Almost a year ago now, I had one of the legs of an LVM mirror fail. Ordinarily, this should not be a problem, however my particular set-up was a little odd. Specifically, I was passing both physical disks through to a kvm/libvirt virtual machine, and then applying LVM there, with both disks as standard mirrors of each other with a handful of ReiserFS partitions on top. While the disks were healthy, this was no problem at all and it worked fine. Unfortunately, when one of the disks started failing, while the host could see that it was clearly broken - it was reporting I/O errors pretty constantly - the guest was just hanging all the time but refused to acknowledge that it was actually dead.
As it was a long time ago now, I don't recall all of the details, but I think that I then removed the disks from the VM and attempted to recover things directly on the host. Unfortunately the host and the guest assigned different UUIDs to the disks, so this was an absolute disaster. After a whole load of fruitless LVM commands I was left with two disks that theoretically contained the same data as each other, one still working flawlessly, but being only one leg of a mirrored pair and LVM was refusing to let me activate any of the logical volumes on it because the other disk was missing. "Yes, of course it's missing! It's a dead disk! Now let me mount this bloody thing so I can copy the data off of it and onto a new (much larger) disk!"
As it was a long time ago now, I don't recall all of the details, but I think that I then removed the disks from the VM and attempted to recover things directly on the host. Unfortunately the host and the guest assigned different UUIDs to the disks, so this was an absolute disaster. After a whole load of fruitless LVM commands I was left with two disks that theoretically contained the same data as each other, one still working flawlessly, but being only one leg of a mirrored pair and LVM was refusing to let me activate any of the logical volumes on it because the other disk was missing. "Yes, of course it's missing! It's a dead disk! Now let me mount this bloody thing so I can copy the data off of it and onto a new (much larger) disk!"