Hi.
[EMAIL PROTECTED] wrote:
Hi all,
I have a HP DAT72 tape and installed it on a new debian sarge box. The problem I have is the following: I defined the tape with hardware compression: - I used the following command: mt -f /dev/nst0 datcompression 1 - I did not enable software compression with bacula.
When I backuped several directories which are holding around 50GB the backup stopped after 33,900,640,561 bytes. This is not even the Amount the tape should save with compression disabled. What do I have to do to enable hardware compression?
First thing is to check that compression is actually enabled: mt status or tapeinfo might tell you.
Second thing is a completely different question: How to determine the usable tape capacity?
Some (sort of) facts: Manufacturers give highly optimistic values: Your example: DAT72 means "raw" capacity of 36 GBytes. In "manufacturer mode" this is 36000000000 Bytes, of which you lose the space occupied by End-Of-Anything marks.
Compression ratios are usually assumed to be 1:2, 1:2.6 (Sony) or even higer (I forgot...), but in fact real-world data compresses quite different. From my backups: I get between 18GB (with 1 GB = 2^30) and 29 GB on a "20/40GB" DLT tape, with an average of about 22GB. Daily incremental backups get great results for marketing purposes: up to 12 GB on a DDS-2 tape (4/8 GB) - mostly log files which compress great. When I store compressed digital media like mp3 of digital video files I have about 3.5 GB on the same sort of tape.
Conclusion: If you want to know how many bytes fit onto a tape you need to measure the right thing:
For example, use 'dd if=/dev/urandom bs=32k of=/dev/st0'. Try with varying block sizes and try with a number of 'mt weof's in between. Try storing _your_ real data using tar, also trying different block sizes and number of files. Then try the same with data compression.
After some days you can draw some fine charts which tell you two important things: How many tapes you need, and that the capacity numbers given by the manufacturers have not much in common with what you encounter on a day-to-day basis. Forget the rest...
I usually wonder when I see a tape marked "Full" with less then half the nominal capacity (often indicates a broken tape) or "Append" at three times the regular capacity (most often by chance the right sort of data).
Thanks for your help,
I don't know if that helps, but it might explain things.
Arno
Oliver
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-- IT-Service Lehmann [EMAIL PROTECTED] Arno Lehmann http://www.its-lehmann.de
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