Quoting Phil Stracchino <[EMAIL PROTECTED]>:
Aleksandar Milivojevic wrote:
Anyhow, if you have enough CPU power, I found that software compression
(using gzip) does better job than hardware compression in the drive.
Interesting -- this is the reverse of my experience. I've found
hardware compression to give me comparable data compression and much
faster actual throughput, with much lower host system CPU load. (And
the host system is an AthlonXP 1700+, so it's no slouch.)
Well, it also depends on the actuall tape drive you have, and on the type of
files being compressed. After all, there's theoreticall maximum the file can
be compressed to, the closer you go to that theoreticall maximum, more CPU
power you need (and it's growing exponentially, sometime a lot more CPU is
needed to get only small decrease in compressed file's size).
Compression algorithms in the drives are optimized to be primarly fast
enough to
compress at the speed data can be written to the tape, with actuall
compression
ratio being secondary objective. I probably needed to include
"usually" and/or
"your experience may vary" in my original text ;-)
Gzip, on the other hand, is optimized to give high compression ratios
at expense
of the speed. You can tune it to some degree using -1 (fast, lower
compression)
to -9 (slow, higher compression) options, with default being -6
(slightly biased
to better compression at expense of speed). The difference between -1 and -9
(in terms of speed) can be as big as two to three times (or negligable,
depending on compressability and the size of actuall files being
compressed). If you have very fast tape drives, and backing up single
machine at a time,
gzip might not be able to compress fast enough.
On the other hand, if I'm not mistaken, software compression is done on the
client side (in file daemon), so it is distributed. If you are doing several
clients in parallel, you should be able to feed the drives with continous data
stream. Plus, network bandwith consumed for your backup will be lower (for
example, you are backing up machine on remote site over slow(er) link). So,
both software and hardware compressions have their cons and pros, and there's
really no definite answer which one is better.
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