[dm-devel] How to force a snapshot status to valid when I know very well it's valid?

[Greg Stark]

> See
> https://fedoraproject.org/wiki/LiveOS_image#Overlay_recovery

THANK YOU. That was enormously helpful and saved my bacon big time.
The lvm snapshot is active and the filesystem is back.

For future reference what I actually did was find the cow dm device
for the invalid lvm snapshot from dmsetup which was the device with
the same name as the snapshot plus "-cow":

# dmsetup status
vg_root-lv_u01_dev: 0 4724490240 snapshot Invalid
vg_root-lv_u01_dev-cow: 0 1048576000 linear

Then I ran the command from that doc to set the valid bit:
# echo $'\x01' | dd of=/dev/mapper/vg_root-lv_u01_dev-cow bs=1 count=1
seek=4 conv=notrunc
1+0 records in
1+0 records out
1 byte (1 B) copied, 2.9e-05 seconds, 34.5 kB/s

Then I didn't know how to reload the dm state so I rebooted the
machine. This caused a bit of a scare because loading the lvm volumes
took about 30 minutes. I don't know why or what happened there. But it
eventually did come up. Then I ran fsck a a few times, the third time
it found no problems (the first time actually seemed to corrupt the
filesystem!).

So actually a few things didn't work as expected. Firstly the lvm
snapshot expiring actually made one of the pv's in the vg invalid:
  --- Physical volume ---
  PV Name               unknown device
  VG Name               vg_root

This caused a lot of confusion since it looked like a hardware
problem. There were no hardware errors and after rebooting that device
seems fine so I think this was caused by dm.

Then rebooting after the bit was twiddled rebooting froze for a very
long time on this step in the redhat boot process:
Setting up Logical Volume Management:
It was about 30 minutes before this appeared:
Setting up Logical Volume Management:   4 logical volume(s) in volume
group "vg_root"

I don't know what could possibly have caused that. It wasn't even
mounting anything at this stage.  I actually went for a drink at this
point sure that I had corrupted the snapshot data structures in some
weird way. One catch is I had done lvextend already -- does the
snapshot dm need to initialize all the blocks of the newly extended
cow table?

-- 
greg