On Fri, Apr 02, 2010 at 10:36:59AM -0600, Robert LeBlanc wrote:
> On Fri, Apr 2, 2010 at 2:44 AM, Matija Nalis <mnalis+bacula AT carnet DOT hr>

> > I think you need to set
> > Prefer Mounted Volumes = no
>

> I guess this is where we need clarification about what is an available
> drive. I took this to mean a drive that has no tape is more available, and
> then a drive that does already have a tape mounted would be next in
> availability.

Hm, it looks to me that any drive which is not doing R/W operation
(no matter if there is a tape in drive or not) is counted as available.
I could be wrong on that, though.

Anyway, the safest way to know is to test it and let the others know
how it goes :)

> It seems that as long as no job is writing to that tape, then
> the drive is available. I do want this setting to yes and not no, however, I
> would like to minimize tape changes, but take advantage of the multiple
> drives.

From what I see in practice, "Prefer Mounted Volumes = yes" would
make sure there is only one drive in each pool that does the writing.

For example, I have pool of 4 drives and I start 10 jobs at the same
time, all using the same pool. I have an concurrency of >10 and
spooling enabled, so all the jobs run at once and start spooling to
disk -- but when they need to despool, one drive will grab a free
tape from Scratch, and all the jobs will wait for their in turn to
write to one tape in one drive, leaving 3 drives idle all the time.
Only when that tape is full, another one is loaded, and the process
repeats.

I think same happens when I disable spooling, but then the 4 jobs all
interleave writes -- but still all of them will write on one tape in
one drive only.

If you set "Prefer Mounted Volumes = no", then all 4 drives get
loaded with 4 fresh tapes (or just use them if right tapes are
already in right drives -- I guess, I have autochanger) and each
tape gets written to at the same time, maximizing drive (and thus,
the tape) usage.

But "no" setting can (or at least could in the past) lead to
deadlocks sometimes (if you have autochanger), when no new jobs will
get serviced because drive A will wait for tape 2 that is currently
in drive B, and at the same time drive B will wait for tape 1 which
is currently in drive A. Then the manual intervention (umount/mount)
is needed (which is a big problem for us as we have lots of jobs/tapes).

The (recommended) alternative is to go semi-manual way -- dedicate
special pool for each drive, and go with "Prefer Mounted Volumes =
yes" Then one can (and indeed, must) specify manually which jobs will
go in which pools (and hence, in which drives) and can optimize it
for maximum parallelism without deadlocks -- but it requires more
planing and is problematic if your backups are more dynamic and hard
to predict, and you have to redesign when you add/upgrade/remove
drives, and your pools might become somewhat harder to manage.