My two cents:

You don't necessarily have to use a tape per client, even if you
collocate.
Collocation is done by storage pool.
You can set the max scratch limit for the storage pool to anything you
want.
I set the scratch limit to 1/5 the number of clients, since we are
backing up desktops and they are (relatively) tiny.

If there are more clients than the max scratch limit, ADSM does stack
more than one client on a tape.
That way it keeps the backups for a given client together on the same
tape, without eating a zillion tapes.
You do still have the potential of a tape mount for every client for
every migration cycle.

As others have said, in deciding whether you want to collocate or not,
consider
1) How many clients you have
2) How many tapes you have
3) Whether you have mechanical or two-legged tape mounters
4) How much free time you have in your backup window
5) How responsive you need to be if a client crashes and requires a full
restore

If you don't collocate, you could in theory have one client's data
scattered across every tape in the pool.
Can you tolerate mounting every tape to do a full restore of the client?


===============================================================
Wanda Prather
Johns Hopkins Applied Physics Lab
301-953-6000 X8769
wanda_prather AT jhuapl DOT edu

"Intelligence has much less practical application than you'd think."
              - Scott Adams/Dilbert
===============================================================






        ----------
        From:   Roger Nadler[SMTP:Roger_Nadler AT sonymusic DOT com]
        Sent:   Monday, January 05, 1998 8:05 AM
        To:     ADSM-L AT vm.marist DOT edu
        Subject:        Re: To Collocate or Not to Collocate?

        <<File: PIC15241.PCX>>


        Collocation has to be turned on or off for a given policy set. I
have found
        that enabling collocation speeds up restores (but how many
restores do you
        actually do?) slows down backups since every client needs to go
through a
        mount of its own tape, and uses alot of tapes. (At least won per
client)



         (Embedded
         image moved   markl AT NSMC.PARTNERS DOT ORG
         to file:      01/05/98 08:19 AM
         PIC15241.PCX)




        Please respond to ADSM-L AT VM.MARIST DOT EDU

        To:   ADSM-L AT VM.MARIST DOT EDU
        cc:    (bcc: Roger Nadler)
        Subject:  To Collocate or Not to Collocate?




        Big question, I finally got reclamation working on our system.
It's amazing
        how many tapes change from full to filling when you change
reclamation from
        60 to 30! Well, now that this is behind me, the next question I
would like
        to throw out is "How usefull is collocation?" Should I enable
it? Will it
        save me that much time on retores? How much longer will it take
to perform
        backups with collocation enabled?? I have to A S S U M E it is
going to
        take a long time for collocation to first take place when it is
enabled.

        Mark