On Wed, 2016-05-11 at 19:25 -0400, Theodore Ts'o wrote: > On Mon, May 09, 2016 at 10:21:21AM -0700, Nikolaus Rath wrote: > > > > > > > > Another way is to use btrfs (or zfs or perhaps LVM snapshots): whenever > > > something goes south in a way that's not trivial to recover, you can > > > restore with a couple commands and reboot. And if unbootable because, > > > for example, someone removed support for your CPU, you boot with > > > subvol=backups/sys-2016-05-07. > > I'd advise against using LVM snapshots. The time for initial activation > > seems to go up exponentially with the amount of data in snapshot > > volumes. I think they are only intended for short-term use > > (e.g. to take a backup). > If what you want to do is a rollback operation after a package > installation goes badly, LVM snapshots are sufficient. They aren't as > convenient as btrfs, but they do work. So what you'd do is do is (a) > create the snapshot, (b) inststall the package. If the package looks > good, then delete the snapshot. > > If you discover that the package hoses your system then to rollback, > shutdown the system to single-user mode, and remount the file system > to be read-only, and then use the command lvconvert --merge to restore > your file system back to the state of the snapshot. This will consume > the snapshot, and leave the file system (presumably ext3 or ext4) in a > potentially confused state, which is why you need to do this with the > file system remounted read-only. Then reboot, and you're all set. What could possibly go wrong? > I there is a yum plugin for Fedora where you reboot, and the lvconvert > --merge is done as part of the reboot (either as the system is > shutting down, or in the initramfs before the file system is mounted). > That's a much more convenient and user-friendly way to do the > rollback; creating such a covenience setup is left as an exercise to > the reader. :-) That makes a lot more sense to me than doing weird things behind the filesystems's back. Implementing a shutdownfs in initramfs-tools is certainly something I'd like to do, though I don't have any solid plans. Ben. -- Ben Hutchings If you seem to know what you are doing, you'll be given more to do.
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