We have 35/70 GB DLT tapes.  We will be backing up about 300 ADSM clients.
We have DEEPLY analyzed storage pool options keeping both backup and
recovery
in mind.

Issues:
Large tapes      (35/70GB)
Many clients     (~300)
Tape mount times (~3 minutes each)

To colocate or not:

Colocation in theory is an excellent idea...after all we should be thinking
in
terms of recovery.
When we looked into colocation we analyzed tape reclamation.  BIG SURPRISE!
Having many clients on one tape can cause MANY mounts.

Example:

Colocation tries to distribute your clients onto their own tapes.
High capacity tapes and not alot of them has caused you to have say 30
clients
on a tape.
But then the tape is full and ADSM continues to distribute client data onto:
another volume where that client's data exists
or
an empty volume
or
the "least full" volume.

Your 30 clients may all end up on 30 different tapes now.

Now you want to reclaim the first tape.  ADSM mounts the tape and looks for
reclamable data.
Once found now ADSM looks for where else that client's data is (what other
tape
volume).
ADSM mounts an output tape and writes out client A's data.
Then ADSM searched the rest of the input volume for client A's data and
moves
it off.
Now ADSM begins again searhing for client B's data and moves it off. Then
client C.....

Now at the point in time when your input (first) tape became full ADSM put
client A's data
and Client B's data on the same 'new' tape... you see how reclamation with
colocation can call
for the same tape twice.  Once for reclaming Client A's data then again for
Client B.

This revelation has caused us to NOT incorporate colocation in our
environment.
If I have misunderstood reclamation w/colocation PLEASE correct me.
We have talked at length with IBM regarding this issue.

Analyze your tape usage before you choose colocation.

Cyndi Flis
Travelers Property Casualty

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We have an automated tape library.
I just check in tape cartridges in "scratch" status, and let ADSM pull
tapes as needed from the scratch pool into the backup and archive tape
pools.
My backup tape pool is colocated.
This works fine because I have more cartridges than clients.

But, we are about to add hundreds of clients, so that I will have more
clients than cartridges.
Each cartridge holds 35 GB, so I should be able to get several clients
per cartridge with no problem, but I would still like to colocate for
speed on restore..

How do other people handle this situation?
I think that probably all I have to do is stop using "scratch" tapes and
manually define a fixed number of cartidge volumes to each pool.  Are
there other considerations?