ADSM-L

Re: Multitasking Reclaim

1998-05-07 14:23:33
Subject: Re: Multitasking Reclaim
From: Lindsay Morris <lhmorris AT US.IBM DOT COM>
Date: Thu, 7 May 1998 14:23:33 -0400
I'll re-post my prior message, with the note that you can conceivably tell the
(imaginary) script to get two MOVE DATA's going at once, if you have four
drives (or three, if you have six drives, or ...)
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Don't set ADSM reclamation to 10% -  It'll spin tapes forever and not reclaim
Don't set ADSM reclamation to 10% -  It'll spin tapes forever and not reclaim
much space.

It turns out that ADSM reclamation makes poor choices regarding which tapes to
reclaim.
If you have reclamation set to 60%, then tapes which are 40% full or less are
candidates, right?

So consider:
if ADSM picks a tape which is 40% full, it'll take maybe 30 minutes to reclaim,
and you'll only free up 60% of a tape.
If ADSM picks a tape which is 1% full, it'll reclaim it in 3 minutes, and
you'll regain 99% of a tape.

Clearly, ADSM should start with tapes which are 1% full, and work its way up
toward the tapes which are 40% full.  It would reclaim a lot more space a lot
faster if it worked this way.

But it doesn't do this.  Instead, it works off a list which is roughly ordered
by total capacity of the tape and completely ignores the utilization.

Two ways to get around this:
1. Use admin schedules to set reclamation to 95% at 6AM, 90% at 6:30, 85% at
7:00, etc.  Tacky, but....
2. Write a script which does a "Query Volume", sorts the output, then does
"Move Data"s on the easiest one first.
I did the latter - it's very effective.

ADSM support understands this problem and has told me they will try to fix the
behavior in some future release.
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