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<h1>Storage Management</h1>
<p>
Libvirt provides storage management on the physical host through
storage pools and volumes.
</p>
<p>
A storage pool is a quantity of storage set aside by an
administrator, often a dedicated storage administrator, for use
by virtual machines. Storage pools are divided into storage
volumes either by the storage administrator or the system
administrator, and the volumes are assigned to VMs as block
devices.
</p>
<p>
For example, the storage administrator responsible for an NFS
server creates a share to store virtual machines' data. The
system administrator defines a pool on the virtualization host
with the details of the share
(e.g. nfs.example.com:/path/to/share should be mounted on
/vm_data). When the pool is started, libvirt mounts the share
on the specified directory, just as if the system administrator
logged in and executed 'mount nfs.example.com:/path/to/share
/vmdata'. If the pool is configured to autostart, libvirt
ensures that the NFS share is mounted on the directory specified
when libvirt is started.
</p>
<p>
Once the pool is started, the files in the NFS share are
reported as volumes, and the storage volumes' paths may be
queried using the libvirt APIs. The volumes' paths can then be
copied into the section of a VM's XML definition describing the
source storage for the VM's block devices. In the case of NFS,
an application using the libvirt APIs can create and delete
volumes in the pool (files in the NFS share) up to the limit of
the size of the pool (the storage capacity of the share). Not
all pool types support creating and deleting volumes. Stopping
the pool (somewhat unfortunately referred to by virsh and the
API as "pool-destroy") undoes the start operation, in this case,
unmounting the NFS share. The data on the share is not modified
by the destroy operation, despite the name. See man virsh for
more details.
</p>
<p>
A second example is an iSCSI pool. A storage administrator
provisions an iSCSI target to present a set of LUNs to the host
running the VMs. When libvirt is configured to manage that
iSCSI target as a pool, libvirt will ensure that the host logs
into the iSCSI target and libvirt can then report the available
LUNs as storage volumes. The volumes' paths can be queried and
used in VM's XML definitions as in the NFS example. In this
case, the LUNs are defined on the iSCSI server, and libvirt
cannot create and delete volumes.
</p>
<p>
Storage pools and volumes are not required for the proper
operation of VMs. Pools and volumes provide a way for libvirt
to ensure that a particular piece of storage will be available
for a VM, but some administrators will prefer to manage their
own storage and VMs will operate properly without any pools or
volumes defined. On systems that do not use pools, system
administrators must ensure the availability of the VMs' storage
using whatever tools they prefer, for example, adding the NFS
share to the host's fstab so that the share is mounted at boot
time.
</p>
<p>
If at this point the value of pools and volumes over traditional
system administration tools is unclear, note that one of the
features of libvirt is its remote protocol, so it's possible to
manage all aspects of a virtual machine's lifecycle as well as
the configuration of the resources required by the VM. These
operations can be performed on a remote host entirely within the
libvirt API. In other words, a management application using
libvirt can enable a user to perform all the required tasks for
configuring the host for a VM: allocating resources, running the
VM, shutting it down and deallocating the resources, without
requiring shell access or any other control channel.
</p>
<p>
Libvirt supports the following storage pool types:
</p>
<ul><li>
<a href="#StorageBackendDir">Directory backend</a>
</li><li>
<a href="#StorageBackendFS">Local filesystem backend</a>
</li><li>
<a href="#StorageBackendNetFS">Network filesystem backend</a>
</li><li>
<a href="#StorageBackendLogical">Logical backend</a>
</li><li>
<a href="#StorageBackendDisk">Disk backend</a>
</li><li>
<a href="#StorageBackendISCSI">iSCSI backend</a>
</li><li>
<a href="#StorageBackendSCSI">SCSI backend</a>
</li><li>
<a href="#StorageBackendMultipath">Multipath backend</a>
</li></ul>
<h2>
<a name="StorageBackendDir" id="StorageBackendDir">Directory pool</a>
</h2>
<p>
A pool with a type of <code>dir</code> provides the means to manage
files within a directory. The files can be fully allocated raw files,
sparsely allocated raw files, or one of the special disk formats
such as <code>qcow</code>,<code>qcow2</code>,<code>vmdk</code>,
<code>cow</code>, etc as supported by the <code>qemu-img</code>
program. If the directory does not exist at the time the pool is
defined, the <code>build</code> operation can be used to create it.
</p>
<h3>Example pool input definition</h3>
<pre>
<pool type="dir">
<name>virtimages</name>
<target>
<path>/var/lib/virt/images</path>
</target>
</pool></pre>
<h3>Valid pool format types</h3>
<p>
The directory pool does not use the pool format type element.
</p>
<h3>Valid volume format types</h3>
<p>
One of the following options:
</p>
<ul><li><code>raw</code>: a plain file</li><li><code>bochs</code>: Bochs disk image format</li><li><code>cloop</code>: compressed loopback disk image format</li><li><code>cow</code>: User Mode Linux disk image format</li><li><code>dmg</code>: Mac disk image format</li><li><code>iso</code>: CDROM disk image format</li><li><code>qcow</code>: QEMU v1 disk image format</li><li><code>qcow2</code>: QEMU v2 disk image format</li><li><code>vmdk</code>: VMWare disk image format</li><li><code>vpc</code>: VirtualPC disk image format</li></ul>
<p>
When listing existing volumes all these formats are supported
natively. When creating new volumes, only a subset may be
available. The <code>raw</code> type is guaranteed always
available. The <code>qcow2</code> type can be created if
either <code>qemu-img</code> or <code>qcow-create</code> tools
are present. The others are dependent on support of the
<code>qemu-img</code> tool.
</p>
<h2>
<a name="StorageBackendFS" id="StorageBackendFS">Filesystem pool</a>
</h2>
<p>
This is a variant of the directory pool. Instead of creating a
directory on an existing mounted filesystem though, it expects
a source block device to be named. This block device will be
mounted and files managed in the directory of its mount point.
It will default to allowing the kernel to automatically discover
the filesystem type, though it can be specified manually if
required.
</p>
<h3>Example pool input</h3>
<pre>
<pool type="fs">
<name>virtimages</name>
<source>
<device path="/dev/VolGroup00/VirtImages"/>
</source>
<target>
<path>/var/lib/virt/images</path>
</target>
</pool></pre>
<h3>Valid pool format types</h3>
<p>
The filesystem pool supports the following formats:
</p>
<ul><li><code>auto</code> - automatically determine format</li><li>
<code>ext2</code>
</li><li>
<code>ext3</code>
</li><li>
<code>ext4</code>
</li><li>
<code>ufs</code>
</li><li>
<code>iso9660</code>
</li><li>
<code>udf</code>
</li><li>
<code>gfs</code>
</li><li>
<code>gfs2</code>
</li><li>
<code>vfat</code>
</li><li>
<code>hfs+</code>
</li><li>
<code>xfs</code>
</li></ul>
<h3>Valid volume format types</h3>
<p>
The valid volume types are the same as for the <code>directory</code>
pool type.
</p>
<h2>
<a name="StorageBackendNetFS" id="StorageBackendNetFS">Network filesystem pool</a>
</h2>
<p>
This is a variant of the filesystem pool. Instead of requiring
a local block device as the source, it requires the name of a
host and path of an exported directory. It will mount this network
filesystem and manage files within the directory of its mount
point. It will default to using NFS as the protocol.
</p>
<h3>Example pool input</h3>
<pre>
<pool type="netfs">
<name>virtimages</name>
<source>
<host name="nfs.example.com"/>
<dir path="/var/lib/virt/images"/>
</source>
<target>
<path>/var/lib/virt/images</path>
</target>
</pool></pre>
<h3>Valid pool format types</h3>
<p>
The network filesystem pool supports the following formats:
</p>
<ul><li><code>auto</code> - automatically determine format</li><li>
<code>nfs</code>
</li></ul>
<h3>Valid volume format types</h3>
<p>
The valid volume types are the same as for the <code>directory</code>
pool type.
</p>
<h2>
<a name="StorageBackendLogical" id="StorageBackendLogical">Logical volume pools</a>
</h2>
<p>
This provides a pool based on an LVM volume group. For a
pre-defined LVM volume group, simply providing the group
name is sufficient, while to build a new group requires
providing a list of source devices to serve as physical
volumes. Volumes will be allocated by carving out chunks
of storage from the volume group.
</p>
<h3>Example pool input</h3>
<pre>
<pool type="logical">
<name>HostVG</name>
<source>
<device path="/dev/sda1"/>
<device path="/dev/sdb1"/>
<device path="/dev/sdc1"/>
</source>
<target>
<path>/dev/HostVG</path>
</target>
</pool></pre>
<h3>Valid pool format types</h3>
<p>
The logical volume pool does not use the pool format type element.
</p>
<h3>Valid volume format types</h3>
<p>
The logical volume pool does not use the volume format type element.
</p>
<h2>
<a name="StorageBackendDisk" id="StorageBackendDisk">Disk volume pools</a>
</h2>
<p>
This provides a pool based on a physical disk. Volumes are created
by adding partitions to the disk. Disk pools are have constraints
on the size and placement of volumes. The 'free extents'
information will detail the regions which are available for creating
new volumes. A volume cannot span across 2 different free extents.
</p>
<h3>Example pool input</h3>
<pre>
<pool type="disk">
<name>sda</name>
<source>
<device path='/dev/sda'/>
</source>
<target>
<path>/dev</path>
</target>
</pool></pre>
<h3>Valid pool format types</h3>
<p>
The disk volume pool accepts the following pool format types, representing
the common partition table types:
</p>
<ul><li>
<code>dos</code>
</li><li>
<code>dvh</code>
</li><li>
<code>gpt</code>
</li><li>
<code>mac</code>
</li><li>
<code>bsd</code>
</li><li>
<code>pc98</code>
</li><li>
<code>sun</code>
</li></ul>
<p>
The <code>dos</code> or <code>gpt</code> formats are recommended for
best portability - the latter is needed for disks larger than 2TB.
</p>
<h3>Valid volume format types</h3>
<p>
The disk volume pool accepts the following volume format types, representing
the common partition entry types:
</p>
<ul><li>
<code>none</code>
</li><li>
<code>linux</code>
</li><li>
<code>fat16</code>
</li><li>
<code>fat32</code>
</li><li>
<code>linux-swap</code>
</li><li>
<code>linux-lvm</code>
</li><li>
<code>linux-raid</code>
</li><li>
<code>extended</code>
</li></ul>
<h2>
<a name="StorageBackendISCSI" id="StorageBackendISCSI">iSCSI volume pools</a>
</h2>
<p>
This provides a pool based on an iSCSI target. Volumes must be
pre-allocated on the iSCSI server, and cannot be created via
the libvirt APIs. Since /dev/XXX names may change each time libvirt
logs into the iSCSI target, it is recommended to configure the pool
to use <code>/dev/disk/by-path</code> or <code>/dev/disk/by-id</code>
for the target path. These provide persistent stable naming for LUNs
</p>
<h3>Example pool input</h3>
<pre>
<pool type="iscsi">
<name>virtimages</name>
<source>
<host name="iscsi.example.com"/>
<device path="demo-target"/>
</source>
<target>
<path>/dev/disk/by-path</path>
</target>
</pool></pre>
<h3>Valid pool format types</h3>
<p>
The iSCSI volume pool does not use the pool format type element.
</p>
<h3>Valid volume format types</h3>
<p>
The iSCSI volume pool does not use the volume format type element.
</p>
<h2>
<a name="StorageBackendSCSI" id="StorageBackendSCSI">SCSI volume pools</a>
</h2>
<p>
This provides a pool based on a SCSI HBA. Volumes are preexisting SCSI
LUNs, and cannot be created via the libvirt APIs. Since /dev/XXX names
aren't generally stable, it is recommended to configure the pool
to use <code>/dev/disk/by-path</code> or <code>/dev/disk/by-id</code>
for the target path. These provide persistent stable naming for LUNs
<span class="since">Since 0.6.2</span>
</p>
<h3>Example pool input</h3>
<pre>
<pool type="scsi">
<name>virtimages</name>
<source>
<adapter name="host0"/>
</source>
<target>
<path>/dev/disk/by-path</path>
</target>
</pool></pre>
<h3>Valid pool format types</h3>
<p>
The SCSI volume pool does not use the pool format type element.
</p>
<h3>Valid volume format types</h3>
<p>
The SCSI volume pool does not use the volume format type element.
</p>
<h2>
<a name="StorageBackendMultipath" id="StorageBackendMultipath">Multipath pools</a>
</h2>
<p>
This provides a pool that contains all the multipath devices on the
host. Volume creating is not supported via the libvirt APIs.
The target element is actually ignored, but one is required to appease
the libvirt XML parser.<br />
<br />
Configuring multipathing is not currently supported, this just covers
the case where users want to discover all the available multipath
devices, and assign them to guests.
<span class="since">Since 0.7.1</span>
</p>
<h3>Example pool input</h3>
<pre>
<pool type="mpath">
<name>virtimages</name>
<target>
<path>/dev/mapper</path>
</target>
</pool></pre>
<h3>Valid pool format types</h3>
<p>
The Multipath volume pool does not use the pool format type element.
</p>
<h3>Valid volume format types</h3>
<p>
The Multipath volume pool does not use the volume format type element.
</p>
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