RE: RAID, journaling, and ASPs, oh my!

Part 1: If you add the 8th drive & migrate to 2 4-drive RAID5 sets, you
haven't really gained anything beyond a little extra redundancy. No
capacity gain.

Part 2: If the usage pattern at your shop is similar to most, the amount
of data to be written to the journals is relatively minimal. You can
also minimize journal content:
http://www.redbooks.ibm.com/abstracts/tips0626.html

A 2780 does support >1 RAID set. If you go the multi-ASP route be aware
that you will have to update your restore procedures to specify the
right ASP when restoring. The default IIRC is the same ASP it was saved
from so if you restore to a single-ASP system the restore of the ASP2
libraries will fail unless you tell it to restore them to ASP1.

From a practicality standpoint there's no reason you couldn't move

seldom-used or static (program) libraries to the journal ASP. If the
ASP is large enough you could experiment with virtual tape & save your
data to virtual tape files in ASP2.

Part 3: Not sure, I suppose it depends on where your load source has to
reside.



All that aside, we used to have a separate ASP for journals but we moved
away from it when we went to 8xx series hardware. By then the RAID
cards and disks were good/fast enough that journal overhead wasn't
noticed. From a performance & admin standpoint I'd stick with a single
ASP.

From a reliability standpoint, you could make the argument for the

second ASP but in my experience it hasn't been necessary. And if the
system crashes due to catastrophic disk failure in ASP1, restoring from
ASP2 will be problematic anyway.