Determining Free Space on Backup Tapes with Veritas Backup Exec

Since there seem to be a lot of questions about this, I just thought I'd point out that there is a way to see how much data has been written to a given tape, regardless of compression. I'm using BENT8.0 in this case.

Using the Reports tab at the bottom of the screen, select the Backups Sets report -- NOT the Media report. This will generate a report of all backup jobs that (apparently) still exist on valid media, listed by media "creation date" as opposed to media name. You can cross-reference name to creation date using backup logs.

The Backup Sets log lists all backups on a given tape, and totals the amount of data on each tape. It's not the world's greatest report, but it does the job. I've seen as much as 18GB on a 12/24GB DAT tape using hardware compression.

Factors that appear to affect the amount of data that will fit on a given tape are:

· the compressibility of the particular files backed up. Images, for example, are notoriously difficult to compress. Text documents and data bases typically compress well.

· the number of separate volumes and backup jobs on a tape, which have space-wasting gaps between them on the tape.

· the speed of the hardware feeding the tape drive (if the drive has to wait for data, it wastes space).

· the number of hard and soft errors experienced, which take up space to correct. I've reduced my soft error count by a factor of 1000 by switching to Sony tapes! I now get about 5 soft errors per GB. I understand other vendors have good tapes as well; it’s worth checking around.

· Presumably, cleaning the drive often helps too. The Media report can supply this metric; it's available elsewhere too.

To summarize, if you suspect that you could be getting better tape performance, here’s how to get it. There’s still no good substitute for tape backups!