I understand how offsite pool reclamation works, but it is not going to
reclaim the entire storage pool unless you set your threshold shockingly
low.  Otherwise, it is only going to skim off the few highly reclaimable
tapes.  I have a lot of high quality tape drives, but not enough to
re-collocate an entire offsite storage pool on even a weekly basis.  Our
disaster recovery plan is...multilayered...like an onion...or an ogre.
TSM is a part of it, but not the first line of defense.
(If our policy was loose enough to allow us to wait until copy pool tape
were full to send them offsite, then it would make more sense, but our
average is about 40% utilized.  We are required to send off tapes every
workday.  Many tapes are so underutilized that they reclaim in a day or
two.)

Collocation does reduce tape contention between nodes and overall tape
mounts during a disaster.  If TSM is your only disaster recovery tool,
and your environment is small enough it is be a great idea, no argument.
But I just completed running Move NodeData on the primary pool of a
department that no longer actively backs up.  It took 2 months, running
two or three nodes at a time.

-----Original Message-----
From: ADSM: Dist Stor Manager [mailto:ADSM-L AT VM.MARIST DOT EDU] On Behalf Of
David E Ehresman
Sent: Tuesday, November 24, 2009 11:35 AM
To: ADSM-L AT VM.MARIST DOT EDU
Subject: Re: [ADSM-L] collocation increases tape mounts while backup

>>I am not sure if collocating a copy pool is beneficial.  It seems like

>>the
copy pool as a disaster recovery pool would be ideal to collocate, but
if you send off tapes every day you end up with a fragmented set of
tapes for each collocation unit anyway.  To clean that up would require
bringing offsite tapes back onsite, exposing you to risk.  It is
possible to do a Move Nodedata in a copy pool, but the tapes have to be
onsite.  Perhaps there are better ways. <<

Collocating a copy pool is VERY beneficial, if you can afford it (and I
might argue, you can't afford not to).  You CAN reclaim a offsite copy
pool; TSM uses onsite primary pool tapes to recreate the copy pool tapes
that are being reclaimed.  Collocating a copy pool does mean more tapes,
more offsite tape movement, and more reclamation.  But if the purpose of
your copy pool is to allow you to continue business after a disaster,
then you most likely will be under pressure to restore as quckly as
possible and a collocated copy pool will certainly make those restores
run faster.

David Ehresman
University of Louisville

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