A snapshot is an image of data taken at a single point in time. Snapshots allow you to view files and folders as they existed at points of time in the past, so you can, for example, recover files that were accidentally deleted or overwritten. You could also compare a current revision of a file with an older revision. Double-Take Availability utilizes snapshot functionality by allowing you to create snapshots of the replicated data stored on the Double-Take Availability target.
Double-Take Availability snapshot functionality ensures that you will always have usable data on the target. For example, if your source server becomes infected with a virus, you can revert to a previous snapshot of the data on the target that was created prior to the virus infection. If you know the data on your target is good data, in a usable state, it will minimize application downtime in the event of a source failure. For example, if the source failed and the data on the target is not good due to an incomplete mirror, you can revert to a good snapshot on the target before failover. Snapshots also allow you to retrieve files that a user may have deleted.
Double-Take Availability uses the Microsoft Volume Shadow Copy service to create snapshots. To access this functionality, your target must be running Windows 2003 Service Pack 1 or later. Your servers must also be using the NTFS file system. Snapshots are taken at the volume level, corresponding to the target volumes contained in your replication set. For example, if your replication set contains d:\data and e:\files, the snapshot will contain all of the data on both the d: and e: volumes. If your replication set only includes d:\data (e:\files exists but is not included in the replication set), the snapshot will only contain the d: volume.
Sometimes taking a snapshot may not be possible. For example, there may not be enough disk space to create and store the snapshot, or maybe the target is too low on memory. If a snapshot fails, an Event message and a Double-Take Availability log message are both created and logged.
There are limitations imposed by Microsoft Volume Shadow Copy that impact Double-Take Availability snapshots. For example, Double-Take Availability maintains only 64 snapshots because Volume Shadow Copy only maintains 64 snapshots. If 64 snapshots exist and another one is taken, the oldest snapshots are deleted to make room for the new one. Another example is that Double-Take Availability snapshots must be created within one minute because Volume Shadow Copy snapshots must be created within one minute. If it takes longer than one minute to create the snapshot, the snapshot will be considered a failure. Additionally, Volume Shadow Copy will not revert snapshots of a volume with operating system files, therefore Double-Take Availability is also unable to revert a volume with operating system files. You must also keep in mind that if you are using extended functionality provided by Volume Shadow Copy, you need to be aware of the impacts that functionality may have on Double-Take Availability. For example, if you change the location where the shadow copies are stored and an error occurs, it may appear to be a Double-Take Availability error when it is in fact a Volume Shadow Copy error. Be sure and review any events created by the VolSnap driver and check your Volume Shadow Copy documentation for details.