Summary
This article describes how to properly use different media pools for base, incremental (and/or differential) backups.
Resolution
A DPX job inherently contains three backup levels: base, differential and incremental. When scheduling a job with a scheme that contains different backup levels (base/incremental or base/differential), the retention period for the different backup levels may not be the same. For example, if you use one of the built-in schedules, you would run a base once a week and differential (or incrementals) on the week days. You may decide to retain the data for the base backups for 3 months, and retain the data for the differential backups for 1 week. When the retention periods are different, you should use different media pools to make better use of your tapes. Once a tape is full, it cannot be re-used until all the jobs on the tape have expired.
To continue with the example above (don't worry about the old media type as the example is still relevant), let's say that the tapes are DLT 70 GB (compressed data) tapes and the data in the job is 15 GB for a base and, on average, 5 GB for an incremental that you run every week day. If you run the job to the same set of tapes (media pool), then you have a base that expires after 90 days, followed by 4 incremental jobs. The first tape will be full after 2 weeks and will expire 86 days after it is full. Your media pool will need to have 14 tapes for this rotation. If you run the base and incremental to different media pools, then you will need two tapes for the incremental pool and four tapes for the base: a total of 6 tapes. You can run 14 incrementals to one tape that will expire 7 days after the last run - long before the second tape will be full; you can run 4 1/3 base to a tape that will expire before you have filled up the fourth tape.