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No LVM on loopbackNo LVM on loopbackPosted Apr 23, 2008 19:55 UTC (Wed) by nix (subscriber, #2304)In reply to: No LVM on loopback by daniel Parent article: Benchmarking Linux filesystems on software RAID 1 (Lone Wolves)
Useful snapshots: i.e. >32 of them, snapshots where snapshotting the root filesystem works... ideally a frontend command like chroot but which snapshots everything below some tree, kicks up a shell in it, and zaps all the snapshots when the shell exits. (Why? So I can back my whole system up via snapshots, reliably. I might well have >32 filesystems: I obviously want to back up / without running a deadlock risk: and the chroot-in-snapshot command is just obviously useful.) So far LVM snapshots have enough limitations that I haven't found a single use for them in practice... it would be nice to have snapshots that work. (Of course we might all go to btrfs and get them for free!)
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No LVM on loopback Posted Apr 23, 2008 21:01 UTC (Wed) by zlynx (subscriber, #2285) [Link] I use LVM on a Fedora Core system at work. I made a little script to snap each of the filesystems, including the root fs, and then mount each snap in a snapshot directory. That script runs just before the nightly backup job which reads off the nightly snapshot. The script also keeps a weekly snapshot. The two snapshots are very convenient for grabbing quick copies of old files. I also took advantage of the ability to assign new LVs (which a snapshot is) to particular PVs so as to distribute the write load across several disks. Because, when a snapshot source is written to, copies of the old blocks have to be written into the snapshots before the old block can be overwritten on the source. That sped things up a lot. In a couple years of doing this I've never had that system lock up during a backup job, so the risk of deadlock must be very low.
No LVM on loopback Posted Apr 24, 2008 20:32 UTC (Thu) by nix (subscriber, #2304) [Link] It is, but still... the problem is apparently that the lvm tools don't lock themselves in memory, and there's a window where I/O to the snapshot origin is disabled. If the OS needs to page in the LVM tools at that point, deadlock. (And that's only the known deadlock.)
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