Hi everybody,

I have read several discussions in the archive about the TSM mirroring
versus OS/Hardware mirroring of the database and/or log.

Both those discussions and the Administrator's Guide mention "partial
page writes". To see if I understand correctly:

- When writing a database or log page to disk, there is a point
  in time when the on-disk structure of a volume is invalid. If the
  process of writing that page is interrupted (e.g. power outage)
  at the "wrong" time, the on-disk structure remains invalid.

- The TSM server can be configured to create a mirrored database or
  log, and, when updating a page on disk, to first update the page on the
  first mirrored copy and then update the page on the second mirrored copy.
  This way, a partial page write can still occur, but by sequentially
  updating the mirrored copies there is at most one mirrored copy that
  is invalid due to the partial page write. The other copy is valid.

- When starting the TSM server, it cannot use an invalid copy of a
  a database volume. If no valid mirror is available, the TSM server
  cannot start and a database restore is necessary.

- A partial page write is a shortcoming of TSM; the on-disk structure
  should always be valid. Page writes should happen atomically. (Of course,
  the responsibility of TSM doesn't need to go further than the "sync"
  procedures of the OS. If the OS says the data is synced to disk, TSM
  can assume it *is* synced to disk. Otherwise, the OS/drivers/hardware
  should be fixed.)


My question is: in recent versions of TSM, do page writes happen atomically
or not? I would like to use the mirroring in our Symmetrix, but if TSM is
still vulnerable to the problem of partial page writes invalidating volumes
I would have to use TSM mirroring.

Thanks,
--
Jurjen Oskam

PGP Key available at http://www.stupendous.org/