Re: btrfs and LVM snapshots (Re: kernel BUG at fs/btrfs/extent-tree.c:1772)

[Goffredo Baroncelli <kreijack@gmail.com>]

On 02/10/2013 01:32 AM, Piotr Pawłow wrote:
> Hello,
>> Yeah you can't mount images, we clear out the chunk tree so
>> nothing works. Let me know if you run into any problems in the
>> future.  Thanks,
> 
> That's surprising, I haven't seen it mentioned anywhere.
> 
> With any other filesystem I could use an LVM snapshot to save the 
> original state, but with a multi-device btrfs it would be very
> risky. Origin and snapshot volumes could easily get mixed up by
> kernel and tools.
> 
> I tried that in qemu, and I've seen btrfs happily mount 1 origin and
> 1 snapshot device as a single FS. And then I've seen it mount both
> origin devices, even though one of them had old content. Naturally
> it complained a lot about bad checksums.

I can confirm that, even with a single-device btrfs filesystem. However
I am curious why you want to use the lvm snapshot capability instead of
the btrfs one.
> 
> Is there any way to avoid such mix-ups? To somehow mark devices so
> that btrfs would know these devices belong together?

Btrfs assume that every device has an "unique" uuid. However when a
device is snapshotted it uuid is copied too, so it is not unique any
more. This is the reason of the btrfs confusing.


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