Stuart D. Gathman wrote:

ZZZZOn Thu, 1 May 2008, Charles Marcus wrote:

In Gerry's scenario here, if the snapshot volume had NOT been on a ram 
disk, would he have had the problem he had or not?


Here is the snip from Fedora/RHEL for activating the VG containing
the root filesystem (note: kernel modules and lvm command are contained in
initrd filesystem, and do not depend on /lib/modules/...):

...
echo "Loading dm-mod.ko module"
insmod /lib/dm-mod.ko
echo "Loading dm-mirror.ko module"
insmod /lib/dm-mirror.ko
echo "Loading dm-zero.ko module"
insmod /lib/dm-zero.ko
echo "Loading dm-snapshot.ko module"
insmod /lib/dm-snapshot.ko
echo Making device-mapper control node
mkdmnod
mkblkdevs
echo Scanning logical volumes
lvm vgscan --ignorelockingfailure

So right ^^^here^^^ is where we need to test if old snapshot was read-only or read-write (how to do that? grep vgdisplay -v for some value?)
So maybe something like this?:

set removemissing flag false
if old snapshot is the only PV missing:
--if old snapshot was read-write:
------stop boot sequence
------show user some info about old snapshot
------prompt whether to remove the old snapshot from the VG so it can be activated
------if response yes:
----------set removemissing flag true
------else
----------alert user that manual action is required to correct problem related to old snapshot before VG can be activated
----------abort boot
--elsif old snapshot was read-only:
------set removemissing flag true
if removemissing flag true:
----vgreduce --removemissing VG PV(old snapshot)

Gerry

echo Activating logical volumes
lvm vgchange -ay --ignorelockingfailure  rootvg
...

This will fail if any of the physical volumes are missing.  More logic
in initrd is required to boot with missing PVs.

So yes, a ram disk is guaranteed to be missing on reboot, and thus to 
fail in the RH/Fedora distros.  There is no problem taking snapshots
of the root fs otherwise.  If there is a robust way to reduce 
missing PVs automagically in the above script from initrd, Fedora could
use the contribution.