formatstorage.html.in

<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 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>
      &lt;pool type="iscsi"&gt;
        &lt;name&gt;virtimages&lt;/name&gt;
        &lt;uuid&gt;3e3fce45-4f53-4fa7-bb32-11f34168b82b&lt;/uuid&gt;
        &lt;allocation&gt;10000000&lt;/allocation&gt;
        &lt;capacity&gt;50000000&lt;/capacity&gt;
        &lt;available&gt;40000000&lt;/available&gt;
        ...</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>
        ...
        &lt;source&gt;
          &lt;host name="iscsi.example.com"/&gt;
          &lt;device path="demo-target"/&gt;
          &lt;vendor name="Acme"/&gt;
          &lt;product name="model"/&gt;
        &lt;/source&gt;
        ...</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 backing directory.
        <span class="since">Since 0.4.1</span></dd>
      <dt><code>adapter</code></dt>
      <dd>Provides the source for pools backed by SCSI adapters. May
        only occur once. Contains a single attribute <code>name</code>
        which is the SCSI adapter name (ex. "host1").
        <span class="since">Since 0.6.2</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).
        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>

      <dt><code>vendor</code></dt>
      <dd>Provides optional information about the vendor of the
        storage device. This contains a single
        attribute <code>name</code> whose value is backend
        specific. <span class="since">Since 0.8.4</span></dd>
      <dt><code>product</code></dt>
      <dd>Provides an optional product name of the storage device.
        This contains a single attribute <code>name</code> whose value
        is backend specific.  <span class="since">Since 0.8.4</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>
        ...
        &lt;target&gt;
          &lt;path&gt;/dev/disk/by-path&lt;/path&gt;
          &lt;permissions&gt;
            &lt;owner&gt;107&lt;/owner&gt;
            &lt;group&gt;107&lt;/group&gt;
            &lt;mode&gt;0744&lt;/mode&gt;
            &lt;label&gt;virt_image_t&lt;/label&gt;
          &lt;/permissions&gt;
          &lt;timestamps&gt;
            &lt;atime&gt;1341933637.273190990&lt;/atime&gt;
            &lt;mtime&gt;1341930622.047245868&lt;/mtime&gt;
            &lt;ctime&gt;1341930622.047245868&lt;/ctime&gt;
          &lt;/timestamps&gt;
          &lt;encryption type='...'&gt;
            ...
          &lt;/encryption&gt;
        &lt;/target&gt;
      &lt;/pool&gt;</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>
      <dt><code>timestamps</code></dt>
      <dd>Provides timing information about the volume. Up to four
        sub-elements are present,
        where <code>atime</code>, <code>btime</code>, <code>ctime</code>
        and <code>mtime</code> hold the access, birth, change and
        modification time of the volume, where known. The used time
        format is &lt;seconds&gt;.&lt;nanoseconds&gt; since the
        beginning of the epoch (1 Jan 1970). If nanosecond resolution
        is 0 or otherwise unsupported by the host OS or filesystem,
        then the nanoseconds part is omitted.  This is a readonly
        attribute and is ignored when creating a volume.
        <span class="since">Since 0.10.0</span>
      </dd>
      <dt><code>encryption</code></dt>
      <dd>If present, specifies how the volume is encrypted.  See
        the <a href="formatstorageencryption.html">Storage Encryption</a> page
        for more information.
      </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>
      &lt;volume&gt;
        &lt;name&gt;sparse.img&lt;/name&gt;
        &lt;key&gt;/var/lib/xen/images/sparse.img&lt;/key&gt;
        &lt;allocation&gt;0&lt;/allocation&gt;
        &lt;capacity unit="T"&gt;1&lt;/capacity&gt;
        ...</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 cannot be set when creating a volume: it is always generated.
        <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. Different types of pools may treat
        sparse volumes differently. For example, the <code>logical</code>
        pool will not automatically expand volume's allocation when it
        gets full; the user is responsible for doing that or configuring
        dmeventd to do so automatically.<br/>
        <br/>
        By default this is specified in bytes, but an optional attribute
        <code>unit</code> can be specified to adjust the passed value.
        Values can be: 'B' or 'bytes' for bytes, 'KB' (kilobytes,
        10<sup>3</sup> or 1000 bytes), 'K' or 'KiB' (kibibytes,
        2<sup>10</sup> or 1024 bytes), 'MB' (megabytes, 10<sup>6</sup>
        or 1,000,000 bytes), 'M' or 'MiB' (mebibytes, 2<sup>20</sup>
        or 1,048,576 bytes), 'GB' (gigabytes, 10<sup>9</sup> or
        1,000,000,000 bytes), 'G' or 'GiB' (gibibytes, 2<sup>30</sup>
        or 1,073,741,824 bytes), 'TB' (terabytes, 10<sup>12</sup> or
        1,000,000,000,000 bytes), 'T' or 'TiB' (tebibytes,
        2<sup>40</sup> or 1,099,511,627,776 bytes), 'PB' (petabytes,
        10<sup>15</sup> or 1,000,000,000,000,000 bytes), 'P' or 'PiB'
        (pebibytes, 2<sup>50</sup> or 1,125,899,906,842,624 bytes),
        'EB' (exabytes, 10<sup>18</sup> or 1,000,000,000,000,000,000
        bytes), or 'E' or 'EiB' (exbibytes, 2<sup>60</sup> or
        1,152,921,504,606,846,976 bytes).  <span class="since">Since
        0.4.1, multi-character <code>unit</code> since
        0.9.11</span></dd>
      <dt><code>capacity</code></dt>
      <dd>Providing the logical capacity for the volume. This value is
        in bytes by default, but a <code>unit</code> attribute can be
        specified with the same semantics as for <code>allocation</code>
        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>
        ...
        &lt;target&gt;
          &lt;path&gt;/var/lib/virt/images/sparse.img&lt;/path&gt;
          &lt;format type='qcow2'/&gt;
          &lt;permissions&gt;
            &lt;owner&gt;107&lt;/owner&gt;
            &lt;group&gt;107&lt;/group&gt;
            &lt;mode&gt;0744&lt;/mode&gt;
            &lt;label&gt;virt_image_t&lt;/label&gt;
          &lt;/permissions&gt;
        &lt;/target&gt;</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> attribute. 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>

    <h3><a name="StorageVolBacking">Backing store elements</a></h3>

    <p>
      A single <code>backingStore</code> element is contained within the top level
      <code>volume</code> element. This tag is used to describe the optional copy
      on write, backing store for the storage volume. It can contain the following
      child elements:
    </p>

    <pre>
        ...
        &lt;backingStore&gt;
          &lt;path&gt;/var/lib/virt/images/master.img&lt;/path&gt;
          &lt;format type='raw'/&gt;
          &lt;permissions&gt;
            &lt;owner&gt;107&lt;/owner&gt;
            &lt;group&gt;107&lt;/group&gt;
            &lt;mode&gt;0744&lt;/mode&gt;
            &lt;label&gt;virt_image_t&lt;/label&gt;
          &lt;/permissions&gt;
        &lt;/backingStore&gt;
      &lt;/volume&gt;</pre>

    <dl>
      <dt><code>path</code></dt>
      <dd>Provides the location at which the backing store can be accessed on
        the local filesystem, as an absolute path. If omitted, there is no
        backing store for this volume.
        <span class="since">Since 0.6.0</span></dd>
      <dt><code>format</code></dt>
      <dd>Provides information about the pool specific backing store 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. The actual format is specified via the type attribute.
        Consult the pool-specific docs for the list of valid
        values. Most file formats require a backing store of the same format,
        however, the qcow2 format allows a different backing store format.
        <span class="since">Since 0.6.0</span></dd>
      <dt><code>permissions</code></dt>
      <dd>Provides information about the permissions of the backing file.
        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 0.6.0</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>
      &lt;pool type="dir"&gt;
        &lt;name&gt;virtimages&lt;/name&gt;
        &lt;target&gt;
          &lt;path&gt;/var/lib/virt/images&lt;/path&gt;
        &lt;/target&gt;
      &lt;/pool&gt;</pre>

    <h3><a name="exampleISCSI">iSCSI based storage pool</a></h3>

    <pre>
      &lt;pool type="iscsi"&gt;
        &lt;name&gt;virtimages&lt;/name&gt;
        &lt;source&gt;
          &lt;host name="iscsi.example.com"/&gt;
          &lt;device path="demo-target"/&gt;
        &lt;/source&gt;
        &lt;target&gt;
          &lt;path&gt;/dev/disk/by-path&lt;/path&gt;
        &lt;/target&gt;
      &lt;/pool&gt;</pre>

    <h3><a name="exampleVol">Storage volume</a></h3>

    <pre>
      &lt;volume&gt;
        &lt;name&gt;sparse.img&lt;/name&gt;
        &lt;allocation&gt;0&lt;/allocation&gt;
        &lt;capacity unit="T"&gt;1&lt;/capacity&gt;
        &lt;target&gt;
          &lt;path&gt;/var/lib/virt/images/sparse.img&lt;/path&gt;
          &lt;permissions&gt;
            &lt;owner&gt;107&lt;/owner&gt;
            &lt;group&gt;107&lt;/group&gt;
            &lt;mode&gt;0744&lt;/mode&gt;
            &lt;label&gt;virt_image_t&lt;/label&gt;
          &lt;/permissions&gt;
        &lt;/target&gt;
      &lt;/volume&gt;</pre>
  </body>
</html>