<?xml version="1.0"?>
<!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>5 Dec 2008</date>
    <productname>DRBD</productname>
    <productnumber>8.3.2</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.
<example><title>A small drbd.conf file</title><programlisting format="linespecific">global { usage-count yes; }
common { syncer { rate 10M; } }
resource r0 {
	protocol C;
	net {
		cram-hmac-alg sha1;
		shared-secret "FooFunFactory";
	}
	on alice {
		device    minor 1;
		disk      /dev/sda7;
		address   10.1.1.31:7789;
		meta-disk internal;
	}
	on bob {
		device    minor 1;
		disk      /dev/sda7;
		address   10.1.1.32:7789;
		meta-disk internal;
	}
}</programlisting></example>
 In this example, there is a single DRBD resource (called r0) which uses
 protocol C for the connection between its devices.
 The device 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.
 </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>
            <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 '<quote>skip</quote>'.
  </para>
          </listitem>
        </varlistentry>
        <varlistentry>
          <term>
            <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>
            <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>syncer</option>,
    a <option>handlers</option>,
    a <option>net</option> and a <option>disk</option> section.
  </para>
          </listitem>
        </varlistentry>
        <varlistentry>
          <term>
            <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>syncer</option>,
    a <option>handlers</option>,
    a <option>net</option> and a <option>disk</option> section.
    Required parameter in this section: <option>protocol</option>.
  </para>
          </listitem>
        </varlistentry>
        <varlistentry>
          <term>
            <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 parameters in this section: <option>device</option>,
    <option>disk</option>, <option>address</option>, <option>meta-disk</option>,
    <option>flexible-meta-disk</option>.
  </para>
          </listitem>
        </varlistentry>
        <varlistentry>
          <term>
            <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>
            <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>,
    <option>flexible-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>
            <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>use-bmbv</option>,
    <option>no-disk-barrier</option>, <option>no-disk-flushes</option>,
    <option>no-disk-drain</option>, <option>no-md-flushes</option>,
    <option>max-bio-bvecs</option>, <option>disk-timeout</option>.
  </para>
          </listitem>
        </varlistentry>
        <varlistentry>
          <term>
            <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>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>
  </para>
          </listitem>
        </varlistentry>
        <varlistentry>
          <term>
            <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>
            <option>syncer</option>
          </term>
          <listitem>
            <para><indexterm significance="normal"><primary>drbd.conf</primary><secondary>syncer</secondary></indexterm>
    This section is used to fine tune the synchronization daemon
    for the device. Please
    refer to <citerefentry><refentrytitle>drbdsetup</refentrytitle><manvolnum>8</manvolnum></citerefentry> for a detailed description
    of this section's parameters.
    Optional parameters:
    <option>rate</option>, <option>after</option>, <option>al-extents</option>,
    <option>use-rle</option>,
    <option>cpu-mask</option>, <option>verify-alg</option>, <option>csums-alg</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>
    and <option>on-no-data-accessible</option>.
  </para>
          </listitem>
        </varlistentry>
        <varlistentry>
          <term>
            <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:
      <variablelist>
        <varlistentry>
	  <term><option>DRBD_RESOURCE</option></term>
	  <listitem><para>is the name of the resource</para></listitem>
        </varlistentry>
        <varlistentry>
	  <term><option>DRBD_MINOR</option></term>
	  <listitem><para>is the minor number of the DRBD device, in decimal.</para></listitem>
        </varlistentry>
        <varlistentry>
	  <term><option>DRBD_CONF</option></term>
	  <listitem><para>
	      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>
        </varlistentry>
        <varlistentry>
	  <term><option>DRBD_PEER_AF</option>,
	  <option>DRBD_PEER_ADDRESS</option>,
	  <option>DRBD_PEERS</option></term>
	  <listitem><para>
	      are the address family (e.g. <option>ipv6</option>),
	      the peer's address and hostnames.
	  </para></listitem>
        </varlistentry>
      </variablelist>
      <option>DRBD_PEER</option> (note the singular form) is deprecated, and superseeded by DRBD_PEERS.
  </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>
            <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 255.
  </para>
            <para>Use <replaceable>minor-count</replaceable>
   if you want to define massively more resources later without reloading
   the DRBD kernel
   module. 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>
            <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>
            <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>
            <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>
            <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>
            <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>
            <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>
            <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>
            <option>meta-disk <replaceable>internal</replaceable></option>
          </term>
          <term>
            <option>flexible-meta-disk <replaceable>internal</replaceable></option>
          </term>
          <term>
            <option>meta-disk <replaceable>device [index]</replaceable></option>
          </term>
          <term>
            <option>flexible-meta-disk <replaceable>device </replaceable></option>
          </term>
          <listitem>
            <para><indexterm significance="normal"><primary>drbd.conf</primary><secondary>meta-disk</secondary></indexterm><indexterm significance="normal"><primary>drbd.conf</primary><secondary>flexible-meta-disk</secondary></indexterm>
    Internal means that the last part of the backing device is used to store
    the meta-data. You must not use <replaceable>[index]</replaceable> with
    internal. Note: Regardless of whether you use the <option>meta-disk</option> or
    the <option>flexible-meta-disk</option> keyword, it will always be of
    the size needed for the remaining storage size.
  </para>
            <para>   You can use a single block <replaceable>device</replaceable> to store
   meta-data of multiple DRBD devices.
   E.g. use meta-disk /dev/sde6[0]; and meta-disk /dev/sde6[1];
   for two different resources. In this case the meta-disk
   would need to be at least 256 MB in size.
  </para>
            <para>   With the <option>flexible-meta-disk</option> keyword you specify
   a block device as meta-data storage. You usually use this with LVM,
   which allows you to have many variable sized block devices.
   The required size of the meta-disk block device is
   36kB + Backing-Storage-size / 32k. Round this number to the next 4kb
   boundary up and you have the exact size.
   Rule of the thumb: 32kByte per 1GByte of storage, round up to the next
   MB.</para>
          </listitem>
        </varlistentry>
        <varlistentry>
          <term>
            <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>
            <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>
                  <option>dont-care</option>
                </term>
                <listitem>
                  <para>	This is the default policy. No fencing actions are taken.
      </para>
                </listitem>
              </varlistentry>
              <varlistentry>
                <term>
                  <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>
                  <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>
            <option>use-bmbv</option>
          </term>
          <listitem>
            <indexterm significance="normal">
              <primary>drbd.conf</primary>
              <secondary>use-bmbv</secondary>
            </indexterm>
            <para>      In case the backing storage's driver has a merge_bvec_fn() function,
       DRBD has to pretend that it can only process IO requests in
      units not larger than 4KiB. (At the time of writing the only known drivers which have such a function
      are: md (software raid driver), dm (device mapper - LVM) and DRBD
      itself).</para>
            <para>      To get the best performance out of DRBD on top of software RAID (or any
      other driver with a merge_bvec_fn() function) you might enable this
      function, if you know for sure that the merge_bvec_fn() function will
      deliver the same results on all nodes of your cluster. I.e. the
      physical disks of the software RAID are of exactly the same
      type. <emphasis>Use this option only if you know what you are
      doing.</emphasis>
    </para>
          </listitem>
        </varlistentry>
        <varlistentry>
          <term>
            <option>no-disk-barrier</option>
          </term>
          <term>
            <option>no-disk-flushes</option>
          </term>
          <term>
            <option>no-disk-drain</option>
          </term>
          <listitem>
            <indexterm significance="normal">
              <primary>drbd.conf</primary>
              <secondary>no-disk-barrier</secondary>
            </indexterm>
            <indexterm significance="normal">
              <primary>drbd.conf</primary>
              <secondary>no-disk-flushes</secondary>
            </indexterm>
            <indexterm significance="normal">
              <primary>drbd.conf</primary>
              <secondary>no-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 the user.
	    </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 disabled by the <option>no-disk-barrier</option> 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 using the <option>no-disk-flushes</option> 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>
            <option>no-md-flushes</option>
          </term>
          <listitem>
            <indexterm significance="normal">
              <primary>drbd.conf</primary>
              <secondary>no-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>no-disk-flushes</option>.
    </para>
          </listitem>
        </varlistentry>
        <varlistentry>
          <term>
            <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 driver of the <replaceable>lower_device</replaceable>
	      does not finish an IO request within <replaceable>disk_timeout</replaceable>,
	      DRBD considers the disk as failed. If DRBD is connected to a remote host,
	      it will reissue local pending IO requests to the peer, and ship all new
	      IO requests to the peer only. The disk state advances to diskless, as soon
	      as the backing block device has finished all IO requests.</para>
	      <para> The default value of is 0, which means that no timeout is enforced.
	      The default unit is 100ms. This option is available since 8.3.12.
	      </para>
          </listitem>
        </varlistentry>


        <varlistentry>
          <term>
            <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>
            <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>
            <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>
            <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>
            <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>
            <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>
            <option>max-buffers <replaceable>number</replaceable></option>
          </term>
          <listitem>
            <para><indexterm significance="normal"><primary>drbd.conf</primary><secondary>max-buffers </secondary></indexterm>
  Maximum number of requests to be allocated by DRBD. Unit is PAGE_SIZE,
  which is 4 KiB on most systems.
  The minimum is hard coded to 32 (=128 KiB).
  For high-performance installations it might help if you
  increase that number. These buffers are used to hold
  data blocks while they are written to disk.
  </para>
          </listitem>
        </varlistentry>
        <varlistentry>
          <term>
            <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 goes into <option>StandAlone</option> mode.)
	      The default value is 0, which disables this feature.
  </para>
          </listitem>
        </varlistentry>
        <varlistentry>
          <term>
            <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>
            <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>
            <option>unplug-watermark <replaceable>number</replaceable></option>
          </term>
          <listitem>
            <indexterm significance="normal">
              <primary>drbd.conf</primary>
              <secondary>unplug-watermark </secondary>
            </indexterm>
            <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. Minimum 16, default 128, maximum
      131072.
    </para>
          </listitem>
        </varlistentry>
        <varlistentry>
          <term>
            <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>
            <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>
            <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>
                  <option>disconnect</option>
                </term>
                <listitem>
                  <para>		  No automatic resynchronization, simply disconnect.
		</para>
                </listitem>
              </varlistentry>
              <varlistentry>
                <term>
                  <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>
                  <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>
                  <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>
                  <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>
                  <option>discard-node-NODENAME</option>
                </term>
                <listitem>
                  <para>		Auto sync to the named node.
		</para>
                </listitem>
              </varlistentry>
            </variablelist>
          </listitem>
        </varlistentry>
        <varlistentry>
          <term>
            <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>
                  <option>disconnect</option>
                </term>
                <listitem>
                  <para>		  No automatic resynchronization, simply disconnect.
		</para>
                </listitem>
              </varlistentry>
              <varlistentry>
                <term>
                  <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>
                  <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>
                  <option>discard-secondary</option>
                </term>
                <listitem>
                  <para>		  Discard the secondary's version.
		</para>
                </listitem>
              </varlistentry>
              <varlistentry>
                <term>
                  <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>
            <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>
                  <option>disconnect</option>
                </term>
                <listitem>
                  <para>		  No automatic resynchronization, simply disconnect.
		</para>
                </listitem>
              </varlistentry>
              <varlistentry>
                <term>
                  <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>
                  <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>
            <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>
            <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>
                  <option>disconnect</option>
                </term>
                <listitem>
                  <para>		  No automatic resynchronization, simply disconnect.
		</para>
                </listitem>
              </varlistentry>
              <varlistentry>
                <term>
                  <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>
                  <option>call-pri-lost</option>
                </term>
                <listitem>
                  <para>                   Call the "pri-lost" 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>
            <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>
            <option>no-tcp-cork</option>
          </term>
          <listitem>
            <indexterm significance="normal">
              <primary>drbd.conf</primary>
              <secondary>no-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, which disables the setting and clearing of
              the TCP_CORK socket option by DRBD.</para>
          </listitem>
        </varlistentry>
        <varlistentry>
          <term>
            <option>on-congestion <replaceable>congestion_policy</replaceable></option>
          </term>
          <term>
	    <option>congestion-fill <replaceable>fill_threshold</replaceable></option>
          </term>
          <term>
	    <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>
            <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>
            <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>
            <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>
            <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>
            <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>
            <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>
            <option>rate <replaceable>rate</replaceable></option>
          </term>
          <listitem>
            <para><indexterm significance="normal"><primary>drbd.conf</primary><secondary>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>
            <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 compatibilty 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>
            <option>after <replaceable>res-name</replaceable></option>
          </term>
          <listitem>
            <para><indexterm significance="normal"><primary>drbd.conf</primary><secondary>after </secondary></indexterm>
  By default, resynchronization of all devices would run in parallel.
  By defining a sync-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>
            <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
  127. (Minimum: 7, Maximum: 3843)
  </para>
          </listitem>
        </varlistentry>
        <varlistentry>
          <term>
            <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>
            <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>
            <option>c-plan-ahead <replaceable>plan_time</replaceable></option>
          </term>
          <term>
	    <option>c-fill-target <replaceable>fill_target</replaceable></option>
          </term>
          <term>
	    <option>c-delay-target <replaceable>delay_target</replaceable></option>
          </term>
          <term>
	    <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>
	    <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>
	      <replaceable>Min_rate</replaceable> has 4096 (4MiB/s) and KiB/s as default unit.
	    </para>
          </listitem>
        </varlistentry>
        <varlistentry>
          <term>
            <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>
            <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>
            <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>
            <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>
            <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>
            <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>
            <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>
            <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>
            <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>
            <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>
            <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>
            <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 requst 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 8.3.2 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>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>