One of the most common misunderstandings with Backup Exec is how media sets work. It's common for people to think that by assigning a job to a media set, the media from that set will be used for the job. In actual fact, this assignment means that any media used in the job will be added or moved to this media set.
A media set is a group of media which have certain attributes applied. The main attributes are:
- Append periods – how long the media should be appendable for after writing
- Overwrite protection period – how long the media should be protected from overwriting
- Vault rules – when the media should be moved to and retrieved from a vault
The way AP, OPP and vaulting rules work is simple, but complex to understand. Append Period is how long you will allow data to be added to this medium. This is measured from the time the media is first added to the media set. Overwrite Protection Period is how long you want the media to be protected from being overwritten, and is measured from the time the tape is last written to. Once the OPP has expired, the media can be overwritten (it becomes recyclable).
The AP and OPP affect how Backup Exec selects which media are used in a job .
If you wish to target specific jobs to specific media, then the best way is to use partitions if using a tape library. Before doing this, it may be easier to let Backup Exec manage the media itself. For more information, see another article on this site (yet to be written).
For more information see pages 213-217 of the Backup Exec 10d Administrator's Guide.