My fix for this would be to remove the d42 mirror, change the vfstab to mount d40, remount /apps using the d40 metadevice, add the d42 mirror. Commands would look something like this:
metadetach d40 d42
metattach d40 d42
Since you have been writing to the c1t0d0s4 device instead of the metadevice, I'm not sure the two mirrors are in sync and mounting d40 without resyncing the mirror could result in loss of data.
So tell me, is slice 4 being mirrored or not? Is the "fix" as easy as just changing vfstab to /dev/md/dsk/d40?
I suspect that action might cause you to fatally corrupt your filesystem.
It is probable c1t0d0s4 is good, c1t1d0s4 is degraded.
I would probably want to take a backup of what you have currently.
I would probably want to be doing a metadetach to break off he bad mirror, then remount thange the vfstab to reference and mount d40, and metattach to resync the 'yffy' mirror d42 from the bad.
Maybe an fsck in there might be good..
IMHO Dont trust what I say ... make a plan and be very very safe and careful.
Edited by: bigdelboy on 22-Jan-2013 08:18 ... okay I see you have had what looks like a good posting while I have been composing this.