Storage pool and volume XML format

Storage pool XML

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.

The is the top level tag for a storage pool document is 'pool'. It has a single attribute type, which is one of dir, fs,netfs,disk,iscsi, logical. This corresponds to the storage backend drivers listed further along in this document. The storage pool XML format is available since 0.4.1

General metadata

      <pool type="iscsi">
        <name>virtimages</name>
        <uuid>3e3fce45-4f53-4fa7-bb32-11f34168b82b</uuid>
        <allocation>10000000</allocation>
        <capacity>50000000</capacity>
        <available>40000000</available>
        ...

name: Providing a name for the pool which is unique to the host. This is mandatory when defining a pool. Since 0.4.1
uuid: Providing an identifier for the pool which is globally unique. This is optional when defining a pool, a UUID will be generated if omitted. Since 0.4.1
allocation: 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. Since 0.4.1
capacity: 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. Since 0.4.1
available: 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. Since 0.4.1

Source elements

A single source element is contained within the top level pool element. This tag is used to describe the source of the storage pool. It can contain the following child elements:

        ...
        <source>
          <host name="iscsi.example.com"/>
          <device path="demo-target"/>
        </source>
	...

device: Provides the source for pools backed by physical devices. May be repeated multiple times depending on backend driver. Contains a single attribute path which is the fully qualified path to the block device node. Since 0.4.1
directory: Provides the source for pools backed by directories. May only occur once. Contains a single attribute path which is the fully qualified path to the block device node. Since 0.4.1
host: Provides the source for pools backed by storage from a remote server. Will be used in combination with a directory or device element. Contains an attribute name which is the hostname or IP address of the server. May optionally contain a port attribute for the protocol specific port number. Since 0.4.1
name: 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. Since 0.4.5
format: Provides information about the format of the pool. This contains a single attribute type 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. Since 0.4.1

Target elements

A single target element is contained within the top level pool element. This tag is used to describe the mapping of the storage pool into the host filesystem. It can contain the following child elements:

        ...
        <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>

path: 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 /dev/ 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 /dev/disk/by-{path,id,uuid,label locations. Since 0.4.1
permissions: 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 mode element contains the octal permission set. The owner element contains the numeric user ID. The group element contains the numeric group ID. The label element contains the MAC (eg SELinux) label string. Since 0.4.1

Device extents

If a storage pool exposes information about its underlying placement / allocation scheme, the device element within the source 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

For storage pools supporting extent information, within each device element there will be zero or more freeExtent elements. Each of these elements contains two attributes, start and end which provide the boundaries of the extent on the device, measured in bytes. Since 0.4.1

Storage volume XML

A storage volume will be either a file or a device node. The storage volume XML format is available since 0.4.1

General metadata

      <volume type="file">
	<name>sparse.img</name>
	<key>/var/lib/xen/images/sparse.img</key>
        <allocation>0</allocation>
	<capacity unit="T">1</capacity>
        ...

name: Providing a name for the volume which is unique to the pool. This is mandatory when defining a volume. Since 0.4.1
key: Providing an identifier for the volume which is globally unique. This is optional when defining a volume, a key will be generated if omitted. Since 0.4.1
allocation: 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 option of deciding to sparsely allocate a volume. It does not have to honour requests for sparse allocation though. Since 0.4.1
capacity: Providing the logical capacity for the volume. This value is in bytes. This is compulsory when creating a volume. Since 0.4.1
source: Provides information about the underlying storage allocation of the volume. This may not be available for some pool types. Since 0.4.1
target: Provides information about the representation of the volume on the local host. Since 0.4.1

Target elements

A single target element is contained within the top level volume element. This tag is used to describe the mapping of the storage volume into the host filesystem. It can contain the following child elements:

        ...
	<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>

path: 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 /dev/ 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 /dev/disk/by-{path,id,uuid,label locations. Since 0.4.1
format: 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 type. Consult the pool-specific docs for the list of valid values. Since 0.4.1
permissions: 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 mode element contains the octal permission set. The owner element contains the numeric user ID. The group element contains the numeric group ID. The label element contains the MAC (eg SELinux) label string. Since 0.4.1

Example configuration

Here are a couple of examples, for a more complete set demonstrating every type of storage pool, consult the storage driver page

File based storage pool

      <pool type="dir">
        <name>virtimages</name>
        <target>
          <path>/var/lib/virt/images</path>
        </target>
      </pool>

iSCSI based storage pool

      <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>

Storage volume

      <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>