I have upgraded this server to kernel 2.6.16.2. The next backup cycle resulted in a minor disaster: The backup process deadlocked at removing the first LVM2 snapshot and the snapshot source volumes were blocking write accesses. A cleanup shutdown was impossible and I had to hard-reset the machine.

After some searching I found out that you apparently need lvm2 2.02.01 or later and devmapper 1.02.02 or later to successfully remove snapshot volumes now. Unfortunately neither of these versions is available for sarge from Debian or backports.org yet, so I had to make my own backports.
As it turned out (see below), it is also necessary to use 2.6.16.12 or to apply the patch from this email to older 2.6.16 versions in order to reliably remove snapshots.

If you are brave enough, you can get the backported packages by adding

deb http://blog.blackdown.de/static/debian/lvm/ sarge main
deb-src http://blog.blackdown.de/static/debian/lvm/ sarge main

to /etc/apt/sources.list.

The repository contains debs for devmapper, dlm, lvm2, and lvm-common. The Release files is signed with my GPG key. If you have a recent apt version, you can authenticate the packages after importing the key with apt-key:

wget http://blog.blackdown.de/static/gpg.asc -O - | \
    sudo apt-key add -

April 15th, 2006: In about 40 backup cycles I’ve seen three lockups with 2.6.16.2 now. Until snapshots get fixed in 2.6.16, I’d recommend to stay with 2.6.15. I’m using 2.6.15.3 again now.

April 24th, 2006: Added note about “dm snapshot: fix kcopyd destructor” patch from Alasdair G Kergon. With this patch snapshots work fine for me again.

May 2nd, 2006: Alasdair G Kergon’s patch has been included in 2.6.16.12.