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