[dm-devel] HOW to install software to mapped device persistently

[V.Kukushkin]

Hi

All we know the good way to make read-only ROOT device be read-write  
looking.
It is the use of dmsetup in initrd. It works perfect. But ..

Question1: HOW to install software to this mapped device persistently ?

For example, if you need to upgrade something and ROOT device is read-write
in fact (e.g. flash) but used as read-only for safety.

I tried to do this by sequence of commands:
(/dev/hdc1 "snapshotted" to / by devmapper)
# mount /dev/hdc1 /mnt/disk
# chroot /mnt/disk
# rpm -Uv <pkg> or whateveer else
# exit
# umount /mnt/disk
Of course, my changes were not seen from "/" because mapper know nothing
about that changes. So, I need to reboot the system to make changes  
visible.
Another and more clear way is to reboot to standard
configuration (without device mapper)and install/update software,
then reboot again to devmapper configuration. Both these ways are  
inacceptible
in production system.

Question2: How to make devmapper to know that base (snapshot'ed) device  
have
been changed ?

1( My thoughs was to remap root from snapshot to disk, upgrade software  
and remap
root back to snapshot. It doesn't work.
2( We may delete snapshot and snapshot-orig and disk mappings, then  
upgrade soft, then restore
snapshotting back. It doen't work also.

The case, I think, in initrd nature of mapping. I.e. mapping devices are
in /inird/dev/mapper instead of /dev/mapper.
My device-mapper configuration as following (by initrd)
--
root-orig: 0 430048 snapshot-origin 254:000
ram: 0 430048 linear 007:000 0
root: 0 430048 snapshot 254:000 254:001 P 8
hd: 0 430048 linear 022:001 0
--
"hd" - base device (compact flash in my case, /dev/hdc1)
"ram" - temporary loop device on file in ramfs.
"root" - snapshot of "hd" to "ram"
"root-orig" - snapshot-orig to "hd"
Rootfs mounted on /dev/mapper/root.

Question3stupid: Where rootfs must be mounted - on /dev/mapper/root or  
/dev/mapper/root-orig ?
-- 
regards,
Vladimir