On Tue, 17 Oct 1995, asdf wrote:

> We are looking into mirroring our ADSM database. Can anyone tell me
> what this will gain us? How does it help you recover from a database
> error? Does the database automatically get synchronized with the
> mirror after you correct the problems with the database volume?
> Does it notify you that there was a problem that needs to addressed?
>
> What does mirroring gain us for the cost of the DASD? Are there any
> additional down sides to mirroring besides requiring twice the disk
> space?
>
This is some info I got from ADSM development on this, if I am
remembering it all correctly.  IBM- please feel free to expand or
correct me...
Mirroring does give you some recovery capabilities, and yes, if a
mirrored volume has an error, you get a message from ADSM--something
like saying the two are unsynched and the bad one is taken offline.
When you bring it back online... adsm automatically resynchs the two.

Without mirroring a severe database error could only be recovered back
to the last DB backup.

With mirroring, depending on the options you picked in the server options
file you may get performance benefits or degradation.  With a DB read,
and mirrors, ADSM will read from either volume, so you can get a performance
improvement.  With DB writes, you can set up the options to write to
the two volumes synchronously or asynchronously.  Obviously, synchronously
will cost a little in performance, but is the safest for "in flight"
updates.  Asynchronously is not as costly performance wise... but
you have the potential to lose an inflight update or have the two volumes
out of synch if you crashed, which would have to be addressed (not sure
how much info adsm would give you on the nature of this and how to fix)