DLT 15/30 runs out of space too early

[Ayman Altounji]

My backup jobs process exactly the same information every night, but some mornings I come in to find a message (Backup Exec software) asking for the next tape for the backup to continue. I can never tell on which nights this will happen.

Strange thing is, this can happen when as little as 16Gb, or as much as 20Gb, has been backed up, while on other nights the whole caboodle (25GB) happily fits on one tape.

I've replaced the tape unit, but the same thing happens. I've tried using brand new cartridges but the same thing happens. And I've checked that the backup jobs are configured in exactly the same way, but the tape span requests are still unpredictable.

I'm confused, please help.

Mike McCreadie

[Ayman Altounji]

I am facing the same problem with a Compaq DLT20/40GB. I notice that when it comes to back up less than 30GB, it works fine. If more that,it backs up a little more than 30GB and then it asks to unload the tape and then load e new one. I 've been trying to fix it the same way as you are but unsuccessfully.

[Ayman Altounji]

I have the same problem on my 20/40 gb DLT.
sometimes everyhing oke, next time it says tape full at appr. 30 gb
I did an upgrade of backup exec, run some patches, but there still is the problem.
I'm now going to install a 40/80 gb, see if this will help.

[Ayman Altounji]

Assuming the system and backup application are performing as expected and that the tape drive compression is enabled, the most likely cause for less than expected backup capacity is the compressibility of the data the drive sees. Verifying whether or not drive compression is performing as expected -OR- if your data set is compressible down to the expected amount is not trivial because of the size of the data set involved. Running a subset of your target backup data through a compression utility may provide clues, however. Be that as it may, you should contact your backup application supplier regarding this issue.

[Ayman Altounji]

OK chaps, I have the answer. A few weeks ago I received a mystery email from someone at Compaq (my reply was bounced) which read:

Hmm here's a question - do you happen to be backing up any type of mail files? Lotus Notes, Exchange, anything?

Funnily enough, I do backup a few pst files every night, so to test this possible solution I archived off virtually all of them and let the nightly backups go off as normal.

I haven't had a problem since.

Perhaps someone at Compaq could explain why this is?

[Ayman Altounji]

This is a classic data compression issue folks... What you backup probably varies quite a bit from time to time - even though you are running the same basic job. The contents are not always identical, otherwise you wouldn't really need a backup.

Have any of you read the old DAT Troubleshooting guide? I know it's not specific to DLT drives, but the section on compression covers the subject for most tape drives just as well.

DAT Troubleshooting Guide