all. You are strongly encouraged to use peer authentication. The HMAC algorithm will
            be used for the challenge response authentication of the peer. You may specify any
            digest algorithm that is named in <option>/proc/crypto</option>.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term xml:id="shared-secret"><option>shared-secret</option></term>

          <listitem>
            <para><indexterm significance="normal">
                <primary>drbd.conf</primary>

                <secondary>shared-secret</secondary>
              </indexterm> The shared secret used in peer authentication. May be up to 64
            characters. Note that peer authentication is disabled as long as no
            <option>cram-hmac-alg</option> (see above) is specified.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term xml:id="after-sb-0pri"><option>after-sb-0pri </option> <replaceable>policy</replaceable></term>

          <listitem>
            <indexterm significance="normal">
              <primary>drbd.conf</primary>

              <secondary>after-sb-0pri</secondary>
            </indexterm>

            <para>possible policies are:</para>

            <variablelist>
              <varlistentry>
                <term xml:id="disconnect"><option>disconnect</option></term>

                <listitem>
                  <para>No automatic resynchronization, simply disconnect.</para>
                </listitem>
              </varlistentry>

              <varlistentry>
                <term xml:id="discard-younger-primary"><option>discard-younger-primary</option></term>

                <listitem>
                  <para>Auto sync from the node that was primary before the split-brain situation
                  happened.</para>
                </listitem>
              </varlistentry>

              <varlistentry>
                <term xml:id="discard-older-primary"><option>discard-older-primary</option></term>

                <listitem>
                  <para>Auto sync from the node that became primary as second during the
                  split-brain situation.</para>
                </listitem>
              </varlistentry>

              <varlistentry>
                <term xml:id="discard-zero-changes"><option>discard-zero-changes</option></term>

                <listitem>
                  <para>In case one node did not write anything since the split brain became
                  evident, sync from the node that wrote something to the node that did not write
                  anything. In case none wrote anything this policy uses a random decision to
                  perform a "resync" of 0 blocks. In case both have written something this policy
                  disconnects the nodes.</para>
                </listitem>
              </varlistentry>

              <varlistentry>
                <term xml:id="discard-least-changes"><option>discard-least-changes</option></term>

                <listitem>
                  <para>Auto sync from the node that touched more blocks during the split brain
                  situation.</para>
                </listitem>
              </varlistentry>

              <varlistentry>
                <term xml:id="discard-node-NODENAME"><option>discard-node-NODENAME</option></term>

                <listitem>
                  <para>Auto sync to the named node.</para>
                </listitem>
              </varlistentry>
            </variablelist>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term xml:id="after-sb-1pri"><option>after-sb-1pri </option> <replaceable>policy</replaceable></term>

          <listitem>
            <indexterm significance="normal">
              <primary>drbd.conf</primary>

              <secondary>after-sb-1pri</secondary>
            </indexterm>

            <para>possible policies are:</para>

            <variablelist>
              <varlistentry>
                <term xml:id="sb1-disconnect"><option>disconnect</option></term>

                <listitem>
                  <para>No automatic resynchronization, simply disconnect.</para>
                </listitem>
              </varlistentry>

              <varlistentry>
                <term xml:id="consensus"><option>consensus</option></term>

                <listitem>
                  <para>Discard the version of the secondary if the outcome of the
                  <option>after-sb-0pri</option> algorithm would also destroy the current
                  secondary's data. Otherwise disconnect.</para>
                </listitem>
              </varlistentry>

              <varlistentry>
                <term xml:id="sb1-violently-as0p"><option>violently-as0p</option></term>

                <listitem>
                  <para>Always take the decision of the <option>after-sb-0pri</option> algorithm,
                  even if that causes an erratic change of the primary's view of the data. This is
                  only useful if you use a one-node FS (i.e. not OCFS2 or GFS) with the
                  <option>allow-two-primaries</option> flag, <emphasis>AND</emphasis> if you
                  really know what you are doing. This is <emphasis>DANGEROUS and MAY CRASH YOUR
                  MACHINE</emphasis> if you have an FS mounted on the primary node.</para>
                </listitem>
              </varlistentry>

              <varlistentry>
                <term xml:id="discard-secondary"><option>discard-secondary</option></term>

                <listitem>
                  <para>Discard the secondary's version.</para>
                </listitem>
              </varlistentry>

              <varlistentry>
                <term xml:id="sb1-call-pri-lost-after-sb"><option>call-pri-lost-after-sb</option></term>

                <listitem>
                  <para>Always honor the outcome of the <option>after-sb-0pri </option> algorithm.
                  In case it decides the current secondary has the right data, it calls the
                  "pri-lost-after-sb" handler on the current primary.</para>
                </listitem>
              </varlistentry>
            </variablelist>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term xml:id="after-sb-2pri"><option>after-sb-2pri </option> <replaceable>policy</replaceable></term>

          <listitem>
            <indexterm significance="normal">
              <primary>drbd.conf</primary>

              <secondary>after-sb-2pri</secondary>
            </indexterm>

            <para>possible policies are:</para>

            <variablelist>
              <varlistentry>
                <term xml:id="sb2-disconnect"><option>disconnect</option></term>

                <listitem>
                  <para>No automatic resynchronization, simply disconnect.</para>
                </listitem>
              </varlistentry>

              <varlistentry>
                <term xml:id="sb2-violently-as0p"><option>violently-as0p</option></term>

                <listitem>
                  <para>Always take the decision of the <option>after-sb-0pri</option> algorithm,
                  even if that causes an erratic change of the primary's view of the data. This is
                  only useful if you use a one-node FS (i.e. not OCFS2 or GFS) with the
                  <option>allow-two-primaries</option> flag, <emphasis>AND</emphasis> if you
                  really know what you are doing. This is <emphasis>DANGEROUS and MAY CRASH YOUR
                  MACHINE</emphasis> if you have an FS mounted on the primary node.</para>
                </listitem>
              </varlistentry>

              <varlistentry>
                <term xml:id="sb2-call-pri-lost-after-sb"><option>call-pri-lost-after-sb</option></term>

                <listitem>
                  <para>Call the "pri-lost-after-sb" helper program on one of the machines. This
                  program is expected to reboot the machine, i.e. make it secondary.</para>
                </listitem>
              </varlistentry>
            </variablelist>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term xml:id="always-asbp"><option>always-asbp</option></term>

          <listitem>
            <para>Normally the automatic after-split-brain policies are only used if current
            states of the UUIDs do not indicate the presence of a third node.</para>

            <para>With this option you request that the automatic after-split-brain policies are
            used as long as the data sets of the nodes are somehow related. This might cause a
            full sync, if the UUIDs indicate the presence of a third node. (Or double faults led
            to strange UUID sets.)</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term xml:id="rr-conflict"><option>rr-conflict </option> <replaceable>policy</replaceable></term>

          <listitem>
            <indexterm significance="normal">
              <primary>drbd.conf</primary>

              <secondary>rr-conflict</secondary>
            </indexterm>

            <para>This option helps to solve the cases when the outcome of the resync decision is
            incompatible with the current role assignment in the cluster.</para>

            <variablelist>
              <varlistentry>
                <term xml:id="rr-disconnect"><option>disconnect</option></term>

                <listitem>
                  <para>No automatic resynchronization, simply disconnect.</para>
                </listitem>
              </varlistentry>

              <varlistentry>
                <term xml:id="violently"><option>violently</option></term>

                <listitem>
                  <para>Sync to the primary node is allowed, violating the assumption that data on
                  a block device are stable for one of the nodes. <emphasis>Dangerous, do not
                  use.</emphasis></para>
                </listitem>
              </varlistentry>

              <varlistentry>
                <term xml:id="call-pri-lost"><option>call-pri-lost</option></term>

                <listitem>
                  <para>Call the <option>pri-lost-after-sb</option> helper
                    program on one of the machines unless that machine can
                    demote to secondary. The helper program is expected to
                    reboot the machine, which brings the node into a secondary
                    role. Which machine runs the helper program is determined
                    by the <option>after-sb-0pri</option> strategy.</para>
                </listitem>
              </varlistentry>
            </variablelist>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term xml:id="data-integrity-alg"><option>data-integrity-alg </option> <replaceable>alg</replaceable></term>

          <listitem>
            <indexterm significance="normal">
              <primary>drbd.conf</primary>

              <secondary>data-integrity-alg</secondary>
            </indexterm>

            <para>DRBD can ensure the data integrity of the user's data on the network by
            comparing hash values. Normally this is ensured by the 16 bit checksums in the headers
            of TCP/IP packets.</para>

            <para>This option can be set to any of the kernel's data digest algorithms. In a
            typical kernel configuration you should have at least one of <option>md5</option>,
            <option>sha1</option>, and <option>crc32c</option> available. By default this is not
            enabled.</para>

            <para>See also the notes on data integrity.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term xml:id="tcp-cork"><option>tcp-cork</option></term>

          <listitem>
            <indexterm significance="normal">
              <primary>drbd.conf</primary>

              <secondary>tcp-cork</secondary>
            </indexterm>

            <para>DRBD usually uses the TCP socket option TCP_CORK to hint to the network stack
            when it can expect more data, and when it should flush out what it has in its send
            queue. It turned out that there is at least one network stack that performs worse when
            one uses this hinting method. Therefore we introducted this option. By setting
            <option>tcp-cork</option> to <option>no</option> you can disable the setting and
            clearing of the TCP_CORK socket option by DRBD.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term xml:id="on-congestion"><option>on-congestion <replaceable>congestion_policy</replaceable></option></term>

          <term xml:id="congestion-fill"><option>congestion-fill <replaceable>fill_threshold</replaceable></option></term>

          <term xml:id="congestion-extents"><option>congestion-extents
          <replaceable>active_extents_threshold</replaceable></option></term>

          <listitem>
            <para>By default DRBD blocks when the available TCP send queue becomes full. That
            means it will slow down the application that generates the write requests that cause
            DRBD to send more data down that TCP connection.</para>

            <para>When DRBD is deployed with DRBD-proxy it might be more desirable that DRBD goes
            into AHEAD/BEHIND mode shortly before the send queue becomes full. In AHEAD/BEHIND
            mode DRBD does no longer replicate data, but still keeps the connection open.</para>

            <para>The advantage of the AHEAD/BEHIND mode is that the application is not slowed
            down, even if DRBD-proxy's buffer is not sufficient to buffer all write requests. The
            downside is that the peer node falls behind, and that a resync will be necessary to
            bring it back into sync. During that resync the peer node will have an inconsistent
            disk.</para>

            <para>Available <replaceable>congestion_policy</replaceable>s are
            <option>block</option> and <option>pull-ahead</option>. The default is
            <option>block</option>. <replaceable>Fill_threshold</replaceable> might be in the
            range of 0 to 10GiBytes. The default is 0 which disables the check.
            <replaceable>Active_extents_threshold</replaceable> has the same limits as
            <option>al-extents</option>.</para>

            <para>The AHEAD/BEHIND mode and its settings are available since DRBD 8.3.10.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term xml:id="wfc-timeout"><option>wfc-timeout <replaceable>time</replaceable></option></term>

          <listitem>
            <para>Wait for connection timeout. <indexterm significance="normal">
                <primary>drbd.conf</primary>

                <secondary>wfc-timeout</secondary>
              </indexterm> The init script <citerefentry>
                <refentrytitle>drbd</refentrytitle>

                <manvolnum>8</manvolnum>
              </citerefentry> blocks the boot process until the DRBD resources are connected. When
            the cluster manager starts later, it does not see a resource with internal
            split-brain. In case you want to limit the wait time, do it here. Default is 0, which
            means unlimited. The unit is seconds.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term xml:id="degr-wfc-timeout"><option>degr-wfc-timeout <replaceable>time</replaceable></option></term>

          <listitem>
            <para><indexterm significance="normal">
                <primary>drbd.conf</primary>

                <secondary>degr-wfc-timeout</secondary>
              </indexterm> Wait for connection timeout, if this node was a degraded cluster. In
            case a degraded cluster (= cluster with only one node left) is rebooted, this timeout
            value is used instead of wfc-timeout, because the peer is less likely to show up in
            time, if it had been dead before. Value 0 means unlimited.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term xml:id="outdated-wfc-timeout"><option>outdated-wfc-timeout <replaceable>time</replaceable></option></term>

          <listitem>
            <para><indexterm significance="normal">
                <primary>drbd.conf</primary>

                <secondary>outdated-wfc-timeout</secondary>
              </indexterm> Wait for connection timeout, if the peer was outdated. In case a
            degraded cluster (= cluster with only one node left) with an outdated peer disk is
            rebooted, this timeout value is used instead of wfc-timeout, because the peer is not
            allowed to become primary in the meantime. Value 0 means unlimited.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term xml:id="wait-after-sb"><option>wait-after-sb</option></term>

          <listitem>
            <para>By setting this option you can make the init script to continue to wait even if
            the device pair had a split brain situation and therefore refuses to connect.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term xml:id="become-primary-on"><option>become-primary-on <replaceable>node-name</replaceable></option></term>

          <listitem>
            <para>Sets on which node the device should be promoted to primary role by the init
            script. The <replaceable>node-name</replaceable> might either be a host name or the
            keyword <option>both</option>. When this option is not set the devices stay in
            secondary role on both nodes. Usually one delegates the role assignment to a cluster
            manager (e.g. heartbeat).</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term xml:id="stacked-timeouts"><option>stacked-timeouts</option></term>

          <listitem>
            <para>Usually <option>wfc-timeout</option> and <option>degr-wfc-timeout</option> are
            ignored for stacked devices, instead twice the amount of <option>connect-int</option>
            is used for the connection timeouts. With the <option>stacked-timeouts</option>
            keyword you disable this, and force DRBD to mind the <option>wfc-timeout</option> and
            <option>degr-wfc-timeout</option> statements. Only do that if the peer of the stacked
            resource is usually not available or will usually not become primary. By using this
            option incorrectly, you run the risk of causing unexpected split brain.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term xml:id="resync-rate"><option>resync-rate <replaceable>rate</replaceable></option></term>

          <listitem>
            <para><indexterm significance="normal">
                <primary>drbd.conf</primary>

                <secondary>resync-rate</secondary>
              </indexterm> To ensure a smooth operation of the application on top of DRBD, it is
            possible to limit the bandwidth which may be used by background synchronizations. The
            default is 250 KB/sec, the default unit is KB/sec. Optional suffixes K, M, G are
            allowed.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term xml:id="use-rle"><option>use-rle</option></term>

          <listitem>
            <para><indexterm significance="normal">
                <primary>drbd.conf</primary>

                <secondary>use-rle</secondary>
              </indexterm> During resync-handshake, the dirty-bitmaps of the nodes are exchanged
            and merged (using bit-or), so the nodes will have the same understanding of which
            blocks are dirty. On large devices, the fine grained dirty-bitmap can become large as
            well, and the bitmap exchange can take quite some time on low-bandwidth links.</para>

            <para>Because the bitmap typically contains compact areas where all bits are unset
            (clean) or set (dirty), a simple run-length encoding scheme can considerably reduce
            the network traffic necessary for the bitmap exchange.</para>

            <para>For backward compatibility reasons, and because on fast links this possibly does
            not improve transfer time but consumes cpu cycles, this defaults to off.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term xml:id="socket-check-timeout"><option>socket-check-timeout <replaceable>value</replaceable></option></term>

          <listitem>
            <para><indexterm significance="normal">
                <primary>drbd.conf</primary>

                <secondary>socket-check-timeout</secondary>
		</indexterm> In setups involving a DRBD-proxy and connections that experience a lot of
	    buffer-bloat it might be necessary to set <option>ping-timeout</option> to an
	    unusual high value. By default DRBD uses the same value to wait if a newly
	    established TCP-connection is stable. Since the DRBD-proxy is usually located
	    in the same data center such a long wait time may hinder DRBD's connect process.
	    </para>
	    <para>In such setups <option>socket-check-timeout</option> should be set to
	    at least to the round trip time between DRBD and DRBD-proxy. I.e. in most
	    cases to 1.</para>
	    <para>
	    The default unit is tenths of a second, the default value is 0 (which causes
	    DRBD to use the value of <option>ping-timeout</option> instead).
	    Introduced in 8.4.5.</para>

          </listitem>
        </varlistentry>

        <varlistentry>
          <term xml:id="resync-after"><option>resync-after <replaceable>res-name</replaceable></option></term>

          <listitem>
            <para><indexterm significance="normal">
                <primary>drbd.conf</primary>

                <secondary>resync-after</secondary>
              </indexterm> By default, resynchronization of all devices would run in parallel. By
            defining a resync-after dependency, the resynchronization of this resource will start
            only if the resource <replaceable>res-name</replaceable> is already in connected state
            (i.e., has finished its resynchronization).</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term xml:id="al-extents"><option>al-extents <replaceable>extents</replaceable></option></term>

          <listitem>
            <para><indexterm significance="normal">
                <primary>drbd.conf</primary>

                <secondary>al-extents</secondary>
              </indexterm> DRBD automatically performs hot area detection. With this parameter you
            control how big the hot area (= active set) can get. Each extent marks 4M of the
            backing storage (= low-level device). In case a primary node leaves the cluster
            unexpectedly, the areas covered by the active set must be resynced upon rejoining of
            the failed node. The data structure is stored in the meta-data area, therefore each
            change of the active set is a write operation to the meta-data device. A higher number
            of extents gives longer resync times but less updates to the meta-data. The default
            number of <replaceable>extents</replaceable> is 1237. (Minimum: 7, Maximum:
            65534)</para>
	    <para>
	      Note that the effective maximum may be smaller, depending on how
	      you created the device meta data, see also
	      <citerefentry><refentrytitle>drbdmeta</refentrytitle><manvolnum>8</manvolnum></citerefentry>.
	      The effective maximum is 919 * (available on-disk activity-log ring-buffer area/4kB -1),
	      the default 32kB ring-buffer effects a maximum of 6433 (covers more than 25 GiB of data).
	      We recommend to keep this well within the amount your backend storage
	      and replication link are able to resync inside of about 5 minutes.
	    </para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term xml:id="al-updates"><option>al-updates <group choice="req" rep="norepeat">
	     <arg choice="plain" rep="norepeat">yes</arg>
	     <arg choice="plain" rep="norepeat">no</arg>
	  </group></option></term>

          <listitem>
            <para><indexterm significance="normal">
                <primary>drbd.conf</primary>
                <secondary>al-updates</secondary>
	      </indexterm> DRBD's activity log transaction writing makes it possible, that
	    after the crash of a primary node a partial (bit-map based) resync is
	    sufficient to bring the node back to up-to-date.
	    Setting <option>al-updates</option> to <option>no</option> might increase
	    normal operation performance but causes DRBD to do a full resync
	    when a crashed primary gets reconnected. The default value is <option>yes</option>.
	    </para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term xml:id="verify-alg"><option>verify-alg <replaceable>hash-alg</replaceable></option></term>

          <listitem>
            <para>During online verification (as initiated by the <command
            moreinfo="none">verify</command> sub-command), rather than doing a bit-wise
            comparison, DRBD applies a hash function to the contents of every block being
            verified, and compares that hash with the peer. This option defines the hash algorithm
            being used for that purpose. It can be set to any of the kernel's data digest
            algorithms. In a typical kernel configuration you should have at least one of
            <option>md5</option>, <option>sha1</option>, and <option>crc32c</option> available. By
            default this is not enabled; you must set this option explicitly in order to be able
            to use on-line device verification.</para>

            <para>See also the notes on data integrity.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term xml:id="csums-alg"><option>csums-alg <replaceable>hash-alg</replaceable></option></term>

          <listitem>
            <para>A resync process sends all marked data blocks from the source to the destination
            node, as long as no <option>csums-alg</option> is given. When one is specified the
            resync process exchanges hash values of all marked blocks first, and sends only those
            data blocks that have different hash values.</para>

            <para>This setting is useful for DRBD setups with low bandwidth links. During the
            restart of a crashed primary node, all blocks covered by the activity log are marked
            for resync. But a large part of those will actually be still in sync, therefore using
            <option>csums-alg</option> will lower the required bandwidth in exchange for CPU
            cycles.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term xml:id="c-plan-ahead"><option>c-plan-ahead <replaceable>plan_time</replaceable></option></term>

          <term xml:id="c-fill-target"><option>c-fill-target <replaceable>fill_target</replaceable></option></term>

          <term xml:id="c-delay-target"><option>c-delay-target <replaceable>delay_target</replaceable></option></term>

          <term xml:id="c-max-rate"><option>c-max-rate <replaceable>max_rate</replaceable></option></term>

          <listitem>
            <para>The dynamic resync speed controller gets enabled with setting
            <replaceable>plan_time</replaceable> to a positive value. It aims to fill the buffers
            along the data path with either a constant amount of data
            <replaceable>fill_target</replaceable>, or aims to have a constant delay time of
            <replaceable>delay_target</replaceable> along the path. The controller has an upper
            bound of <replaceable>max_rate</replaceable>.</para>

            <para>By <replaceable>plan_time</replaceable> the agility of the controller is
            configured. Higher values yield for slower/lower responses of the controller to
            deviation from the target value. It should be at least 5 times RTT. For regular data
            paths a <replaceable>fill_target</replaceable> in the area of 4k to 100k is
            appropriate. For a setup that contains drbd-proxy it is advisable to use
            <replaceable>delay_target</replaceable> instead. Only when
            <replaceable>fill_target</replaceable> is set to 0 the controller will use
            <replaceable>delay_target</replaceable>. 5 times RTT is a reasonable starting value.
            <replaceable>Max_rate</replaceable> should be set to the bandwidth available between
            the DRBD-hosts and the machines hosting DRBD-proxy, or to the available
            disk-bandwidth.</para>

            <para>The default value of <replaceable>plan_time</replaceable> is 0, the default unit
            is 0.1 seconds. <replaceable>Fill_target</replaceable> has 0 and sectors as default
            unit. <replaceable>Delay_target</replaceable> has 1 (100ms) and 0.1 as default unit.
            <replaceable>Max_rate</replaceable> has 10240 (100MiB/s) and KiB/s as default
            unit.</para>

            <para>The dynamic resync speed controller and its settings are available since DRBD
            8.3.9.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term xml:id="c-min-rate"><option>c-min-rate <replaceable>min_rate</replaceable></option></term>

          <listitem>
            <para>A node that is primary and sync-source has to schedule application IO requests
            and resync IO requests. The <replaceable>min_rate</replaceable> tells DRBD use only up
            to min_rate for resync IO and to dedicate all other available IO bandwidth to
            application requests.</para>

            <para>Note: The value 0 has a special meaning. It disables the limitation of resync IO
            completely, which might slow down application IO considerably. Set it to a value of 1,
            if you prefer that resync IO never slows down application IO.</para>

            <para>Note: Although the name might suggest that it is a lower bound for the dynamic
            resync speed controller, it is not. If the DRBD-proxy buffer is full, the dynamic
            resync speed controller is free to lower the resync speed down to 0, completely
            independent of the <option>c-min-rate</option> setting.</para>

            <para>The default value of <replaceable>min_rate</replaceable> is 250,
	    in units of KiB/s</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term xml:id="on-no-data-accessible"><option>on-no-data-accessible
          <replaceable>ond-policy</replaceable></option></term>

          <listitem>
            <para>This setting controls what happens to IO requests on a degraded, disk less node
            (I.e. no data store is reachable). The available policies are
            <option>io-error</option> and <option>suspend-io</option>.</para>

            <para>If <replaceable>ond-policy</replaceable> is set to <option>suspend-io</option>
            you can either resume IO by attaching/connecting the last lost data storage, or by the
            <command moreinfo="none">drbdadm resume-io <replaceable>res</replaceable></command>
            command. The latter will result in IO errors of course.</para>

            <para>The default is <option>io-error</option>. This setting is available since DRBD
            8.3.9.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term xml:id="cpu-mask"><option>cpu-mask <replaceable>cpu-mask</replaceable></option></term>

          <listitem>
            <para><indexterm significance="normal">
                <primary>drbd.conf</primary>

                <secondary>cpu-mask</secondary>
              </indexterm> Sets the cpu-affinity-mask for DRBD's kernel threads of this device.
            The default value of <replaceable>cpu-mask</replaceable> is 0, which means that DRBD's
            kernel threads should be spread over all CPUs of the machine. This value must be given
            in hexadecimal notation. If it is too big it will be truncated.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term xml:id="pri-on-incon-degr"><option>pri-on-incon-degr <replaceable>cmd</replaceable></option></term>

          <listitem>
            <para><indexterm significance="normal">
                <primary>drbd.conf</primary>

                <secondary>pri-on-incon-degr</secondary>
              </indexterm> This handler is called if the node is primary, degraded and if the
            local copy of the data is inconsistent.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term xml:id="pri-lost-after-sb"><option>pri-lost-after-sb <replaceable>cmd</replaceable></option></term>

          <listitem>
            <para><indexterm significance="normal">
                <primary>drbd.conf</primary>

                <secondary>pri-lost-after-sb</secondary>
              </indexterm> The node is currently primary, but lost the after-split-brain auto
            recovery procedure. As as consequence, it should be abandoned.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term xml:id="pri-lost"><option>pri-lost <replaceable>cmd</replaceable></option></term>

          <listitem>
            <para><indexterm significance="normal">
                <primary>drbd.conf</primary>

                <secondary>pri-lost</secondary>
              </indexterm> The node is currently primary, but DRBD's algorithm thinks that it
            should become sync target. As a consequence it should give up its primary role.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term xml:id="fence-peer"><option>fence-peer <replaceable>cmd</replaceable></option></term>

          <listitem>
            <para><indexterm significance="normal">
                <primary>drbd.conf</primary>

                <secondary>fence-peer</secondary>
              </indexterm> The handler is part of the <option>fencing</option> mechanism. This
            handler is called in case the node needs to fence the peer's disk. It should use other
            communication paths than DRBD's network link.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term xml:id="local-io-error"><option>local-io-error <replaceable>cmd</replaceable></option></term>

          <listitem>
            <para><indexterm significance="normal">
                <primary>drbd.conf</primary>

                <secondary>local-io-error</secondary>
              </indexterm> DRBD got an IO error from the local IO subsystem.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term xml:id="initial-split-brain"><option>initial-split-brain <replaceable>cmd</replaceable></option></term>

          <listitem>
            <para><indexterm significance="normal">
                <primary>drbd.conf</primary>

                <secondary>initial-split-brain</secondary>
              </indexterm> DRBD has connected and detected a split brain situation. This handler
            can alert someone in all cases of split brain, not just those that go
            unresolved.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term xml:id="split-brain"><option>split-brain <replaceable>cmd</replaceable></option></term>

          <listitem>
            <para><indexterm significance="normal">
                <primary>drbd.conf</primary>

                <secondary>split-brain</secondary>
              </indexterm> DRBD detected a split brain situation but remains unresolved. Manual
            recovery is necessary. This handler should alert someone on duty.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term xml:id="before-resync-target"><option>before-resync-target <replaceable>cmd</replaceable></option></term>

          <listitem>
            <para><indexterm significance="normal">
                <primary>drbd.conf</primary>

                <secondary>before-resync-target</secondary>
              </indexterm> DRBD calls this handler just before a resync begins on the node that
            becomes resync target. It might be used to take a snapshot of the backing block
            device.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term xml:id="after-resync-target"><option>after-resync-target <replaceable>cmd</replaceable></option></term>

          <listitem>
            <para><indexterm significance="normal">
                <primary>drbd.conf</primary>

                <secondary>after-resync-target</secondary>
              </indexterm> DRBD calls this handler just after a resync operation finished on the
            node whose disk just became consistent after being inconsistent for the duration of
            the resync. It might be used to remove a snapshot of the backing device that was
            created by the <option>before-resync-target</option> handler.</para>
          </listitem>
        </varlistentry>
      </variablelist>
    </refsect2>

    <refsect2>
      <title>Other Keywords</title>

      <variablelist>
        <varlistentry>
          <term xml:id="include"><option>include <replaceable>file-pattern</replaceable></option></term>

          <listitem>
            <para><indexterm significance="normal">
                <primary>drbd.conf</primary>

                <secondary>include</secondary>
              </indexterm> Include all files matching the wildcard pattern
            <replaceable>file-pattern</replaceable>. The <option>include</option> statement is
            only allowed on the top level, i.e. it is not allowed inside any section.</para>
          </listitem>
        </varlistentry>
      </variablelist>
    </refsect2>
  </refsect1>

  <refsect1 id="data-integrity">
    <title>Notes on data integrity</title>

    <para>There are two independent methods in DRBD to ensure the integrity of the mirrored data.
    The online-verify mechanism and the <option>data-integrity-alg</option> of the
    <option>network</option> section.</para>

    <para>Both mechanisms might deliver false positives if the user of DRBD modifies the data
    which gets written to disk while the transfer goes on. This may happen for swap, or for
    certain append while global sync, or truncate/rewrite workloads, and not necessarily poses a
    problem for the integrity of the data. Usually when the initiator of the data transfer does
    this, it already knows that that data block will not be part of an on disk data structure, or
    will be resubmitted with correct data soon enough.</para>

    <para>The <option>data-integrity-alg</option> causes the receiving side to log an error about
    "Digest integrity check FAILED: Ns +x\n", where N is the sector offset, and x is the size of
    the request in bytes. It will then disconnect, and reconnect, thus causing a quick resync. If
    the sending side at the same time detected a modification, it warns about "Digest mismatch,
    buffer modified by upper layers during write: Ns +x\n", which shows that this was a false
    positive. The sending side may detect these buffer modifications immediately after the
    unmodified data has been copied to the tcp buffers, in which case the receiving side won't
    notice it.</para>

    <para>The most recent (2007) example of systematic corruption was an issue with the TCP
    offloading engine and the driver of a certain type of GBit NIC. The actual corruption happened
    on the DMA transfer from core memory to the card. Since the TCP checksum gets calculated on
    the card, this type of corruption stays undetected as long as you do not use either the online
    <option>verify</option> or the <option>data-integrity-alg</option>.</para>

    <para>We suggest to use the <option>data-integrity-alg</option> only during a pre-production
    phase due to its CPU costs. Further we suggest to do online <option>verify</option> runs
    regularly e.g. once a month during a low load period.</para>
  </refsect1>

  <refsect1>
    <title>Version</title>

    <simpara>This document was revised for version 8.4.0 of the DRBD distribution.</simpara>
  </refsect1>

  <refsect1>
    <title>Author</title>

    <simpara>Written by Philipp Reisner <email>philipp.reisner@linbit.com</email> and Lars
    Ellenberg <email>lars.ellenberg@linbit.com</email>.</simpara>
  </refsect1>

  <refsect1>
    <title>Reporting Bugs</title>

    <simpara>Report bugs to <email>drbd-user@lists.linbit.com</email>.</simpara>
  </refsect1>

  <refsect1>
    <title>Copyright</title>

    <simpara>Copyright 2001-2008 LINBIT Information Technologies, Philipp Reisner, Lars Ellenberg.
    This is free software; see the source for copying conditions. There is NO warranty; not even
    for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.</simpara>
  </refsect1>

  <refsect1>
    <title>See Also</title>

    <para>
      <citerefentry><refentrytitle>drbd</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>drbddisk</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>drbdsetup</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>drbdmeta</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>drbdadm</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
      <ulink url="http://www.drbd.org/users-guide/"><citetitle>DRBD User's Guide</citetitle></ulink>,
      <ulink url="http://www.drbd.org/"><citetitle>DRBD web site</citetitle></ulink>
    </para>

  </refsect1>
</refentry>