How do you know your restore times are too long without collocation?  We
recommend collocation when one knows the restore times are too long.
 Usually, this can be determined well into the implementation, after data
has the chance to "scatter" to many tapes, by trying a large restore and
timing it.

Questions to ask:

1. What are the typical restores for the environment?  Most sites
experience 99% of restores as single files.  The other one percent can be
attributed to larger, more complex restores.  Sometimes even an entire
disk.

2. What is the restore time requirement for these large restores?
 Typically, for a single disk two to four hours is the norm.  Some
applications and environments will have shorter times, but what can most of
us do now with existing backup tools?

3. Are all environments and disks created equal?  Probably not. Some stuff
is much more critical than others.

Let's take the case where a disk's data has been scattered to 30 tapes.
 Let's restore the disk.  In your environment, I'll assume you have a 3494,
that'll be 30 tape mounts.  Probably two minutes per mount (perhaps
shorter), so 60 minutes just to mount tapes.  How long will it take to get
4, 9, 18 GB of 3590 tapes?  Another hour, perhaps a little longer.  This
will probably meet a 2 hour and almost certainly a 4 hour restore time.
 Collocation was not necessary.

How about collocating, but setting the value of maxscratch on the pool to
something lower?  This will result in collocation, but a sharing of tapes.
 In your case, you've got a bunch of 4 GB clients and 10 GB (nominal)
tapes.  Set maxscratch to the number of client you have divided by three
since you will get compression.  Keep an eye on the pool to make sure you
don't run out of tapes.  This technique will force data onto the fewest
tapes possible, but still maintain a single client's data on a small number
of tapes.

Collocation should be used when needed.  The cost, in tape and tape mounts
during backups, is only partially outweighed by the necessity of having it
for fast restores.  Larger tapes exacerbate the problem even further.

Bottom line: don't use collocation unless you have business justifiable
reason to use it.

Kelly J. Lipp
Storage Solutions Specialists, Inc.
www.storsol.com
lipp AT storsol DOT com
(719)531-5926