[CentOS] GFS/LVM/RAID1 recovery question

I have a (CentOS4.5) cluster in which the servers mount a GFS partition
which is an LVM2 logical volume created as a mirror of two iSCSI-
connected drives (with a third for the log).  The LV was created using a
command along the lines of:

  lvcreate -m 1 ... /dev/sdb /dev/sdc /dev/sdd

where sd[bc] are the mirrored (iSCSI) PVs in the VG and sdd is the log.
I have this working and can write data to the filesystem on one machine
in the cluster and see it appear elsewhere etc.

What I now want to do is to test what happens when I disable one
of the mirrors and then restore it with clean disks.

However, my google-foo is weak today and I can't find any information on
how to get LVM to tag the broken half of the mirror as failed and then
how to get it to resynchronise the mirrors once the disks are replaced.

Can anyone explain how to do this?  Is it even possible?

James