HP Storage Mirroring Software - Manual (Page 630)

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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 Recover
utilizes snapshot functionality by allowing you to create snapshots of the replicated data
stored on the Storage Mirroring Recover target.
Storage Mirroring Recover 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 Recover 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 Recover log
message are both created and logged.
There are limitations imposed by Microsoft Volume Shadow Copy that impact Storage
Mirroring Recover snapshots. For example, Storage Mirroring Recover 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 Recover 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 Recover
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