Phillip raises a valid point, which I can reinforce.  We also have
configured our TSM DB and LOG volumes on IBM FAStT (900s), using
RAID5 LUNs.  We mirror pairs of them using TSM mirroring.  We
recently had a convoluted situation that caused our server to crash,
and it would not come up due to a corrupted log.  We were able to
bring up the server in "verify" mode and it was able to recover the
log using the mirror copies.

Details:  We have a rack which contains our TSM AIX server along with
a CISCO SAN Director.  The rack has 4 power circuits.  All equipment
had dual power supplies, however - due to a human oversight - two of
the RIO drawers had their redundant power supplies plugged into the
same circuit.  I know - a preventable error, but nonetheless, this
resulted in log corruption that would have caused us much more grief
had we not had the mirror copy.  A power supply in the SAN Director
blew, tripping the circuit that also had both redundant power
supplies in the RIO drawer, taking down our AIX server hard.

..Paul

At 02:30 PM 1/20/2006, Ford, Phillip wrote:

I believe there is still a reason to mirror at the TSM level.  If you mirror
at the hardware level only and TSM writes a corrupt entry to the DB or log,
you are hosed since the hardware mirror will write it to both copies.  If
TSM is doing the mirror, it can see that it had a problem writing to one
copy and not write the corrupt data to the second copy.  TSM can then use
the good copy to fix the corrupt mirror.  Now I do not know how or when this
case can come up but that is the reason I was told to mirror at the TSM
level.  We use EMC for the storage.  All our EMC storage is mirrored or
raided.  We then also mirror at the TSM level.  This is over kill but we do
not have any unprotected disks to use so we double do it.  Have not had a
problem but we may be wasting disks.


--
Phillip
320-4462

--
Paul Zarnowski                            Ph: 607-255-4757
Manager, Storage Systems                  Fx: 607-255-8521
719 Rhodes Hall, Ithaca, NY 14853-3801    Em: psz1 AT cornell DOT edu