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<html>
<body>
<h1>Storage pool and volume XML format</h1>
<ul id="toc"></ul>
<h2><a name="StoragePool">Storage pool XML</a></h2>
<p>
Although all storage pool backends share the same public APIs and
XML format, they have varying levels of capabilities. Some may
allow creation of volumes, others may only allow use of pre-existing
volumes. Some may have constraints on volume size, or placement.
</p>
<p>
The is the top level tag for a storage pool document is 'pool'. It has
a single attribute <code>type</code>, which is one of <code>dir</code>,
<code>fs</code>,<code>netfs</code>,<code>disk</code>,<code>iscsi</code>,
<code>logical</code>. This corresponds to the storage backend drivers
listed further along in this document.
The storage pool XML format is available <span class="since">since 0.4.1</span>
</p>
<h3><a name="StoragePoolFirst">General metadata</a></h3>
<pre>
<pool type="iscsi">
<name>virtimages</name>
<uuid>3e3fce45-4f53-4fa7-bb32-11f34168b82b</uuid>
<allocation>10000000</allocation>
<capacity>50000000</capacity>
<available>40000000</available>
...</pre>
<dl>
<dt><code>name</code></dt>
<dd>Providing a name for the pool which is unique to the host.
This is mandatory when defining a pool. <span class="since">Since 0.4.1</span></dd>
<dt><code>uuid</code></dt>
<dd>Providing an identifier for the pool which is globally unique.
This is optional when defining a pool, a UUID will be generated if
omitted. <span class="since">Since 0.4.1</span></dd>
<dt><code>allocation</code></dt>
<dd>Providing the total storage allocation for the pool. This may
be larger than the sum of the allocation of all volumes due to
metadata overhead. This value is in bytes. This is not applicable
when creating a pool. <span class="since">Since 0.4.1</span></dd>
<dt><code>capacity</code></dt>
<dd>Providing the total storage capacity for the pool. Due to
underlying device constraints it may not be possible to use the
full capacity for storage volumes. This value is in bytes. This
is not applicable when creating a pool. <span class="since">Since 0.4.1</span></dd>
<dt><code>available</code></dt>
<dd>Providing the free space available for allocating new volumes
in the pool. Due to underlying device constraints it may not be
possible to allocate the entire free space to a single volume.
This value is in bytes. This is not applicable when creating a
pool. <span class="since">Since 0.4.1</span></dd>
</dl>
<h3><a name="StoragePoolSource">Source elements</a></h3>
<p>
A single <code>source</code> element is contained within the top level
<code>pool</code> element. This tag is used to describe the source of
the storage pool. It can contain the following child elements:
</p>
<pre>
...
<source>
<host name="iscsi.example.com"/>
<device path="demo-target"/>
</source>
...</pre>
<dl>
<dt><code>device</code></dt>
<dd>Provides the source for pools backed by physical devices.
May be repeated multiple times depending on backend driver. Contains
a single attribute <code>path</code> which is the fully qualified
path to the block device node. <span class="since">Since 0.4.1</span></dd>
<dt><code>directory</code></dt>
<dd>Provides the source for pools backed by directories. May
only occur once. Contains a single attribute <code>path</code>
which is the fully qualified path to the block device node.
<span class="since">Since 0.4.1</span></dd>
<dt><code>host</code></dt>
<dd>Provides the source for pools backed by storage from a
remote server. Will be used in combination with a <code>directory</code>
or <code>device</code> element. Contains an attribute <code>name</code>
which is the hostname or IP address of the server. May optionally
contain a <code>port</code> attribute for the protocol specific
port number. <span class="since">Since 0.4.1</span></dd>
<dt><code>name</code></dt>
<dd>Provides the source for pools backed by storage from a
named element (e.g., a logical volume group name).
remote server. Contains a string identifier.
<span class="since">Since 0.4.5</span></dd>
<dt><code>format</code></dt>
<dd>Provides information about the format of the pool. This
contains a single attribute <code>type</code> whose value is
backend specific. This is typically used to indicate filesystem
type, or network filesystem type, or partition table type, or
LVM metadata type. All drivers are required to have a default
value for this, so it is optional. <span class="since">Since 0.4.1</span></dd>
</dl>
<h3><a name="StoragePoolTarget">Target elements</a></h3>
<p>
A single <code>target</code> element is contained within the top level
<code>pool</code> element. This tag is used to describe the mapping of
the storage pool into the host filesystem. It can contain the following
child elements:
</p>
<pre>
...
<target>
<path>/dev/disk/by-path</path>
<permissions>
<owner>0744</owner>
<group>0744</group>
<mode>0744</mode>
<label>virt_image_t</label>
</permissions>
</target>
</pool></pre>
<dl>
<dt><code>path</code></dt>
<dd>Provides the location at which the pool will be mapped into
the local filesystem namespace. For a filesystem/directory based
pool it will be the name of the directory in which volumes will
be created. For device based pools it will be the name of the directory in which
devices nodes exist. For the latter <code>/dev/</code> may seem
like the logical choice, however, devices nodes there are not
guaranteed stable across reboots, since they are allocated on
demand. It is preferable to use a stable location such as one
of the <code>/dev/disk/by-{path,id,uuid,label</code> locations.
<span class="since">Since 0.4.1</span>
</dd>
<dt><code>permissions</code></dt>
<dd>Provides information about the default permissions to use
when creating volumes. This is currently only useful for directory
or filesystem based pools, where the volumes allocated are simple
files. For pools where the volumes are device nodes, the hotplug
scripts determine permissions. It contains 4 child elements. The
<code>mode</code> element contains the octal permission set. The
<code>owner</code> element contains the numeric user ID. The <code>group</code>
element contains the numeric group ID. The <code>label</code> element
contains the MAC (eg SELinux) label string.
<span class="since">Since 0.4.1</span>
</dd>
</dl>
<h3><a name="StoragePoolExtents">Device extents</a></h3>
<p>
If a storage pool exposes information about its underlying
placement / allocation scheme, the <code>device</code> element
within the <code>source</code> element may contain information
about its available extents. Some pools have a constraint that
a volume must be allocated entirely within a single constraint
(eg disk partition pools). Thus the extent information allows an
application to determine the maximum possible size for a new
volume
</p>
<p>
For storage pools supporting extent information, within each
<code>device</code> element there will be zero or more <code>freeExtent</code>
elements. Each of these elements contains two attributes, <code>start</code>
and <code>end</code> which provide the boundaries of the extent on the
device, measured in bytes. <span class="since">Since 0.4.1</span>
</p>
<h2><a name="StorageVol">Storage volume XML</a></h2>
<p>
A storage volume will be either a file or a device node.
The storage volume XML format is available <span class="since">since 0.4.1</span>
</p>
<h3><a name="StorageVolFirst">General metadata</a></h3>
<pre>
<volume type="file">
<name>sparse.img</name>
<key>/var/lib/xen/images/sparse.img</key>
<allocation>0</allocation>
<capacity unit="T">1</capacity>
...</pre>
<dl>
<dt><code>name</code></dt>
<dd>Providing a name for the volume which is unique to the pool.
This is mandatory when defining a volume. <span class="since">Since 0.4.1</span></dd>
<dt><code>key</code></dt>
<dd>Providing an identifier for the volume which is globally unique.
This is optional when defining a volume, a key will be generated if
omitted. <span class="since">Since 0.4.1</span></dd>
<dt><code>allocation</code></dt>
<dd>Providing the total storage allocation for the volume. This
may be smaller than the logical capacity if the volume is sparsely
allocated. It may also be larger than the logical capacity if the
volume has substantial metadata overhead. This value is in bytes.
If omitted when creating a volume, the volume will be fully
allocated at time of creation. If set to a value smaller than the
capacity, the pool has the <strong>option</strong> of deciding
to sparsely allocate a volume. It does not have to honour requests
for sparse allocation though. <span class="since">Since 0.4.1</span></dd>
<dt><code>capacity</code></dt>
<dd>Providing the logical capacity for the volume. This value is
in bytes. This is compulsory when creating a volume.
<span class="since">Since 0.4.1</span></dd>
<dt><code>source</code></dt>
<dd>Provides information about the underlying storage allocation
of the volume. This may not be available for some pool types.
<span class="since">Since 0.4.1</span></dd>
<dt><code>target</code></dt>
<dd>Provides information about the representation of the volume
on the local host. <span class="since">Since 0.4.1</span></dd>
</dl>
<h3><a name="StorageVolTarget">Target elements</a></h3>
<p>
A single <code>target</code> element is contained within the top level
<code>volume</code> element. This tag is used to describe the mapping of
the storage volume into the host filesystem. It can contain the following
child elements:
</p>
<pre>
...
<target>
<path>/var/lib/virt/images/sparse.img</path>
<permissions>
<owner>0744</owner>
<group>0744</group>
<mode>0744</mode>
<label>virt_image_t</label>
</permissions>
</target>
</volume></pre>
<dl>
<dt><code>path</code></dt>
<dd>Provides the location at which the volume can be accessed on
the local filesystem, as an absolute path. This is a readonly
attribute, so shouldn't be specified when creating a volume.
<span class="since">Since 0.4.1</span></dd>
<dt><code>format</code></dt>
<dd>Provides information about the pool specific volume format.
For disk pools it will provide the partition type. For filesystem
or directory pools it will provide the file format type, eg cow,
qcow, vmdk, raw. If omitted when creating a volume, the pool's
default format will be used. The actual format is specified via
the <code>type</code>. Consult the pool-specific docs for the
list of valid values. <span class="since">Since 0.4.1</span></dd>
<dt><code>permissions</code></dt>
<dd>Provides information about the default permissions to use
when creating volumes. This is currently only useful for directory
or filesystem based pools, where the volumes allocated are simple
files. For pools where the volumes are device nodes, the hotplug
scripts determine permissions. It contains 4 child elements. The
<code>mode</code> element contains the octal permission set. The
<code>owner</code> element contains the numeric user ID. The <code>group</code>
element contains the numeric group ID. The <code>label</code> element
contains the MAC (eg SELinux) label string.
<span class="since">Since 0.4.1</span>
</dd>
</dl>
<h2><a name="examples">Example configuration</a></h2>
<p>
Here are a couple of examples, for a more complete set demonstrating
every type of storage pool, consult the <a href="storage.html">storage driver page</a>
</p>
<h3><a name="exampleFile">File based storage pool</a></h3>
<pre>
<pool type="dir">
<name>virtimages</name>
<target>
<path>/var/lib/virt/images</path>
</target>
</pool></pre>
<h3><a name="exampleISCSI">iSCSI based storage pool</a></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><a name="exampleVol">Storage volume</a></h3>
<pre>
<volume type="file">
<name>sparse.img</name>
<allocation>0</allocation>
<capacity unit="T">1</capacity>
<target>
<path>/var/lib/virt/images/sparse.img</path>
<permissions>
<owner>0744</owner>
<group>0744</group>
<mode>0744</mode>
<label>virt_image_t</label>
</permissions>
</target>
</volume></pre>
</body>
</html>
|
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