If you have sufficient disk capacity, the integrity gains are worth
the relatively small loss of performance (or so it seems to us). We
have two large backup servers - the one on the newest box with most
disk capacity is mirrored; the other (poor one) is not.  An example
of why it is a good idea follows :

As luck would have it, we took a hit a few weeks ago on the poor one.
The DB was on SSA raid-5 with hot spare protection.  A disk failed.
The hot spare cut in ok and all was well - except the disk failure was
so unusual that the SSA adapter code did not cope at all well and was
so busy trying to bring the dead disk back into life it was throwing
away data the TSM server sent it.  TSM was returned I/O errors, threw
up it's hands in horror and crashed.

The server wouldn't restart; it crashed due to a partial write failure.

The roll-forward recovery failed at the last moment when it reached
the same partial write point and crashed (I wish for a true point in
time recovery ...).  The only answer was to lose 14 hours work and
discard the contents of the recovery log, reverting to the last tape
backup.  If we had had a mirrored DB we may have been ok.

[And it took quite a while to get to the bottom of what had actually
happened - but that is another story.]

My vote for mirroring and when we (hopefully) upgrade the server this
summer we will plan to mirror the DB.

Best wishes, Sheelagh
--
Sheelagh Treweek
Sheelagh Treweek
Oxford University Computing Services
Email: sheelagh.treweek AT oucs.ox.ac DOT uk
Phone: +44 (0)1865 273205 Fax:-273275