1. How many clients do you have to support, and how much will they have to
back up daily?
In this environment, a client MUST be able to get a tape drive when he does
anything with ADSM. Obviously, then, you could only support two concurrent
users. This can be controlled through the server options file, but it is
something to think about. There are two "knobs" here - number of concurrent
sessions, and % available for scheduled sessions.
One possibility with scheduled sessions MAY be (I haven't done a lot of
research on this one) is to have your clients use schedmode=prompted when
establishing their schedules. This will cause the server to contact the
clients for a backup, instead of the client coming back randomly. Why would
you do this? If you have 3 clients that want to schedule a backup, and they
have very little to back up, then it probably isn't a big deal - the odds of
them all asking at the same time are most likely not very great, depending on
the duration of the schedule window. But if these 3 users have lots to back
up (let's say the average session lasts an hour), and the window for backups
is 2 hours (50% of a 4 hour backup schedule), then there is a good
probability that the third session will attempt to start before the other two
have completed.
There are 2 methods of scheduling - pooled and prompted. Polled (the
default) will randomly assign times, and the clients come back at these
times. Prompted, however, has the server contact the client, and start the
backup. Therefore, when the window opens, ADSM would start two sessions, and
not attempt to contact the third, until one of the others was finished. If
you used polled, and the # of sessions was 2, the third session would try
again later,depending on what the retry time interval was (default of 20
min). You have to be careful here, though - if the retry bumps you out of
the schedule window, then no backup is taken. It seems to me that prompted
would maximize the number of sessions in a window.
2. How does copypool processing work with this?
Making a copypool tape is a critical process, since it will require both tape
drives. The process itself is straight forward - copy the files from tape to
the backup tape. It is NOT a straight copy - but that's not important here.
The issue is that while you are doing this, drives are unavailable for other
uses. The good news here is that you control the backup process - so you can
use the admin scheduler to kick off the backup tape creation at a time when
no other users are expected.
As an aside here, if you are thinking about using copypools, I would start
doing them immediately in your shop. We implemented them later, and had a
lot of tape mount activity to work through, even though we are MVS and have
lots of drives available. It sounds to me as though it would be better to
implement a copypool strategy as soon as possible for you.
3. How does reclaimation work here?
As with copypools, tape reclaimation also requires both drives. Unlike
copypool processing, reclaimation can be initiated by the server, and as
such, can start at the time you least want it. BUT you can limit this. For
example, you could set reclaimation at 100%, thus stopping it entirely -
tapes will not be reclaimed. Of course, you may have a lot of tapes with
only one file on them, which may not be desireable in your situation.
Another alternative is to turn the threshold up and down at certain times of
the day, to control the process. That allows you to start reclaims at a good
time, and turn it off when it does not suit you. The thing to remember is
that expiration processing will drive your reclaims - when you run expire
inventory, files will be deleted from the tapes, and if a tape falls below
the threshold, then it will spawn a reclaim.
Hope these items help.
Jerry Lawson
jlawson AT thehartford DOT com
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