Quoting Trevor Morrison <[EMAIL PROTECTED]>:
Hi,
I have an Exabyte-8900 20/40 GB drive and it does a great job backing
up my boxes. My question is: After about 25 GB, Bacula says the tape
is full and wants another tape. According to the LCD display on the
drive itself, compression is turned on. So, how can I tell Bacula to
write up to 40 GB worth of data to the tape before changing?
The 40GB is relatively unrealistic 1:2 compression. The real compression (and
tape's capacity) that you'll get depends on the files you actually back
up. If
they were already compressed, you won't get any compression in the drive (you
can't compress compressed files), so you'll fit only about 20GB worth of data
on the drive (same as not using compression). If you are backing up
text files
(which can be nicely compressed) you'd be closer to 40GB. In general
case, you
could expect your compression ratio to be somewhere between 25% and 75%, but
very rarely to be what manufacturer told you.
Anyhow, if you have enough CPU power, I found that software compression (using
gzip) does better job than hardware compression in the drive. If you are
compressing files that can actually be compressed. See the "compression"
option in the FileSet resource chapter. However, unlike hardware compression
in the drive, compressing compressed file (or extremely small file,
like couple
of bytes) with gzip will produce larger file then original ;-)
For example, try this:
echo a > a; ls -l a
for a in 1 2 3 4 5; do gzip -v a; ls -l a.gz; mv a.gz a; done
rm a
Moral of the story, know your files before compressing them ;-)
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