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A VMware snapshot is a copy of the virtual machine's disk file (VMDK) at a given point in time. Snapshots provide a change log for the virtual disk and are used to restore a VM to a particular point in time when a failure or system error occurs. Snapshots alone do not provide backup.
Any data that was writable on a VM becomes read-only when the snapshot is taken. VMware administrators can take multiple snapshots of a VM to create multiple possible point-in-time restore points. When a VM reverts to a snapshot, current disk and memory states are deleted and the snapshot becomes the new parent snapshot for that VM. The snapshot file cannot exceed the size of the original disk file, and it requires some overhead disk space. Snapshots will grow rapidly with high disk-write activity volume. Most snapshots are deleted within an hour and VMware recommends deleting snapshots within 24 hours.
Snapshot file formats include *--delta.vmdk file, *.vmsd file and *.vmsn file. Administrators create snapshots in VMware vSphere's Snapshot Manager or with the vmware-cmd command line utility. Deleting, or committing, snapshots merges all of the delta files into the VMDK. If delta files remain in the VM's directory after deletion, the snapshot did not delete properly.
VMware recommends the following best practices regarding snapshots:
- Do not keep a single snapshot for more than 72 hours. While VMware supports up to 32 snapshots in a chain, try to limit chains to three snapshots.
- Do not rely upon snapshots for I/O intensive VMs with rapid data changes, because significant data inconsistencies will occur when the VM is restored.