[linux-lvm] LVM-Snapshot + dm_crypt

[Torben Viets <Viets@web.de>]

Hello,

I've got 3 harddisks with 3* 60GB partitions in a software raid 5 
(/dev/md0), on this device I have installed a volume group (/dev/data/) 
and a logical volume mp3-crypt (the -crypt means that it is encrypt with 
dm_crypt). Now I decrypt the mp3 lv with:
cryptsetup create mp3 /dev/data/mp3-crypt
and then mount it with
mount /dev/mapper/mp3 /mnt/mp3

until now everything works fine, but now I want to create a snapshot of 
/dev/mapper/mp3.
My idea was that I make a volume group backup and then say: save the 
snapshot on the volume group, but I have no idea how I can say lvcreate 
on which volume group it has to save the snapshot.

If I make a
lvcreate -s -nsnap -L1G /dev/data/mp3-crypt
then I have a snapshot but it is encryptet and I can't mount it, even if 
I make a new cryptsetup for this device, every times mount says: "You 
must specifes a file system"

if I make a
lvcreate -s -nsnap -L1G /dev/mapper/mp3 (which ist my unencrypted device)
then I get a: "Volume group "mapper" doesn't exist"

How can I make a snapshot of a dm_crypt device?

Distribution is Debian sarge with kernel-2.6.10

Thanks for your help.

greetings
Viets