With the ADSM V2.1 servers, the requirements for mirroring have
slightly changed.  Also, with Raid devices such as the RAMAC, the need
for mirroring is also modified.

The ADSM software mirroring function is mainly for high data
availability.  If you lose a controller or disk drive, the mirror can
take over.  Thus you need to decide what your availability
requirements are, and what failure scenarios you are will to take
risks on.  RAID devices protect against a disk failure, but if the
subsystem fails you are exposed.  Mirrors can cover this exposure.

With V2.1, you can set triggers to do full and incremental backups of
the ADSM DB, with full forward recovery IF the log is still available.
Thus, if the DB is on raid, the need to mirror is decreased based on
your availability needs.

We recommend that you mirror the log, even on raid devices.  With our
DB backup, you can safely backup/recover the DB if your device fails.
You may suffer some availability, but that is your call.  If you lose
the log, you will need to use our volume history file to audit the
appropriate media volumes to return to a consistent state.  Running
audits is a job you would like to avoid if at all possible.

RAID lowers the need for mirroring, but does not eliminate it.  Your
decision should be based on your availability requirements and the
risk levels you find acceptable.  The ADSM log is critical to the
operation of the server, and should be protected accordingly.

Paul Bradshaw