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Veritas 10.x compression/reporting problems

I have an install that is giving me problems.
Win2003 server running Veritas ver 10 build 5484, sp 4. Seagate Travan 40 gig drive - 20 native/40 compressed, PCI drive. I was backing up about 24 to 25 gig onto 1 tape, when last week it took two tapes. The first issue is that when it ran onto two tapes the first tape reports that it used 18.9 Gig, had 16.4 gig available with a compression ratio of 6.59:1. The second tape was written to for about 1 minute, then we went into the verify mode. At this point I was at no service packs on veritas so I installed SP4. I removed some files from the backup schedule so I could get back to 1 tape. On the latest backup it reported 17.8 used capacity, 17.5 available capacity, total capacity 19.3, with a compression ratio 10.0:1. I have the jobs set to overwrite, no append. I did a quick erase of the tapes before the jobs ran last week.
My problems are why does veritas report bogus compression ratios, and how can I really tell how much of the tape is available? The graphic representation does not give me a good picture of how much is used/free.
I think we were running onto 2 tapes because we are backing up large PST files towords the end of the job, and this may cause problems estimating tape usage, etc...
I am using the veritas drivers and no errors are being reported.

I posted on Veritas discussion forums but got no real help......

Thanks - Wayne
1 Solution
When compression is used there is no way to get an acurate figure on the amount of tape left. The drive does all those calculations based of the native capacity, so compression really screws it up.

One way to find out just how much is going to one tape is to run a job you know will fill the tape and have it cancel instead of spanning.

As you know the amount of compression is up to the type of data more than anything else.

I have no idea about the inability of Backup Exec to report the correct figures.

Oh one more thing that is just a side issue and this also I'm sure you know but just in case, Travan tapes should be retentioned every so often. It takes awhile to do it but with Travan it is important. Certainly any time it gets a read error retention the tape and retry the operation. But like I said this is just a side issue and it is doubtful it will have any effect on this problem.
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