If you find your client task suddenly dysfunctional, it may have been
preempted.  I encountered this when I was running an incremental backup on an
AIX system (Server: 2.1.0.7 Client: 2.1.0.4) to a 3494 with two 3590 drives.
The client just stopped dead.  After a while I did a Ctrl-C, which failed to
get back to the Unix prompt.  I had to do a 'kill -9' to terminate the client
session.  Looking in the server Activity Log I found the following:

09/13/96   09:22:18      ANR0492I All drives in use. Session 909 for node 
<CLIENT>
                          (AIX) being preempted by higher priority operation.
09/13/96   09:22:18      ANR0490I Canceling session 909 for node <CLIENT> (AIX) 
.
09/13/96   09:22:45      ANR0487W Session 909 for node <CLIENT> (AIX) 
terminated -
                          preempted by another operation.

HSM needed a tape drive to recall a file and rudely caused the backup
operation to get discarded.  And it seems there is a defect wherein either the
server fails to communicate this to the client so that it can end with
indication, or the client fails to handle the event properly.
So watch out for this.

I think we would all like to see the software better written such that it
would SUSPEND an operation for the duration of the preemption and then RESUME
it, rather than blithely DISCARD it!  Losing an overnight backup of important
data because of something like this is not exactly a good way to do things.

     Richard Sims, Boston University OIT