One of the more compelling reasons to use ESX/ESXi Server for virtual server management is the feature called Snapshot backups. Snapshot backups provide the ESX/ESXi Server administrator with the ability to essentially freeze a virtual machine (VMDK virtual server) at a point in time while scheduled updates or upgrades are being attempted. Snapshot backups are managed using the VMWare VI Client, discussed in a earlier post.
When a snapshot is taken all updates (regardless of origin) which occur to the virtual machine will be written to a set of snapshot files, rather than the VMDK virtual machine image; thus providing the administrator with the opportunity to copy the VMDK virtual machine image. A snapshot can be taken for two reasons; 1) if an administrator wants to copy the VMDK virtual machine, or 2) if the administrator wants to apply updates or upgrades (update from Microsoft, application upgrades, etc) to the virtual machine.
Once the VMDK virtual machine has been copied successfully, using simply copy/paste commands against the VMDK Virtual machine, or once the updates/upgrades have been verified as having applied successfully, then the snapshot is customarily deleted. Snapshot deletion will force ESX/ESXi Server to apply all queued updates/ upgrades (queued in the snapshot set of files) to the actual VMDK virtual machine.
If the updates/upgrades failed for some reason then the administrator can revert the VMDK virtual machine back to the point in time when the snapshot was originally taken. If the VMDK virtual machine is reverted to the point in time when the snapshot was originally taken then all updates/upgrades after that point in time will be lost, including the failed updates/ upgrades.
Snapshot backups are indeed a handy tool for virtual machine management. How many times have you applied an upgrade only to sadly find out that the upgrade failed and it needed to be backed out.