Dean Roy asked:

> (1) If collocation is turned off, can multiple clients backup data
>      simultaneuosly to the same volume? Or are requests from multiple
>      clients queued up and handled one at a time as a drive becomes
>      available?

No, they can't backup simultaneously to the same tape volume.  (If it is a
disk volume, of course they can.) If all the drives are in use, the second
request will go on a wait queue.  But if you have another tape drive
available, they can back up to different tapes at the same time.  For a
non-collocated storage pool, here is the volume selection algorithm when
a backup operation starts up and needs a tape to backup to:

1. A mounted, idle (not currently being used) volume
2. The "fullest" unused volume in the storage pool.  If scratch is being
   used, it will ask for a scratch tape if all the unused volumes are full.
3. If it gets here, all volumes that have space on them are in use.  It will
   look for the volume that currently has the fewest outstanding waiters.

Again...the above is for non-collocated pools.  Collocation has a different
volume selection algorithm.

> (2) If I only have 2 drives available, and reclamation is running, what
>      happens if a backup or restore request comes in?
>      Same question but what if both drives are doing a stgpool backup?

For a Version 1 server, the operations will go on the "mount point" wait
queue.  In Version 2, "mount point preemption" has been introduced.  In
your example above, a "restore" operation has preemption authority over
reclamation or storage pool backup.  The restore operation will preempt
(cancel) the reclamation or stgpool backup process and snatch up one of
the drives.  A backup operation does not have "preemption" authority and
would go on the wait queue.  But since reclamation or stgpool backup was
tying up two drives, the other drive would be available for the backup
operation and it would jump off the wait queue.

Dave Crockett - ADSM development