Storage Management

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:

Directory pool

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 qcow,qcow2,vmdk, cow, 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.

Example pool input definition

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

Valid pool format types

The directory pool does not use the pool format type element.

Valid volume format types

One of the following options:

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 either qemu-img or qcow-create tools are present. The others are dependent on support of the qemu-img tool.

Filesystem pool

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.

Example pool input

      <pool type="fs">
        <name>virtimages</name>
        <source>
          <device path="/dev/VolGroup00/VirtImages"/>
        </source>
        <target>
          <path>/var/lib/virt/images</path>
        </target>
      </pool>

Valid pool format types

The filesystem pool supports the following formats:

Valid volume format types

The valid volume types are the same as for the directory pool type.

Network filesystem pool

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 NFS as the protocol.

Example pool input

      <pool type="netfs">
        <name>virtimages</name>
        <source>
          <host name="nfs.example.com"/>
          <dir path="/var/lib/virt/images"/>
        </source>
        <target>
          <path>/var/lib/virt/images</path>
        </target>
      </pool>

Valid pool format types

The network filesystem pool supports the following formats:

Valid volume format types

The valid volume types are the same as for the directory pool type.

Logical volume pools

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.

Example pool input

      <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>

Valid pool format types

The logical volume pool does not use the pool format type element.

Valid volume format types

The logical volume pool does not use the volume format type element.

Disk volume pools

This provides a pool based on a physical disk. Volumes are created by adding partitions to the disk. Disk pools are 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 2 different free extents.

Example pool input

      <pool type="disk">
        <name>sda</name>
        <source>
          <device path='/dev/sda'/>
        </source>
        <target>
          <path>/dev</path>
        </target>
      </pool>

Valid pool format types

The disk volume pool accepts the following pool format types, representing the common partition table types:

The dos or gpt formats are recommended for best portability - the latter is needed for disks larger than 2TB.

Valid volume format types

The disk volume pool accepts the following volume format types, representing the common partition entry types:

iSCSI volume pools

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

Example pool input

      <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>

Valid pool format types

The iSCSI volume pool does not use the pool format type element.

Valid volume format types

The iSCSI volume pool does not use the volume format type element.

SCSI volume pools

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

Example pool input

      <pool type="scsi">
        <name>virtimages</name>
        <source>
          <adapter name="host0"/>
        </source>
        <target>
          <path>/dev/disk/by-path</path>
        </target>
      </pool>

Valid pool format types

The SCSI volume pool does not use the pool format type element.

Valid volume format types

The SCSI volume pool does not use the volume format type element.

Multipath pools

This provides a pool that contains all the multipath devices on the 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

Example pool input

      <pool type="mpath">
        <name>virtimages</name>
        <target>
          <path>/dev/mapper</path>
        </target>
      </pool>

Valid pool format types

The Multipath volume pool does not use the pool format type element.

Valid volume format types

The Multipath volume pool does not use the volume format type element.

RBD pools

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. Since 0.9.13

Example pool input

      <pool type="rbd">
        <name>myrbdpool</name>
        <source>
          <name>rbdpool</name>
            <host name='1.2.3.4' port='6789'/>
            <host name='my.ceph.monitor' port='6789'/>
            <host name='third.ceph.monitor' port='6789'/>
            <auth username='admin' type='ceph'>
              <secret uuid='2ec115d7-3a88-3ceb-bc12-0ac909a6fd87'/>
            </auth>
        </source>
      </pool>

Example volume output

       <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>

Example disk attachement

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.

Valid pool format types

The RBD pool does not use the pool format type element.

Valid volume format types

The RBD pool does not use the volume format type element.

Sheepdog pools

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. Since 0.9.13

Example pool input

      <pool type="sheepdog">
        <name>mysheeppool</name>
        <source>
          <name>mysheeppool</name>
          <host name='localhost' port='7000'/>
        </source>
      </pool>

Example volume output

       <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>

Example disk attachment

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.

Valid pool format types

The Sheepdog pool does not use the pool format type element.

Valid volume format types

The Sheepdog pool does not use the volume format type element.