2-phase commit allows you to recover from HW faliure and power loss and still
have a consistent DB.
The ADSM Database is composed of 2 types of files, the DB and the LOG. These
should be placed on
separate volumes. Updates to the DB are grouped into transactions, for
simplicity a set of updates.
A 2-phase commit scheme works the following way, for the discussion assume we
modify DB pages 22, 23
1) start transaction
2) read 22 from DB and write to LOG
3) update 22' in DB and write 22' to log
4) same as 2), 3) for page 23
5) commit
6) free LOG space

The difference from other products is that ADSM can take an unplanned hit on
the DB, eg power
outage, and still be able to undo parly completed entries using the "before
Images" in the LOG.
A corollary to all this is that when you eliminate step 6), by putting the ADSM
LOG in rollforward
mode (ADSM setup option) you get a true redo log and can rebuild the DB
starting fom an
old DBbackup and reapplying the LOGs.

For managing important data you need an industrial strength product: ADSM

Regards, Giulio John Tarella , IBM Global Services, Italy