Re: BTRFS messes up snapshot LV with origin

[Goffredo Baroncelli <kreijack@inwind.it>]

On 2014-11-17 07:59, Brendan Hide wrote:
> 
> That leaves two aspects of this issue which I view as two separate bugs:
> a) Btrfs cannot gracefully handle separate filesystems that have the same UUID. At all.
> b) Grub appears to pick the wrong filesystem when presented with two filesystems with the same UUID.
> 
> I feel a) is a btrfs bug.
> I feel b) is a bug that is more about "ecosystem design" than grub being silly.

Regarding a)
IIRC, btrfs collects the filesystem information by UUID; if two 
filesystems have the same UUID (like the LVM-snapshot case), the
last filesystem discovered overwrite the first one.

The filesystem discovering is done in user-space; so it should be simple
to skip a filesystem on a LVM-snapshot.

Regarding b)
I am bit confused: if I understood correctly, the root filesystem was
picked from a LVM-snapshot, so grub-probe *correctly* reported that
the root device is the snapshot.
The problem was that during the boot filesystem discovering: first
scanned the *real* device, then the LVM-snapshot; the latter
overwrote the former so the system booted from the LVM-snapshot.

My conclusion is that we should improve the btrfs scan so:
- in udev rules, a partition that is a LVM snapshot by default 
should be not scanned by "btrfs dev scan"
- "btrfs dev scan", during the partition discovery should skip the 
lvm-snapshot.

BR
G.Baroncelli



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