Fedora and LVM

Posted Oct 31, 2012 18:54 UTC (Wed) by paulj (subscriber, #341)
In reply to: Fedora and LVM by paulj
Parent article: Fedora and LVM

Oh, this is so useful, that I'm surprised that Fedora doesn't make use of LVM snapshots to do revertable upgrades.

If LVM snapshots could be promoted to writeable LVs, this process would be even easier. There doesn't seem be a way though. Since Solaris switched to ZFS and IPS, it does upgrades this way I think - through writeable snapshots.

Fedora and LVM

Posted Oct 31, 2012 20:11 UTC (Wed) by dtlin (subscriber, #36537) [Link] (1 responses)

Not sure what you mean by "writable LV" – LVM snapshots are already writable. If you want an wholly independent LV – perhaps you could just copy the snapshot to a new LV.

lvconvert --merge merges a snapshot back into the origin, for when you only want the snapshot's contents and not the origin's anymore.

(Hypothetically, at least. I've not yet tried it myself.)

Fedora and LVM

Posted Oct 31, 2012 22:28 UTC (Wed) by paulj (subscriber, #341) [Link]

They're writable now? That's changed then, and good news for me. Thanks! I'd heard DM might have merge back support somewhere, but hadn't noticed the LVM UI for it was in lvconvert - I've been looking at lvchange the whole time.

Thanks :)

Fedora and LVM

Posted Nov 1, 2012 12:10 UTC (Thu) by Cato (guest, #7643) [Link] (1 responses)

LVM snapshots are as buggy as a very buggy thing ... search "lvm bugs snapshots" for more and see the comment from the schroot developer above. They are also very slow because they multiply number of I/Os required for a changed block.

Seriously, do a lot of testing before you rely on LVM snapshots. Or just use btrfs.

Fedora and LVM

Posted Nov 1, 2012 23:02 UTC (Thu) by agk (guest, #23332) [Link]

> LVM snapshots are as buggy as a very buggy thing

They are pretty robust nowadays: the last kernel race I can recall that could lead to incorrect data was found a few years ago now.

But they are not designed to be efficient if they grow large or if you have many of them; and make sure they never run out of space (use the snapshot_autoextend* lvm.conf settings / --monitor y / dmeventd).