Implementing Backup and Recovery: The Readiness Guide for the Enterprise

Capacity

After selecting the appropriate drive technology that provides the performance and cartridge capacity you need, you next want to look at how many cartridges you will need to have available. This involves all the elements we have looked at so far. The number of cartridges required depends on the amount of data that you are backing up, the frequency of your backups, your retention periods, and the capacity of the media used to store your backups. A simple formula that can be used is as follows:

Number of tapes = (Total data to back up × Frequency of backups × Retention period)/Tape capacity

Following is an example:

• Total amount of data = 100 GB

• Full backups per month = 4

• Retention period for full backups = 6 months

• Incremental backups per month = 30

• Retention period for incremental backups = 1 month

Preliminary calculations:

• Size of full backups = 100 GB × 4 per month × 6 months = 2.4 TB

• Size of incremental backups = (20 percent of 100 GB) × 30 × 1 month = 600 GB

• Total data stored = 2.4 TB + 600 GB = 3 TB

Solution:

• Tape drive = DLT 7000

• Tape capacity without compression = 31.5 GB

• Total tapes needed for full backups = 2.4 TB / 31.5 GB = 76.2 = 77

• Total tapes needed for incremental backups = 600 GB / 31.5 GB = 19.1 = 20

• Total tapes needed = 77 + 20 = 97

By looking at this example, you would expect to have a minimum of 97 active cartridges at any given time. This also assumes that all the cartridges will be filled to capacity and there will be no unused tape. These calculations are based on no compression. This does give you an idea of the steps necessary to plan for an appropriately sized tape library. We would never recommend implementing an enterprise backup strategy that does not include a robotic tape library with a barcode reader. Without these, the management can become overwhelming and very susceptible to human error. It is much better to turn over media management to an enterprise backup application.

When figuring out how many slots are required to support your environment, do not forget to include some slots for cleaning tapes and at least two for the catalog backups. Actually, you will want to reserve twice as many slots for catalog backups as are needed so you can keep a copy of the catalog. If you are including an off-site storage solution of some type (vaulting) as part of your backup strategy, you need to include this in your total capacity calculations, since creating duplicate copies requires additional tapes.