Hp Storage Mirroring Software User Manual download pdf (Page 599)

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Snapshots Page 598 of 677

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 servers must 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, for some operating system versions 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 to be aware of the impacts that

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