A virtual machine (VM) snapshot is a record of a running virtual machine at a point in time. When you take a snapshot of a VM, its storage information (the data on the hard drive) and metadata (configuration information) is also saved. Where necessary, I/O is temporarily halted while the snapshot is being taken to ensure that a self-consistent disk image can be captured.
Unlike VM exports, snapshots can be created without first shutting down the VM. A snapshot is similar to a normal VM template but it contains all the storage and configuration information for the original VM, including networking information. Snapshots provide a fast way of creating templates that can be exported for backup purposes and then restored, or that can be used to quickly create new VMs.
Snapshots are supported on all storage types, though for LVM-based storage types (XenServer version 5.5 onwards) the storage repository must have been upgraded if it was created on an older version of XenServer, and the volume must be in the default format; see Upgrading older SRs.
XenCenter supports all three types of VM snapshots: disk-only, quiesced, and disk and memory. See Take a VM Snapshot for more information.
Disk-only snapshots store a VM's configuration information (metadata) and disks (storage), allowing them to be exported and restored for backup purposes. This type of snapshot is crash-consistent and can be performed on all VM types, including Linux VMs.
Quiesced snapshots take advantage of the Windows Volume Shadow Copy Service (VSS) to generate application-consistent point-in-time snapshots. The VSS framework helps VSS-aware applications (for example Microsoft Exchange or Microsoft SQL Server) flush data to disk and prepare for the snapshot before it is taken. Quiesced snapshots are therefore safer to restore, but can have a greater performance impact on a system while they are being taken. They may also fail under load, so more than one attempt to take the snapshot may be required.
Disk and memory snapshots
In addition to saving the VM's metadata and disks, disk and memory snapshots also save the VM's memory state (RAM). Reverting back to a disk and memory snapshot does not require a reboot of the VM, and VMs can be running or suspended when the snapshot is taken. Disk and memory snapshots can be useful if you are upgrading or patching software, or want to test a new application, but also want the option to be able to get back to the current, pre-change state (RAM) of the VM.
Note that if you take snapshots of a VM and subsequently delete the original VM, you can still access those snapshots: in Folder View in the Resources pane, click to expand the Types group and then expand the Snapshots group to see all available snapshots.