Hi Zoltan,

We too have seen poor performance on random access DISK volumes where
multiple volumes are on the same underlying RAID set. A few things I've
noticed:

* TSM is ignorant of the underlying filesystem structure. This means it
  will write new data and migrate/backup old data from the same filesystems
  at the same time, ignoring the fact that there's eligible data to
  migrate/backup on idle filesystems. You can observe this behavior on
  Linux using iostat.

* Performance gets extraordinarily bad when the DISK pool fills up. You can
  easily get into a death spiral, where you can't migrate fast enough to
  keep from shoe-shining your tape drives to accomodate new data. The
  problem is when the pool itself fills up, not the underlying filesystem,
  so leaving headroom in the filesystem doesn't help you at all. My guess
  is TSM is having to try to span multiple DISK volumes when the pool fills
  up, which is pretty intensive.

We're going to be replacing our disk spool in the next couple weeks. Rather
than having small RAID-5 sets, we'll probably just have jumbo RAID-60 sets
and just have a couple DISK volumes per pool.

--
-- Skylar Thompson (skylar2 AT u.washington DOT edu)
-- Genome Sciences Department, System Administrator
-- Foege Building S046, (206)-685-7354
-- University of Washington School of Medicine