It has been my experience that you should keep the log and database volumes
on
different drives froom each other. The reason being, the log file is
actually a
backup to the database, so it makes sense to keep them on different physical
media.  As far as placing either log or database volumes on disks with other
things on them, this is strictly a performance consideration.  ADSM queries
the
database for every file that is backed up so it gets hit pretty hard.  I run
my
log and database volumes on raw partitions and let ADSM provide the
mirroring.
Hope this helps.

Richard