On Tue, 2004-05-18 at 10:14, Joe Konecny wrote:
> Am I correct in thinking that if I am backing
> up only the server running amanda then it makes
> no sense to use the holding disk?

Not exactly.  If you are backing up files from the holding disk, for
example in other directories, then it probably makes no sense to use the
holding disk.  For filesystems other than the holding disk, you may
still get better tape performance by using the holding disk because
dumps may stream to tape better than dumping directly to tape.

Offhand, I'd say use the holding disk.  If you really care, then try it
without the holding disk and compare the statistics with and without
using the holding disk.

In a nutshell, the 2 questions are: 1) How fast does your tapedrive
gobble up data? 2) How fast can the backup process feed data to the
buffer(s)?  The goal is to keep the tapedrive "streaming" as much as
possible, which means keeping a full buffer, which means your backup
process better generate data faster than the tapedrive can absorb it. 
Otherwise, every time the buffer goes empty, the tapedrive has to stop
and reposition, which takes time and may eat up space on the tape,
making for slower backups that use up more tape to store the same amount
of data.

If the amanda server is the only client, I'd probably turn on hardware
compression on the tapedrive, turn off software compression, and maybe
dump directly to tape.  If there are other clients involved, and you
want the bandwidth savings of software compression client-side (which I
do) then I'd stick to software compression for everything and use the
holding disk on the amanda server, since the software compression would
slow down the backup process and increase the chance of emptying the
buffer.

Why then use software compression for everything?  I'm not sure if it's
a good idea, or necessarily possible, to mix hardware compressed and
uncompressed data on the same tape, or to get amanda to somehow switch
the tapedrive between those modes.  I am sure that applying hardware
compression to data that is already software compressed will use at
least as much, and probably more (and possibly a lot more) tape.

-- 
Jonathan Dill <jfdill AT jfdill DOT com>