Re: BTRFS messes up snapshot LV with origin

[Chris Murphy <lists@colorremedies.com>]

On Tue, Nov 25, 2014 at 8:22 PM, Duncan <1i5t5.duncan@cox.net> wrote:
> From my perspective, however, btrfs is simply incompatible with lvm
> snapshots, because the basic assumptions are incompatible.  Btrfs assumes
> UUIDs will be exactly what they say on the label, /unique/, while lvm's
> snapshot feature directly breaks that uniqueness by copying the (former)
> UUID, thus making the former UUID no longer unique and thus no longer
> truly UUID.

The seed device has a mechanism to change volume UUID without
rewriting a bunch of stuff in the original, the gotcha is that it
requires adding a device.

man fsfreeze says "fsfreeze is unncessary for device-mapper devices.
The device-mapper (and LVM) automatically freezes filesystem on the
device when a snapshot creation is requested." So if it's possible to
communicate snapshotting/freezing to the fs at snapshot time, then
maybe btrfs could 'btrfstune -S 1' the volume in the snapshot. That
way that snapshot actually contains a btrfs seed device, which is read
only. At least the snapshot copy isn't going to get obliterated in an
accident; even though most people would probably want the origin LV to
be protected while considering the snapshot disposable.



-- 
Chris Murphy