Wanda has identified the critical choices;  there is no single, right answer
because the performance factors intersect at different points for nearly
every situation -- if only because most sites are at different points along
the evolutionary development of IT infrastructure, different sized disks for
RAID, different RAID technologies (ESS vs. EMC vs. native/local drives,
etc.), and different performance ranges for a given TSM server box.

And it does matter how you decide to carve up your disk pool resources. Just
as different tape capacities & technology will influence your choices for
how tape pools are configured, variety in the type and capacity AND number
of channels accessing a set of disks will dictate the kind of performance
you can achieve.  So, if you think your TSM server system can handle 25 (50
in today's terms) sessions concurrently, then you measure the disk I/O
performance choices against that level of concurrency.  Most folks (these
days) disregard the effect of distributing many logical (TSM) volumes over
some number of physical volumes;  increasing the number of logical volumes
per physical gets that many concurrent writes queued to the device.  The
more of each, the more sessions can be sustained without waiting for
"mount-point".

I rather NOT use RAID-5 for TSM disk pool volumes... Why pay the penalty for
calc & writing parity when you don't expect to read it more than once?!?
This data is so transient (in general, it exists for less than 48 hours)
it's not worth sacrificing ANY performance -- we really want to get the
backups AND migration done as quickly as possible, right?  These days, I
recommend RAID-0, striping without parity, if any at all;  on most moderate
sized Unix machines, no striping at all, and as many smaller drives as can
be handled -- these days that translates to 36 or 72 GB drives, fill the
drive bays, get as much SCSI separation as possible... do the math on
dividing each physical drive into some 7-10 logical drives.  For RAID
anything, Gianluca's 7 or 8 physical drives per RAID config is a good
high-end count.


Don France
Technical Architect -- Tivoli Certified Consultant
San Jose, Ca
(408) 257-3037
mailto:don_france AT att DOT net

Professional Association of Contract Employees
(P.A.C.E. -- www.pacepros.com)