HP StorageWorks Storage Mirroring user's guide (T2558-96073, February 2008)

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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. 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.