Hitachi Copy-on Write SnapShot, User Manual

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Planning and Design
2–3
Hitachi Simple Modular Storage Copy-on-Write SnapShot User’s Guide
Ideally, a business desires no data loss. In the real world, disasters occur
and data is lost. You or your organization’s decision makers must decide
the number of business transactions, the number of hours required to
key in lost data, and so on.
• If losing 4 hours of business transaction is acceptable, but not more,
backups should be planned every 4 hours. If 24 hours of business
transaction can be lost, backups may be planned every 24 hours.
Determining how often copies should be made is one of the factors used to
determine data pool size. The more time that elapses between snapshots,
the more data accumulates in the data pool. Copy frequency may need to
be modified to reduce the data pool size
Selecting a Reasonable Time Between Snapshots
The length of time between snapshots, if too short or too long, can cause
problems.
• When short periods are indicated by your company’s business needs,
consider also that snapshots taken too frequently could make it
impossible to recognize logical errors in the storage system. This would
result in snapshots of bad data. How long does it take to notice and
correct such logical errors? The time span for snapshots should provide
ample time to locate and correct logical errors in the storage system.
• When longer periods between snapshots are indicated by business
needs, consider that the longer the period, the more data accumulates
in the data pool. Longer periods between backups require more space
in the data pool.
This effect is multiplied if more than one V-VOL is used. If you have two
snapshots of the P-VOL, then two V-VOLs are tracking changes to the P-VOL
at the same time.
Establishing How Long a Copy Is Held (Copy Lifespan)
Copy lifespan is the length of time a copy (V-VOL) is held, before a new
backup is made to the volume. Lifespan is determined by two factors:
• Your organization’s data retention policy for holding onto backup
copies.
• Secondary business uses of the backup data.
When you determine the snapshot’s lifespan, you can then calculate the
number of V-VOLs your system requires.
Lifespan Based on Backup Requirements
• If the snapshot is to be used for tape backups, the minimum lifespan
must be => the time required to copy the data to tape. For example:
Hours to copy a V-VOL to tape = 3 hours
V-VOL lifespan => 3 hours