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Colorado 20GB not Compressing Data

Kenneth Hart
Occasional Visitor

Colorado 20GB not Compressing Data

Win2K Server,SP3, w/ internal Colorado 20GB Travan on IDE#2. NTBackup asks for second tape when data to backup is more than 10gig. I am using command line switch /HC:on yet data is not being compressed. ??
2 REPLIES
Joseph T. Wyckoff
Honored Contributor

Re: Colorado 20GB not Compressing Data

I do not know about your specific drive, and there may indeed be an issue - check service packs for NT (and specific patches for NTBackup), and drivers for your SCSI card (IDE in this case too) and tape drive as a starting point.

Look also, for some DIP switch settings on your tape drive - QIC drives used to do that... back a few years ago when I last touched one. Check also formware for the tape drive.

That said... compression varies - I see this question with great frequency, across all technologies.

Firstly, remember that the backup software has some overhead - it has to keep some 'accounting', catalog, and other information on the tape. That should be a small part of the puzzle, with luck, but it may be relevant. 10gb may actually be 11gb, assuming 10% overhead...

In some cases the data you are backing up is not compressable (it is already compressed, or very random.)

In other cases, there is a compression versus performance tradeoff, and the manufacturer of the tape technology has chosen performance as the ideal; we see this with DLT, and I think LTO:

A rewind operation takes a long time, relatively speaking. In order not to suffer a rewind penalty, it is common to keep the tape streaming, and simply write an empty block on the tape, if there is no actual data to write. Such an empty block, not only is not compressed, but uses space where compressed data could go, had it been available. Generally, after a several such empty blocks in a row, the tape technology (firmware) will force a rewind, and re-use the space, but if the blocks were periodic, and not sequential, a lot of space could be 'wasted' in the name of performance.

I don't think this applies to you, but it must be noted that compression problems can be pretty subtle.

Try a different tape - if you have bad blocks on the tape, this could be your answer.

Finally, look at the tape technology. Often times marketing folks will advertise 2:1 or even 4:1 compression. In my experience that is not realistic, although opinions, and experiences, vary.

I am not familiar with your TRAVAN, so a slightly different example here. If your tape technology promised 35/70, like a dlt 7000 - thats 35gb 'uncompressed' and 70gb with 2:1 compression, then anything over 35GB implies that you are in fact seeing compression.

If your technology is 10/20 - anything over 10gb implies compression is present.

Good Luck.
Omniback and NT problems? double check name resolution, DNS/HOSTS...
harry d brown jr
Honored Contributor

Re: Colorado 20GB not Compressing Data

A 20GB tape drive can ONLY store 10GB of data. The BOGUS "rating" of 20GB is a MARKETING SCAM for the unknowing. If you get a 2:1 compression ratio then you can store 20GB on the 10GB tape drive! But if you are backing up binary and compressed files, you be lucky to see any compression at all. If some cases a compressed file will actually grow in size when you try to compress it again!

Maybe it's time for a bigger tape drive?

live free or die
harry
Live Free or Die