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Andrea Bolognani 96777db719 docs: Other fixes to :since: tags

Make sure that they're entirely contained within a single line
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Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>

2024-02-26 12:10:27 +01:00

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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 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, scsi (all since 0.4.1 ), mpath ( since 0.7.1 ), rbd ( since 0.9.13 ), sheepdog (since 0.10.0, removed in 8.8.0 ), gluster ( since 1.2.0 ), zfs ( since 1.2.8 ), vstorage ( since 3.1.0 ), or iscsi-direct ( since 4.7.0 ). This corresponds to the storage backend drivers listed further along in this document.

Storage pool 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

Features

Some pools support optional features:

...
<features>
  <cow state='no'>
</features>
...

Valid features are:

cow: Controls whether the filesystem performs copy-on-write (COW) for images in the pool. This may only be set for directory / filesystem pools on the btrfs filesystem. If not set then libvirt will attempt to disable COW on any btrfs filesystems. Since 6.6.0.

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. The set of child elements that it will contain depend on the pool type, but come from the following child elements:

...
<source>
  <host name="iscsi.example.com"/>
  <device path="iqn.2013-06.com.example:iscsi-pool"/>
  <auth type='chap' username='myname'>
    <secret usage='mycluster_myname'/>
  </auth>
  <vendor name="Acme"/>
  <product name="model"/>
</source>
...

...
<source>
  <device path='/dev/mapper/mpatha' part_separator='no'/>
  <format type='gpt'/>
</source>
...

...
<source>
  <adapter type='scsi_host' name='scsi_host1'/>
</source>
...

...
<source>
  <adapter type='scsi_host'>
    <parentaddr unique_id='1'>
      <address domain='0x0000' bus='0x00' slot='0x1f' addr='0x2'/>
    </parentaddr>
  </adapter>
</source>
...

...
<source>
  <adapter type='fc_host' parent='scsi_host5' wwnn='20000000c9831b4b' wwpn='10000000c9831b4b'/>
</source>
...

...
  <source>
    <host name='localhost'/>
    <dir path='/var/lib/libvirt/images'/>
    <format type='nfs'/>
    <protocol ver='3'/>
  </source>
...

device

Provides the source for pools backed by physical devices (pool types fs, logical, disk, iscsi, iscsi-direct, zfs, vstorage). May be repeated multiple times depending on backend driver. Contains a required attribute path which is either the fully qualified path to the block device node or for iscsi or iscsi-direct the iSCSI Qualified Name (IQN). Since 0.4.1

An optional attribute part_separator for each path may be supplied. Valid values for the attribute may be either "yes" or "no". This attribute is to be used for a disk pool type using a path to a device mapper multipath device. Setting the attribute to "yes" causes libvirt to attempt to generate and find target volume path's using a "p" separator. The default algorithm used by device mapper is to add the "p" separator only when the source device path ends with a number; however, it's possible to configure the devmapper device to not use 'user_friendly_names' thus creating partitions with the "p" separator even when the device source path does not end with a number. Since 1.3.1

dir

Provides the source for pools backed by directories (pool types dir, netfs, gluster), or optionally to select a subdirectory within a pool that resembles a filesystem (pool type gluster). May only occur once. Contains a single attribute path which is the fully qualified path to the backing directory or for a netfs pool type using format type "cifs", the path to the Samba share without the leading slash. Since 0.4.1

adapter

Provides the source for pools backed by SCSI adapters (pool type scsi). May only occur once.

name

The SCSI adapter name (e.g. "scsi_host1", although a name such as "host1" is still supported for backwards compatibility, it is not recommended). The scsi_host name to be used can be determined from the output of a virsh nodedev-list scsi_host command followed by a combination of lspci and virsh nodedev-dumpxml scsi_hostN commands to find the scsi_hostN to be used. Since 0.6.2

It is further recommended to utilize the parentaddr element since it's possible to have the path to which the scsi_hostN uses change between system reboots. Since 1.2.7

type

Specifies the adapter type. Valid values are "scsi_host" or "fc_host". If omitted and the name attribute is specified, then it defaults to "scsi_host". To keep backwards compatibility, this attribute is optional only for the "scsi_host" adapter, but is mandatory for the "fc_host" adapter. Since 1.0.5 A "fc_host" capable scsi_hostN can be determined by using virsh nodedev-list --cap fc_host. Since 1.2.8

Note: Regardless of whether a "scsi_host" adapter type is defined using a name or a parentaddr, it should refer to a real scsi_host adapter as found through a virsh nodedev-list scsi_host and virsh nodedev-dumpxml scsi_hostN on one of the scsi_host's displayed. It should not refer to a "fc_host" capable scsi_hostN nor should it refer to the vHBA created for some "fc_host" adapter. For a vHBA the nodedev-dumpxml output parent setting will be the "fc_host" capable scsi_hostN value. Additionally, do not refer to an iSCSI scsi_hostN for the "scsi_host" source. An iSCSI scsi_hostN's nodedev-dumpxml output parent field is generally "computer". This is a libvirt created parent value indicating no parent was defined for the node device.

wwnn and wwpn

The required "World Wide Node Name" (wwnn) and "World Wide Port Name" (wwpn) are used by the "fc_host" adapter to uniquely identify the vHBA device in the Fibre Channel storage fabric. If the vHBA device already exists as a Node Device, then libvirt will use it; otherwise, the vHBA will be created using the provided values. It is considered a configuration error use the values from the HBA as those would be for a "scsi_host" type pool instead. The wwnn and wwpn have very specific format requirements based on the hypervisor being used, thus care should be taken if you decide to generate your own to follow the standards; otherwise, the pool will fail to start with an opaque error message indicating failure to write to the vport_create file during vport create/delete due to "No such file or directory". Since 1.0.4

parent

Used by the "fc_host" adapter type to optionally specify the parent scsi_host device defined in the Node Device database as the NPIV virtual Host Bus Adapter (vHBA). The value provided must be a vport capable scsi_host. The value is not the scsi_host of the vHBA created by 'virsh nodedev-create', rather it is the parent of that vHBA. If the value is not provided, libvirt will determine the parent based either finding the wwnn,wwpn defined for an existing scsi_host or by creating a vHBA. Providing the parent attribute is also useful for the duplicate pool definition checks. This is more important in environments where both the "fc_host" and "scsi_host" source adapter pools are being used in order to ensure a new definition doesn't duplicate using the scsi_hostN of some existing storage pool. Since 1.0.4

parent_wwnn and parent_wwpn

Instead of the parent to specify which scsi_host to use by name, it's possible to provide the wwnn and wwpn of the parent to be used for the vHBA in order to ensure that between reboots or after a hardware configuration change that the scsi_host parent name doesn't change. Both the parent_wwnn and parent_wwpn must be provided. Since 3.0.0

parent_fabric_wwn

Instead of the parent to specify which scsi_host to use by name, it's possible to provide the fabric_wwn on which the scsi_host exists. This provides flexibility for choosing a scsi_host that may be available on the fabric rather than requiring a specific parent by wwnn or wwpn to be available. Since 3.0.0

managed

An optional attribute to instruct the SCSI storage backend to manage destroying the vHBA when the pool is destroyed. For configurations that do not provide an already created vHBA from a 'virsh nodedev-create', libvirt will set this property to "yes". For configurations that have already created a vHBA via 'virsh nodedev-create' and are using the wwnn/wwpn from that vHBA and optionally the scsi_host parent, setting this attribute to "yes" will allow libvirt to destroy the node device when the pool is destroyed. If this attribute is set to "no" or not defined in the XML, then libvirt will not destroy the vHBA. Since 1.2.11

parentaddr

Used by the "scsi_host" adapter type instead of the name attribute to more uniquely identify the SCSI host. Using a combination of the unique_id attribute and the address element to formulate a PCI address, a search will be performed of the /sys/class/scsi_host/hostNN links for a matching PCI address with a matching unique_id value in the /sys/class/scsi_host/hostNN/unique_id file. The value in the "unique_id" file will be unique enough for the specific PCI address. The hostNN will be used by libvirt as the basis to define which SCSI host is to be used for the currently booted system. Since 1.2.7

address: The PCI address of the scsi_host device to be used. Using a PCI address provides consistent naming across system reboots and kernel reloads. The address will have four attributes: domain (a 2-byte hex integer, not currently used by qemu), bus (a hex value between 0 and 0xff, inclusive), slot (a hex value between 0x0 and 0x1f, inclusive), and function (a value between 0 and 7, inclusive). The PCI address can be determined by listing the /sys/bus/pci/devices and the /sys/class/scsi_host directories in order to find the expected scsi_host device. The address will be provided in a format such as "0000:00:1f:2" which can be used to generate the expected PCI address "domain='0x0000' bus='0x00' slot='0x1f' function='0x0'". Optionally, using the combination of the commands 'virsh nodedev-list scsi_host' and 'virsh nodedev-dumpxml' for a specific list entry and converting the resulting path element as the basis to formulate the correctly formatted PCI address.
unique_id: Required parentaddr attribute used to determine which of the scsi_host adapters for the provided PCI address should be used. The value is determine by contents of the unique_id file for the specific scsi_host adapter. For a PCI address of "0000:00:1f:2", the unique identifier files can be found using the command find -H /sys/class/scsi_host/host*/unique_id | xargs grep '[0-9]'. Optionally, the virsh nodedev-dumpxml scsi_hostN' of a specific scsi_hostN list entry will list the unique_id value.

host

Provides the source for pools backed by storage from a remote server (pool types netfs, iscsi, iscsi-direct, rbd, sheepdog, gluster). 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. Duplicate storage pool definition checks may perform a cursory check that the same host name by string comparison in the new pool does not match an existing pool's source host name when combined with the directory or device element. Name resolution of the provided hostname or IP address is left to the storage driver backend interactions with the remote server. See the storage driver page for any restrictions for specific storage backends. Since 0.4.1

initiator

Required by the iscsi-direct pool in order to provide the iSCSI Qualified Name (IQN) to communicate with the pool's device target IQN. There is one sub-element iqn with the name attribute to describe the IQN for the initiator. Since 4.7.0

auth

If present, the auth element provides the authentication credentials needed to access the source by the setting of the type attribute (pool types iscsi, iscsi-direct, rbd). The type must be either "chap" or "ceph". Use "ceph" for Ceph RBD (Rados Block Device) network sources and use "iscsi" for CHAP (Challenge-Handshake Authentication Protocol) iSCSI targets. Additionally a mandatory attribute username identifies the username to use during authentication as well as a sub-element secret with a mandatory attribute type, to tie back to a libvirt secret object that holds the actual password or other credentials. The domain XML intentionally does not expose the password, only the reference to the object that manages the password. The secret element requires either a uuid attribute with the UUID of the secret object or a usage attribute matching the key that was specified in the secret object. Since 0.9.7 for "ceph" and 1.1.1 for "chap"

name

Provides the source for pools backed by storage from a named element (pool types logical, rbd, sheepdog, gluster). Contains a string identifier. Since 0.4.5

format

Provides information about the format of the pool (pool types fs, netfs, disk, logical). 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

protocol

For a netfs Storage Pool provide a mechanism to define which NFS protocol version number will be used to contact the server's NFS service. The attribute ver accepts the version number to use. Since 5.1.0

vendor

Provides optional information about the vendor of the storage device. This contains a single attribute name whose value is backend specific. Since 0.8.4

product

Provides an optional product name of the storage device. This contains a single attribute name whose value is backend specific. Since 0.8.4

Storage pool target elements

A single target element is contained within the top level pool element for some types of pools (pool types dir, fs, netfs, logical, disk, iscsi, scsi, mpath, zfs). 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>107</owner>
    <group>107</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, as an absolute path. For a filesystem/directory based pool it will be a fully qualified name of the directory in which volumes will be created. For device based pools it will be a fully qualified 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. For logical and zfs pool types, a provided value is ignored and a default path generated. For a Multipath pool (type mpath), the provided value is ignored and the default value of "/dev/mapper" is used. Since 0.4.1
permissions: This is currently only useful for directory or filesystem based pools, which are mapped as a directory into the local filesystem namespace. It provides information about the permissions to use for the final directory when the pool is built. There are 4 child elements. The mode element contains the octal permission set. The mode defaults to 0711 when not provided. The owner element contains the numeric user ID. The group element contains the numeric group ID. If owner or group aren't specified when creating a directory, the UID and GID of the libvirtd process are used. The label element contains the MAC (eg SELinux) label string. Since 0.4.1 For running directory or filesystem based pools, these fields will be filled with the values used by the existing directory. Since 1.2.16

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

Refresh overrides

The optional refresh element can control how the pool and associated volumes are refreshed (pool type rbd). The allocation attribute of the volume child element controls the method used for computing the allocation of a volume. The valid attribute values are default to compute the actual usage or capacity to use the logical capacity for cases where computing the allocation is too expensive. The following XML snippet shows the syntax:

<pool type="rbd">
  <name>myrbdpool</name>
...
  <source/>
...
  <refresh>
    <volume allocation='capacity'/>
  </refresh>
...
</pool>

Since 5.2.0

Storage Pool Namespaces

Usage of Storage Pool Namespaces provides a mechanism to provide pool type specific data in a free form or arbitrary manner via XML syntax targeted solely for the needs of the specific pool type which is not otherwise supported in standard XML. For the "fs" and "netfs" pool types this provides a mechanism to provide additional mount options on the command line. For the "rbd" pool this provides a mechanism to override default settings for RBD configuration options.

Usage of namespaces comes with no support guarantees. It is intended for developers testing out a concept prior to requesting an explicitly supported XML option in libvirt, and thus should never be used in production.

fs:mount_opts

Provides an XML namespace mechanism to optionally utilize specifically named options for the mount command via the "-o" option for the fs or netfs type storage pools. In order to designate that the Storage Pool will be using the mechanism, the pool element must be modified to provide the XML namespace attribute syntax as follows:

xmlns:fs='http://libvirt.org/schemas/storagepool/fs/1.0'

The fs:mount_opts defines the mount options by specifying multiple fs:option subelements with the attribute name specifying the mount option to be added. The value of the named option is not checked since it's possible options don't exist on all distributions. It is expected that proper and valid options will be supplied for the target host.

The following XML snippet shows the syntax required in order to utilize for a netfs pool:

<pool type="netfs" xmlns:fs='http://libvirt.org/schemas/storagepool/fs/1.0'>
  <name>nfsimages</name>
...
  <source>
...
  </source>
...
  <target>
...
  </target>
  <fs:mount_opts>
    <fs:option name='sync'/>
    <fs:option name='lazytime'/>
  </fs:mount_opts>
</pool>
...

Since 5.1.0.

rbd:config_opts

Provides an XML namespace mechanism to optionally utilize specifically named options for the RBD configuration options via the rados_conf_set API for the rbd type storage pools. In order to designate that the Storage Pool will be using the mechanism, the pool element must be modified to provide the XML namespace attribute syntax as follows:

xmlns:rbd='http://libvirt.org/schemas/storagepool/rbd/1.0'

The rbd:config_opts defines the configuration options by specifying multiple rbd:option subelements with the attribute name specifying the configuration option to be added and value specifying the configuration option value. The name and value for each option is only checked to be not empty. The name and value provided are not checked since it's possible options don't exist on all distributions. It is expected that proper and valid options will be supplied for the target host.

The following XML snippet shows the syntax required in order to utilize

<pool type="rbd" xmlns:rbd='http://libvirt.org/schemas/storagepool/rbd/1.0'>
  <name>myrbdpool</name>
...
  <source>
...
  </source>
...
  <target>
...
  </target>
...
  <rbd:config_opts>
    <rbd:option name='client_mount_timeout' value='45'/>
    <rbd:option name='rados_mon_op_timeout' value='20'/>
    <rbd:option name='rados_osd_op_timeout' value='10'/>
  </rbd:config_opts>
</pool>

Since 5.1.0.

Storage volume XML

A storage volume will generally be either a file or a device node; since 1.2.0, an optional output-only attribute type lists the actual type (file, block, dir, network, netdir or ploop), which is also available from virStorageVolGetInfo(). The storage volume XML format is available since 0.4.1

Storage volume 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. For a disk pool, the name must be combination of the source device path device and next partition number to be created. For example, if the source device path is /dev/sdb and there are no partitions on the disk, then the name must be sdb1 with the next name being sdb2 and so on. Since 0.4.1
key: Providing an identifier for the volume which identifies a single volume. In some cases it's possible to have two distinct keys identifying a single volume. This field cannot be set when creating a volume: it is always generated. 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. Different types of pools may treat sparse volumes differently. For example, the logical 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. By default this is specified in bytes, but an optional attribute unit can be specified to adjust the passed value. Values can be: 'B' or 'bytes' for bytes, 'KB' (kilobytes, 10³ or 1000 bytes), 'K' or 'KiB' (kibibytes, 2¹⁰ or 1024 bytes), 'MB' (megabytes, 10⁶ or 1,000,000 bytes), 'M' or 'MiB' (mebibytes, 2²⁰ or 1,048,576 bytes), 'GB' (gigabytes, 10⁹ or 1,000,000,000 bytes), 'G' or 'GiB' (gibibytes, 2³⁰ or 1,073,741,824 bytes), 'TB' (terabytes, 10¹² or 1,000,000,000,000 bytes), 'T' or 'TiB' (tebibytes, 2⁴⁰ or 1,099,511,627,776 bytes), 'PB' (petabytes, 10¹⁵ or 1,000,000,000,000,000 bytes), 'P' or 'PiB' (pebibytes, 2⁵⁰ or 1,125,899,906,842,624 bytes), 'EB' (exabytes, 10¹⁸ or 1,000,000,000,000,000,000 bytes), or 'E' or 'EiB' (exbibytes, 2⁶⁰ or 1,152,921,504,606,846,976 bytes). Since 0.4.1, multi-character unit since 0.9.11.
capacity: Providing the logical capacity for the volume. This value is in bytes by default, but a unit attribute can be specified with the same semantics as for allocation This is compulsory when creating a volume. Since 0.4.1
physical: This output only element provides the host physical size of the target storage volume. The default output unit will be in bytes. Since 3.0.0
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

Storage volume 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>
  <format type='qcow2'/>
  <permissions>
    <owner>107</owner>
    <group>107</group>
    <mode>0744</mode>
    <label>virt_image_t</label>
  </permissions>
  <timestamps>
    <atime>1341933637.273190990</atime>
    <mtime>1341930622.047245868</mtime>
    <ctime>1341930622.047245868</ctime>
  </timestamps>
  <encryption type='...'>
    ...
  </encryption>
  <compat>1.1</compat>
  <nocow/>
  <clusterSize unit='KiB'>64</clusterSize>
  <features>
    <lazy_refcounts/>
    <extended_l2/>
  </features>
</target>

path

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. Since 0.4.1

format

Provides information about the pool specific volume format. For disk pools it will provide the partition table format type, but is not preserved after a pool refresh or libvirtd restart. Use extended in order to create an extended disk extent partition. 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 attribute. Consult the storage driver page for the list of valid volume format type values for each specific pool. The format will be ignored on input for pools without a volume format type value and the default pool format will be used. Since 0.4.1

permissions

Provides information about the 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. There are 4 child elements. The mode element contains the octal permission set. The mode defaults to 0600 when not provided. The owner element contains the numeric user ID. The group element contains the numeric group ID. If owner or group aren't specified when creating a supported volume, the UID and GID of the libvirtd process are used. The label element contains the MAC (eg SELinux) label string. For existing directory or filesystem based volumes, these fields will be filled with the values used by the existing file. Since 0.4.1

timestamps

Provides timing information about the volume. Up to four sub-elements are present, where atime, btime, ctime and mtime hold the access, birth, change and modification time of the volume, where known. The used time format is <seconds>.<nanoseconds> 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. Since 0.10.0

encryption

If present, specifies how the volume is encrypted. See the Storage Encryption page for more information.

compat

Specify compatibility level. So far, this is only used for type='qcow2' volumes. Valid values are 0.10 and 1.1 so far, specifying QEMU version the images should be compatible with. If the feature element is present, 1.1 is used. Since 1.1.0 If omitted, 0.10 is used. Since 1.1.2

nocow

Turn off COW of the newly created volume. So far, this is only valid for a file image in btrfs file system. It will improve performance when the file image is used in VM. To create non-raw file images, it requires QEMU version since 2.1. Since 1.2.7

clusterSize

Changes the qcow2 cluster size which can affect image file size and performance. Since 7.4.0

features

Format-specific features. Only used for qcow2 now. Valid sub-elements are:

<lazy_refcounts/> - allow delayed reference counter updates. Since 1.1.0
<extended_l2/> - enables subcluster allocation for qcow2 images. QCOW2 clusters are split into 32 subclusters decreasing the size of L2 cache needed. It's recommended to increase clusterSize.

Backing store elements

A single backingStore element is contained within the top level volume 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:

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

path: 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. Since 0.6.0
format: 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. Since 0.6.0
permissions: Provides information about the permissions of the backing file. See volume permissions documentation for explanation of individual fields. Since 0.6.0

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="iqn.2013-06.com.example:iscsi-pool"/>
    <auth type='chap' username='myuser'>
      <secret usage='libvirtiscsi'/>
    </auth>
  </source>
  <target>
    <path>/dev/disk/by-path</path>
  </target>
</pool>

Storage volume

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

Storage volume using LUKS

<volume>
  <name>MyLuks.img</name>
  <capacity unit="G">5</capacity>
  <target>
    <path>/var/lib/virt/images/MyLuks.img</path>
    <format type='raw'/>
    <encryption format='luks'>
      <secret type='passphrase' uuid='f52a81b2-424e-490c-823d-6bd4235bc572'/>
    </encryption>
  </target>
</volume>

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