Yes, collocation works properly if you have MAXSCRATCH set to fewer
tapes than you have clients. I have 500+ clients backing up into a 400
slot library. With high-capacity tapes, it makes sense. I used
calculations such as you did to come up with an initial MAXSCRATCH that
would leave me enough slots for my other tape pools.
The rules are:
If collocation is on, when TSM starts migration (or backup to tape) for
a given client's data, it tries to put the data on a non-full tape where
data already exists for that client. If there isn't one, it starts a
new tape unless the pool has already gotten up to MAXSCRATCH, in which
case it puts the data on the least-full tape available in the pool.
Advantages:
In my experience, restores run significantly faster with collocated
tapes, even with stacked clients. If you have 4-5 clients stacking on 1
3590 tape, it doesn't cost as much time to skip over the other client's
data as it does to do extra mounts, even with the 3590's blazing speed.
(After all, you'll be skipping over some of your own client's inactive
data, and that doesn't hurt much, does it?)
The alternative, you have to consider, is potentially mounting EVERY
TAPE in the library to get a large restore done.
It's also very easy to set up. Just turn on collocation and set
MAXSCRATCH for the pool you already have, it will start collocating as
it sits now. Existing tapes will gradually get collocated as they go
through the reclaim process. And if you don't like the results, just
turn collocation off again.
Disadvantages:
With collocation on you get a LOT more tape mounts during migration,
during reclaim of offsite storage pools, and during backup stgpool
(going from your onsite collocated pool to your offsite non-collcated
copy pool). The 3590's are certainly capable of taking the beating
required (not all tapes drives are) and it shouldn't be a problem unless
you are real short on drives or your schedule is already very tight.
(You can reduce the amount of extra tape activity somewhat by trying to
schedule your copy storage pool before migration happens, so that most
of the data goes from disk to tape, rather than tape to tape.)
Another disadvantage is that you have to MONITOR the MAXSCRATCH value,
and keep bumping it up. After all, your tapes will start to fill up
over time, and there will be tapes that are full but not yet eligible
for reclaim, so you will have fewer FILLING tapes in the pool. So you
either have to increase MAXSCRATCH or reduce RECLAIM%, or you have fewer
and fewer filling tapes and gradually lose the benefits of collocation.
It takes experimentation to figure out what the balance is for your site
for MAXSCRATCH vs. RECLAIM%. I have a script that checks the number of
tapes in FILLING status and emails me so that I know before things get
hosed up.
Trying it out is easy and pretty much harmless!
Hope that helps!
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