Snapshot in Citrix XenServer

Taken from Citrix XenCenter 5.6 :)

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 Upgrade an SR.

Types of snapshot
Three types of VM snapshots are supported: regular, quiesced, and snapshot with memory.

1. Regular Snapshots. Regular snapshots are crash-consistent and can be performed on all VM types, including Linux VMs.

2. Quiesced Snapshots. 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. XenServer supports quiesced snapshots on Windows Server 2003 and Windows Server 2008 for both 32-bit and 64-bit variants. Windows 2000, Windows XP and Windows Vista are not supported. For further detail on quiesced snapshots, refer to the Advanced Notes for Quiesced Snapshots section later in this chapter.

3. Snapshots with memory. In addition to saving the VM's memory (storage) and metadata, snapshots with memory also save the VM's state (RAM). This 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. Reverting back to a snapshot with memory, does not require a reboot of the VM. You can take a snapshot with memory of a running or suspended VM.

Accessing orphaned snapshots
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.