A common problem in virtualization is snapshots. The name “snapshot” makes us (novice or otherwise!) think of a picture in time, which sometimes leads to the belief that the snapshot is “taken” and then stored somewhere, though that’s not how snapshots really work.

In reality, snapshots create a psuedo-consistent state of the virtual disk at that point in time. Subsequent writes in a snapshotted state are redirected to delta files. If you are performing an upgrade, a snapshot is helpful, allowing you to restore the prior system state if there are problems. After a few days, the snapshot loses its value as a restore becomes increasingly unlikely because you would lose the application changes as well. Snapshots also play a role in backups, where they are used temporarily to provide the psuedo-consistent state for the backup utility before the snapshot is deleted.

When a snapshot is deleted, that delta is applied to the base virtual disk(s), playing back through the transactions. Large snapshots take a long time to delete and affect system performance until the consolidation is complete. They can also affect the VM during normal operation as the delta file size increases.
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