Commit or revert a Linux LVM snapshot?
I'm about to perform an experimental upgrade on my CentOS 5 server. If the upgrade fails, I want to be able to back out the changes to the filesystem. This scenario seems similar to the example in Section 3.8 of the LVM HOWTO for LVM2 read-write snapshots - but the example is rather lacking in the actual how-to.
How would I commit the changes, merging them back into the original partition?
How would I revert the changes, restoring the filesystem back to its original state? Should I assume that I'll need to restart several services, if not outright reboot?
Is it possible to snapshot only certain directories on a partition, or is it a partition-wide operation?
LVM2 / device mapper snapshots merge functionality is available if you are running Linux 2.6.33+ and using LVM 2.0.58+:
lvconvert --merge
See this post: http://www.jonnor.com/2010/02/lvm-snapshot-merging-avaliable/
It references http://kernelnewbies.org/Linux_2_6_33 (look at section 5, MD/DM) and LVM changelog at 2.0.58: ftp://sources.redhat.com/pub/lvm2/WHATS_NEW
But I can't tell you yet how to use it properly ;-)
I just tried a snapshot-based upgrade with Ubuntu. And yeah, I needed to reboot several times. First rename the original root-lv to something else, so you can give the snapshot the original name (since an upgrade creates a lot of change and changes are faster on the snapshot than on the original):
# lvrename lvm root root-old
# lvcreate -n root -s lvm/root-old -L 10G
The size should be chosen appropriately. Then reboot, so the 'new' lvm/ubuntu gets mounted as root and you can perform the upgrade. Now you can test the new version and even change to the old system by
# lvrename lvm root root-new
# lvrename lvm root-old root
# reboot
If you want to drop the upgrade, just run (from the old system)
# lvremove lvm/root-new
If you want to commit the changes, just run (from the old system)
# lvconvert --merge lvm/root-new
or, from the new system
# lvrename lvm root root-new
# lvconvert --merge lvm/root-new
# lvrename lvm root-old root
followed by a reboot. The system will refuse to do the merge right away, since the volumes are open. So the merge will be started during the boot and continued while you already can work with the system.
Oh, and by the way: When changing between the systems, remember to use the appropriate kernel. Since /boot is not part of lvm, the old and new kernels would be placed there side-by-side.