On Wed, 7 Jan 2009, Craig White wrote:

> funny thing is that amanda developers are adamant that you disable
> hardware compression and use software compression instead.

Amanda devs are adament about a few things and aren't always correct

Hardware compression on older technologies left a lot to be desired,
however LTO hardware compression is fast and comparable to gzip/fast in
terms of efficiency.

> Obviously it takes longer and more cpu power to compress the files in
> software before storing them on the tape and if you leave hardware
> compression on and use software compression too, the files probably grow
> in size.

Modern tape hardware compression has a "step-aside" algoritm - output data
is compared to input data on the fly and if there is no gain from
compression then it's not used and the data is written directly to tape.

> Commercial backup software just seems to always use hardware
> compression.

See above.

Software compression was probably worthwhile when CPUs and disks could
trivially outrun tape drives. That hasn't been the case for at least half
a decade.

AB




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