Chapter 13. Managing Storage for Virtual Machines

A storage pool is a quantity of storage set aside for use by guest virtual machines. Storage pools are divided into storage volumes. Each storage volume is assigned to a guest virtual machine as a block device on a guest bus.

Storage pools and volumes are managed using libvirt. With libvirt's remote protocol, it is possible to manage all aspects of a guest virtual machine's life cycle, as well as the configuration of the resources required by the guest virtual machine. These operations can be performed on a remote host. As a result, a management application, such as the Virtual Machine Manager, using libvirt can enable a user to perform all the required tasks for configuring the host physical machine for a guest virtual machine. These include allocating resources, running the guest virtual machine, shutting it down, and de-allocating the resources, without requiring shell access or any other control channel.

The libvirt API can be used to query the list of volumes in the storage pool or to get information regarding the capacity, allocation, and available storage in the storage pool. A storage volume in the storage pool may be queried to get information such as allocation and capacity, which may differ for sparse volumes.

For storage pools that support it, the libvirt API can be used to create, clone, resize, and delete storage volumes. The APIs can also be used to upload data to storage volumes, download data from storage volumes, or wipe data from storage volumes.

Once a storage pool is started, a storage volume can be assigned to a guest using the storage pool name and storage volume name instead of the host path to the volume in the domain XML.

Storage pools can be stopped (destroyed). This removes the abstraction of the data, but keeps the data intact.

For example, an NFS server that uses mount -t nfs nfs.example.com:/path/to/share /path/to/data. A storage administrator responsible could define an NFS Storage Pool on the virtualization host to describe the exported server path and the client target path. This will allow libvirt to perform the mount either automatically when libvirt is started or as needed while libvirt is running. Files with the NFS Server exported directory are listed as storage volumes within the NFS storage pool.

When the storage volume is added to the guest, the administrator does not need to add the target path to the volume. He just needs to add the storage pool and storage volume by name. Therefore, if the target client path changes, it does not affect the virtual machine.

When the storage 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 storage pool is configured to autostart, libvirt ensures that the NFS shared disk is mounted on the directory specified when libvirt is started.

Once the storage pool is started, the files in the NFS shared disk are reported as storage volumes, and the storage volumes' paths may be queried using the libvirt API. The storage volumes' paths can then be copied into the section of a guest virtual machine's XML definition that describes the source storage for the guest virtual machine's block devices. In the case of NFS, an application that uses the libvirt API can create and delete storage volumes in the storage pool (files in the NFS share) up to the limit of the size of the pool (the storage capacity of the share).

Not all storage pool types support creating and deleting volumes. Stopping the storage pool (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 what the name of the command suggests. For more details, see man virsh.