On Mon, Nov 17, 2014 at 08:04:05PM +0100, Goffredo Baroncelli wrote: > 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. IMHO if the device UUID search finds multiple devices with the same device UUID, it should ignore _all_ of them as the identification problem is unsolvable without further user input. This is what the 'device=' mount option is for. > 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. That would mean I can't do this: 1. lvm snapshot of ext4 filesystem 2. btrfs-convert the snapshot 3. mount the snapshot, make sure it's OK 4. merge LVM snapshot to overwrite original ext4 filesystem which would be a shame since that's the only way I ever convert ext3/4 filesystems to btrfs (btrfs-convert is a little buggy still). > BR > G.Baroncelli > > > > -- > gpg @keyserver.linux.it: Goffredo Baroncelli <kreijackATinwind.it> > Key fingerprint BBF5 1610 0B64 DAC6 5F7D 17B2 0EDA 9B37 8B82 E0B5 > -- > To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in > the body of a message to majordomo@xxxxxxxxxxxxxxx > More majordomo info at http://vger.kernel.org/majordomo-info.html
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