Libvirt provides storage management on the physical host through storage pools and volumes.
A storage pool is a quantity of storage set aside by an administrator, often a dedicated storage administrator, for use by virtual machines. Storage pools are divided into storage volumes either by the storage administrator or the system administrator, and the volumes are assigned to VMs as block devices.
For example, the storage administrator responsible for an NFS server creates a share to store virtual machines' data. The system administrator defines a pool on the virtualization host with the details of the share (e.g. nfs.example.com:/path/to/share should be mounted on /vm_data). When the pool is started, libvirt mounts the share on the specified directory, just as if the system administrator logged in and executed 'mount nfs.example.com:/path/to/share /vmdata'. If the pool is configured to autostart, libvirt ensures that the NFS share is mounted on the directory specified when libvirt is started.
Once the pool is started, the files in the NFS share are reported as volumes, and the storage volumes' paths may be queried using the libvirt APIs. The volumes' paths can then be copied into the section of a VM's XML definition describing the source storage for the VM's block devices. In the case of NFS, an application using the libvirt APIs can create and delete volumes in the pool (files in the NFS share) up to the limit of the size of the pool (the storage capacity of the share). Not all pool types support creating and deleting volumes. Stopping the pool (somewhat unfortunately referred to by virsh and the API as "pool-destroy") undoes the start operation, in this case, unmounting the NFS share. The data on the share is not modified by the destroy operation, despite the name. See man virsh for more details.
A second example is an iSCSI pool. A storage administrator provisions an iSCSI target to present a set of LUNs to the host running the VMs. When libvirt is configured to manage that iSCSI target as a pool, libvirt will ensure that the host logs into the iSCSI target and libvirt can then report the available LUNs as storage volumes. The volumes' paths can be queried and used in VM's XML definitions as in the NFS example. In this case, the LUNs are defined on the iSCSI server, and libvirt cannot create and delete volumes.
Storage pools and volumes are not required for the proper operation of VMs. Pools and volumes provide a way for libvirt to ensure that a particular piece of storage will be available for a VM, but some administrators will prefer to manage their own storage and VMs will operate properly without any pools or volumes defined. On systems that do not use pools, system administrators must ensure the availability of the VMs' storage using whatever tools they prefer, for example, adding the NFS share to the host's fstab so that the share is mounted at boot time.
If at this point the value of pools and volumes over traditional system administration tools is unclear, note that one of the features of libvirt is its remote protocol, so it's possible to manage all aspects of a virtual machine's lifecycle as well as the configuration of the resources required by the VM. These operations can be performed on a remote host entirely within the libvirt API. In other words, a management application using libvirt can enable a user to perform all the required tasks for configuring the host for a VM: allocating resources, running the VM, shutting it down and deallocating the resources, without requiring shell access or any other control channel.
Libvirt supports the following storage pool types:
Contents
A pool with a type of dir provides the means to manage files within a directory. The files can be fully allocated raw files, sparsely allocated raw files, or one of the special disk formats such as qcow2, vmdk, etc as supported by the qemu-img program. If the directory does not exist at the time the pool is defined, the build operation can be used to create it.
<pool type="dir">
<name>virtimages</name>
<target>
<path>/var/lib/virt/images</path>
</target>
</pool>
The directory pool does not use the pool format type element.
One of the following options:
raw: a plain file
bochs: Bochs disk image format
cloop: compressed loopback disk image format
cow: User Mode Linux disk image format
dmg: Mac disk image format
iso: CDROM disk image format
qcow: QEMU v1 disk image format
qcow2: QEMU v2 disk image format
qed: QEMU Enhanced Disk image format
vmdk: VMware disk image format
vpc: VirtualPC disk image format
When listing existing volumes all these formats are supported natively. When creating new volumes, only a subset may be available. The raw type is guaranteed always available. The qcow2 type can be created if the qemu-img tool is present. The others are dependent on support of the qemu-img tool.
This is a variant of the directory pool. Instead of creating a directory on an existing mounted filesystem though, it expects a source block device to be named. This block device will be mounted and files managed in the directory of its mount point. It will default to allowing the kernel to automatically discover the filesystem type, though it can be specified manually if required.
<pool type="fs">
<name>virtimages</name>
<source>
<device path="/dev/VolGroup00/VirtImages"/>
</source>
<target>
<path>/var/lib/virt/images</path>
</target>
</pool>
The filesystem pool supports the following formats:
auto - automatically determine format
ext2
ext3
ext4
ufs
iso9660
udf
gfs
gfs2
vfat
hfs+
xfs
ocfs2
vmfs
The valid volume types are the same as for the directory pool type.
This is a variant of the filesystem pool. Instead of requiring a local block device as the source, it requires the name of a host and path of an exported directory. It will mount this network filesystem and manage files within the directory of its mount point. It will default to using auto as the protocol, which generally tries a mount via NFS first.
<pool type="netfs">
<name>virtimages</name>
<source>
<host name="nfs.example.com"/>
<dir path="/var/lib/virt/images"/>
<format type='nfs'/>
</source>
<target>
<path>/var/lib/virt/images</path>
</target>
</pool>
The network filesystem pool supports the following formats:
auto - automatically determine format
nfs
glusterfs - use the glusterfs FUSE file system. For now, the dir specified as the source can only be a gluster volume name, as gluster does not provide a way to directly mount subdirectories within a volume. (To bypass the file system completely, see the Gluster pool).
cifs - use the SMB (samba) or CIFS file system. The mount will use "-o guest" to mount the directory anonymously.
The valid volume types are the same as for the directory pool type.
This provides a pool based on an LVM volume group. For a pre-defined LVM volume group, simply providing the group name is sufficient, while to build a new group requires providing a list of source devices to serve as physical volumes. Volumes will be allocated by carving out chunks of storage from the volume group.
<pool type="logical">
<name>HostVG</name>
<source>
<device path="/dev/sda1"/>
<device path="/dev/sdb1"/>
<device path="/dev/sdc1"/>
</source>
<target>
<path>/dev/HostVG</path>
</target>
</pool>
The logical volume pool supports only the lvm2 format, although not supplying a format value will result in automatic selection of thelvm2 format.
The logical volume pool does not use the volume format type element.
This provides a pool based on a physical disk. Volumes are created by adding partitions to the disk. Disk pools have constraints on the size and placement of volumes. The 'free extents' information will detail the regions which are available for creating new volumes. A volume cannot span across two different free extents. It will default to using dos as the pool source format.
<pool type="disk">
<name>sda</name>
<source>
<device path='/dev/sda'/>
</source>
<target>
<path>/dev</path>
</target>
</pool>
The disk volume pool accepts the following pool format types, representing the common partition table types:
dos
dvh
gpt
mac
bsd
pc98
sun
lvm2
The formats dos ("msdos" in parted terminology, good for BIOS systems) or gpt (good for UEFI systems) are recommended for best portability - the latter is needed for disks larger than 2TB. Note that the lvm2 format refers to the physical volume format (i.e. the whole disk is a physical volume - not the usual usage of LVM where physical volumes are partitions). This is not really a partition table and such pool cannot be built by libvirt, only detected.
Building a pool of a certain format depends on its availability in parted.
The disk volume pool accepts the following volume format types, representing the common partition entry types:
none
linux
fat16
fat32
linux-swap
linux-lvm
linux-raid
extended
This provides a pool based on an iSCSI target. Volumes must be pre-allocated on the iSCSI server, and cannot be created via the libvirt APIs. Since /dev/XXX names may change each time libvirt logs into the iSCSI target, it is recommended to configure the pool to use /dev/disk/by-path or /dev/disk/by-id for the target path. These provide persistent stable naming for LUNs
The libvirt iSCSI storage backend does not resolve the provided host name or IP address when finding the available target IQN's on the host; therefore, defining two pools to use the same IQN on the same host will fail the duplicate source pool checks.
<pool type="iscsi">
<name>virtimages</name>
<source>
<host name="iscsi.example.com"/>
<device path="iqn.2013-06.com.example:iscsi-pool"/>
</source>
<target>
<path>/dev/disk/by-path</path>
</target>
</pool>
The iSCSI volume pool does not use the pool format type element.
The iSCSI volume pool does not use the volume format type element.
This is a variant of the iSCSI pool. Instead of using iscsiadm, it uses libiscsi. It requires a host, a path which is the target IQN, and an initiator IQN.
<pool type="iscsi-direct">
<name>virtimages</name>
<source>
<host name="iscsi.example.com"/>
<device path="iqn.2013-06.com.example:iscsi-pool"/>
<initiator>
<iqn name="iqn.2013-06.com.example:iscsi-initiator"/>
</initiator>
</source>
</pool>
The iSCSI direct volume pool does not use the pool format type element.
The iSCSI direct volume pool does not use the volume format type element.
This provides a pool based on a SCSI HBA. Volumes are preexisting SCSI LUNs, and cannot be created via the libvirt APIs. Since /dev/XXX names aren't generally stable, it is recommended to configure the pool to use /dev/disk/by-path or /dev/disk/by-id for the target path. These provide persistent stable naming for LUNs Since 0.6.2
<pool type="scsi">
<name>virtimages</name>
<source>
<adapter name="host0"/>
</source>
<target>
<path>/dev/disk/by-path</path>
</target>
</pool>
The SCSI volume pool does not use the pool format type element.
The SCSI volume pool does not use the volume format type element.
This provides a pool that contains all the multipath devices on the host. Therefore, only one Multipath pool may be configured per host. Volume creating is not supported via the libvirt APIs. The target element is actually ignored, but one is required to appease the libvirt XML parser.
Configuring multipathing is not currently supported, this just covers the case where users want to discover all the available multipath devices, and assign them to guests. Since 0.7.1
<pool type="mpath">
<name>virtimages</name>
<target>
<path>/dev/mapper</path>
</target>
</pool>
The Multipath volume pool does not use the pool format type element.
The Multipath volume pool does not use the volume format type element.
This storage driver provides a pool which contains all RBD images in a RADOS pool. RBD (RADOS Block Device) is part of the Ceph distributed storage project.
This backend only supports QEMU with RBD support. Kernel RBD which exposes RBD devices as block devices in /dev is not supported. RBD images created with this storage backend can be accessed through kernel RBD if configured manually, but this backend does not provide mapping for these images.
Images created with this backend can be attached to QEMU guests when QEMU is build with RBD support (Since QEMU 0.14.0). The backend supports cephx authentication for communication with the Ceph cluster. Storing the cephx authentication key is done with the libvirt secret mechanism. The UUID in the example pool input refers to the UUID of the stored secret.
The port attribute for a Ceph monitor does not have to be provided. If not provided librados will use the default Ceph monitor port. Since 0.9.13
<pool type="rbd">
<name>myrbdpool</name>
<source>
<name>rbdpool</name>
<host name='1.2.3.4'/>
<host name='my.ceph.monitor'/>
<host name='third.ceph.monitor' port='6789'/>
<auth username='admin' type='ceph'>
<secret uuid='2ec115d7-3a88-3ceb-bc12-0ac909a6fd87'/>
</auth>
</source>
</pool>
<volume>
<name>myvol</name>
<key>rbd/myvol</key>
<source>
</source>
<capacity unit='bytes'>53687091200</capacity>
<allocation unit='bytes'>53687091200</allocation>
<target>
<path>rbd:rbd/myvol</path>
<format type='unknown'/>
<permissions>
<mode>00</mode>
<owner>0</owner>
<group>0</group>
</permissions>
</target>
</volume>
RBD images can be attached to QEMU guests when QEMU is built with RBD support. Information about attaching a RBD image to a guest can be found at format domain page.
The RBD pool does not use the pool format type element.
Only raw volumes are supported.
This provides a pool based on a Sheepdog Cluster. Sheepdog is a distributed storage system for QEMU/KVM. It provides highly available block level storage volumes that can be attached to QEMU/KVM virtual machines. The cluster must already be formatted. Introduced in 0.9.13 removed in 8.8.0.
<pool type="sheepdog">
<name>mysheeppool</name>
<source>
<name>mysheeppool</name>
<host name='localhost' port='7000'/>
</source>
</pool>
<volume>
<name>myvol</name>
<key>sheep/myvol</key>
<source>
</source>
<capacity unit='bytes'>53687091200</capacity>
<allocation unit='bytes'>53687091200</allocation>
<target>
<path>sheepdog:myvol</path>
<format type='unknown'/>
<permissions>
<mode>00</mode>
<owner>0</owner>
<group>0</group>
</permissions>
</target>
</volume>
Sheepdog images can be attached to QEMU guests. Information about attaching a Sheepdog image to a guest can be found at the format domain page.
The Sheepdog pool does not use the pool format type element.
The Sheepdog pool does not use the volume format type element.
This provides a pool based on native Gluster access. Gluster is a distributed file system that can be exposed to the user via FUSE, NFS or SMB (see the Network filesystem pool for that usage); but for minimal overhead, the ideal access is via native access (only possible for QEMU/KVM compiled with libgfapi support). The cluster and storage volume must already be running, and it is recommended that the volume be configured with gluster volume set $volname storage.owner-uid=$uid and gluster volume set $volname storage.owner-gid=$gid for the uid and gid that qemu will be run as. It may also be necessary to set rpc-auth-allow-insecure on for the glusterd service, as well as gluster set $volname server.allow-insecure on, to allow access to the gluster volume. Since 1.2.0
A gluster volume corresponds to a libvirt storage pool. If a gluster volume could be mounted as mount -t glusterfs localhost:/volname /some/path, then the following example will describe the same pool without having to create a local mount point. Remember that with gluster, the mount point can be through any machine in the cluster, and gluster will automatically pick the ideal transport to the actual bricks backing the gluster volume, even if on a different host than the one named in the host designation. The <name> element is always the volume name (no slash). The pool source also supports an optional <dir> element with a path attribute that lists the absolute name of a subdirectory relative to the gluster volume to use instead of the top-level directory of the volume.
<pool type="gluster">
<name>myglusterpool</name>
<source>
<name>volname</name>
<host name='localhost'/>
<dir path='/'/>
</source>
</pool>
Libvirt storage volumes associated with a gluster pool correspond to the files that can be found when mounting the gluster volume. The name is the path relative to the effective mount specified for the pool; and the key is a string that identifies a single volume uniquely. Currently the key attribute consists of the URI of the volume but it may be changed to a UUID of the volume in the future.
<volume> <name>myfile</name> <key>gluster://localhost/volname/myfile</key> <source> </source> <capacity unit='bytes'>53687091200</capacity> <allocation unit='bytes'>53687091200</allocation> </volume>
Files within a gluster volume can be attached to QEMU guests. Information about attaching a Gluster image to a guest can be found at the format domain page.
The Gluster pool does not use the pool format type element.
The valid volume types are the same as for the directory pool type.
This provides a pool based on the ZFS filesystem. Initially it was developed for FreeBSD, and since 1.3.2 experimental support for ZFS on Linux version 0.6.4 or newer is available.
A pool could either be created manually using the zpool create command and its name specified in the source section or :since:` since 1.2.9` source devices could be specified to create a pool using libvirt.
Please refer to the ZFS documentation for details on a pool creation.
Since 1.2.8
<pool type="zfs">
<name>myzfspool</name>
<source>
<name>zpoolname</name>
<device path="/dev/ada1"/>
<device path="/dev/ada2"/>
</source>
</pool>
The ZFS volume pool does not use the pool format type element.
The ZFS volume pool does not use the volume format type element.
This provides a pool based on Virtuozzo storage. Virtuozzo Storage is a highly available distributed software-defined storage with built-in replication and disaster recovery. More detailed information about Virtuozzo storage and its management can be found here: Virtuozzo Storage).
Please refer to the Virtuozzo Storage documentation for details on storage management and usage.
In order to create storage pool with Virtuozzo Storage backend you have to provide cluster name and be authorized within the cluster.
<pool type="vstorage">
<name>myvstoragepool</name>
<source>
<name>clustername</name>
</source>
<target>
<path>/mnt/clustername</path>
</target>
</pool>
The Vstorage volume pool does not use the pool format type element.
The valid volume types are the same as for the directory pool.