On Sun, 26 Feb 2006, Ian Levesque wrote:
I've noticed a pattern in my backups when two jobs are simultaneously
writing to the same tape. When the tape ends, Bacula doesn't respond
gracefully. This is only a problem when two jobs are running on the tape,
not when a single job is running.
Are you spooling to disk or writing directly to tape?
I'm writing directly to tape. Do you suppose that this may be remedied by
spooling? My initial tests with spooling showed a performance hit
On one job there's a performance hit _while writing_
On multiple jobs there's a performance hit _while reading_ because all the
jobs are interleaved, resulting in shoeshining.
On multiple jobs, using slightly staggered start times, one job can be
spooling to disk while the other is dumping to tape (this will naturally
happen after a couple of spool/dump operatiopns anyway), givening an
overall speed increase, especially on incrementals, differentials
or backups of clients with sub-Gb/s connectivity
Advantage 1: Backup jobs are clustered on the tape, making reading a far
cleaner streaming affair (read spool dump, skip to next section, no shoe
shining).
Advantage 2: On incremental/differential jobs it pretty much eliminates
shoeshining while running the job.
Advantage 3: Only one job is ever accessing the tape at a time, so the
issue you've hit won't happen.
It's clear you've tickled a bug and I hope you've filed it on bugs.bacula.org.
However spooling will work around it and as soon as you're running
multiple simultaneous jobs you should consider using it anyway as it
usually gives _overall_ (not individual) speed advantages.
AB
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