Jesús Guerrero wrote:
In my opinion, if we really want to speak about a way to implement that kind of snapshoting, we should start thinking about providing a better integration with lvm, from the root. lvm can take care of the snapshots on a non-expensive way, and it would be relatively easy to implement. However a lot of stuff would need to be re-documented, starting from the handbook, and the init system.
LVM snapshotting is extremely wasteful - it has no knowledge of the underlying structure of a filesystem. For example, if I moved all the files around on a fairly full ext3 filesystem, an LVM snapshot would consume the full size of the filesystem. However, a filesystem-level solution wouldn't need to store a single byte of data since nothing actually changed.
Also - a snapshot restoration obliterates ALL data on the partition that has changed since the snapshot was taken - so unless the essential files are on a separate partition it won't work out well.
LVM snapshots really seem to be a solution to atomic backups - if you unmount, snapshot, and remount a filesystem then you can run a self-consistent backup off of the snapshot with minimal downtime. The wasted space isn't a big deal since the snapshot would be deleted before it grew too far.
Finally - I'm not to eager to try out lvm2 again anytime soon - I lost a ton of data when something glitched and wiped out data across multiple lvm partitions. I know that the error must have been in the lvm layer (not md or the filesystem), because when I fscked and repaired one filesystem, another filesystem instantly became hosed (on a separate lvm partition). Somehow the partitions had gotten scrambled together and the fsck was crossing partition boundaries. Plus, dmesg was dumping all kinds of compliants at the md layer about the lvm device trying to access out-of-range clusters. Googling I found a few other reports of similar behavior - it seems extremely rare, but very nasty.
Fortunately the most important stuff on my PC was backed up (good planning), but it was still a pain - I lost tons of DVR recordings since I don't back that up (not worth the cost vs the value of the data). Now I just run ext3 on top of md and I haven't had any problems.
You're right that btrfs will definitely help. However, being able to create a personal stage1 tarball at will would certainly also be useful, and it wouldn't actually consume much disk space.
