HP Storage Mirroring V5.1 Software - Manual (Page 95)

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Snapshots
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. Storage Mirroring utilizes snapshot functionality by
allowing you to create snapshots of the replicated data stored on the Storage Mirroring target.
Storage Mirroring 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.
Storage Mirroring 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 Storage Mirroring log message are both created and logged.
There are limitations imposed by Microsoft Volume Shadow Copy that impact Storage Mirroring
snapshots. For example, Storage Mirroring 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 Storage Mirroring
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 Storage Mirroring 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 Storage Mirroring. For example, if you change the location where the shadow copies are stored
and an error occurs, it may appear to be a Storage Mirroring 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.