ADSM-L

Re: Question about Re-Use Delay

2000-02-03 00:26:14
Subject: Re: Question about Re-Use Delay
From: Simon Watson <simon.s.watson AT SHELL.COM DOT BN>
Date: Thu, 3 Feb 2000 13:26:14 +0800
Ming,

The
reclamation period for offsite pools,
the reuse delay for those pools and
the policy that says how many versions to keep
have nothing whatsoever to do with each other.  They are all set
independantly and are set based on different requirements.

Set your number of versions based on the user requirement, ie how many
versions would they like to be able to recover.

The reuse delay period is for real disasters.  Basically when a tape is
no longer used, all the data has been expired, or it has been
reclaimed, it will go into a pending state.  This stops it from being
overwritten until the reuse delay period has ended, however the current
copy of the database no longer has any record of what is on those
tapes.  They are only useful if you load a previously backed up version
of the database, which with ADSM, nobody ever really wants to have to
do.

The reclamation period can be anything you like, however should have
some relationship with how often your expire inventory command runs.
There is absolutely no point running reclamations more often than your
expire inventory.  Expire inventory updates the Database, and actually
epires all those entries that are no longer required, based on the
policy, in your case, it keeps only 21 versions.  This will gradually
increase the Percentage Reclaimable of each tape, as more & more data
is no longer of any use.  Once this Percentage Reclaimable reaches a
certain limit on a given tape it is worth reclaiming that particular
tape.  There is not much point having hundreds of tapes hanging around
with only 1% useful data on them.  For example I run Expire Inventory
every day.  This keeps the Database cleaned up.  For storage pools
within my library I have a reclamation threshold of approx 50%.  Once
there is only 50% of data that is still useful on the tape, or 50%
unused space, then it will reclaim automatically.  Since my onsite
tapes are Collocated & my offsite tapes are not I only run reclamations
on the offsite (copypool) tapes over the weekend, as even to reclaim 1
offsite tapes it will generate many mounts of the onsite tapes.  It is
more efficient to reclaim a whole bunch of offsite tapes in one pass.
So I leave my offsite pool reclamation threshold quite high during the
week (90%) and drop it to 50% over the weekend.

I hope this helps.

Simon

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