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To avoid limiting your scalability (few environments are static or shrink with
regards to the amount of managed data)
To avoid building additional complexity into the design and implementation,
which will drive the cost of implementation higher
Because the price point for libraries and tape drives have fallen significantly in
the recent past
Two-drive systems allow for quicker reclamation and pool copies and reduced
drive outages, but restores coming from tape may be delayed due to other
competing tape activity. Systems with three or more tape drives can handle
restores from tape occurring while tape reclamation or other tape processing is in
progress.
As for the number of tape drives, a critical consideration is how quickly you need
to restore one or more clients. More tape drives allow you to restore more clients
in a given time. Collocation allows as many clients to be restored simultaneously
as you have tape drives. If you have stringent restore requirements, collocation
and multiple tape drives make sense. You could also consider features like
backup sets and logical volume backups for highly time dependent servers to
improve their restore window.
To calculate the rate of a restore operation, divide the amount of storage to be
restored by the sustained data rate of the tape drive (not the instantaneous, or
burst, rate) quoted by the manufacturer. If this number does not allow you to
meet your service levels, collocation, more tape drives, faster tape drives, a new
service level, or another backup strategy may be required.
We recommend a minimum of a library with a barcode reader and at least two
tape drives for all production-level Tivoli Storage Manager solutions.
2.10 Tape volumes
Tape volumes are used to store on-site copies of data, off-site copies of data,
and database backups. Additional tape volumes are required because all tape
volumes are not filled to capacity. Some volumes are required to stock a scratch
pool so that mounts for unused tape volumes can be satisfied. If you are
considering backup sets and image backups, you may need to add more tapes
to your scratch pool.
2.10.1 On-site volumes
You must consider how many tapes you will need to hold your backup, archive,
and image data in a specified time frame. Each of those have different expiration
Chapter 2. Implementation planning
53
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