On Thu, Jun 20, 2013 at 08:22:12AM -0500, Kevin O'Kelley wrote: > Thank you for your reply. I appreciate it. Unfortunately this issue > is a deal killer for us. The ability to take very fast snapshots and > replicate them to another site is key for us. We just can't us Btrfs > with this setup. That's too bad. Good luck and thank you. If you want to make fast atomic incremental copies of btrfs to a remote system, then btrfs send/receive may be what you're looking for. Hugo. > Sent from my iPhone > > On Jun 20, 2013, at 5:56 AM, Hugo Mills <hugo@xxxxxxxxxxxxx> wrote: > > > On Thu, Jun 20, 2013 at 10:41:53AM +0000, Gabriel de Perthuis wrote: > >>>> Instead of redirecting to a different block device, Btrfs could and > >>>> should refuse to mount an already-mounted superblock when the block > >>>> device doesn't match, somewhere in or below btrfs_mount. Registering > >>>> extra, distinct superblocks for an already mounted raid is a different > >>>> matter, but that isn't done through the mount syscall anyway. > >>> > >>> The problem here is that you could quite legitimately mount > >>> /dev/sda (with UUID=AA1234) on, say, /mnt/fs-a, and /dev/sdb (with > >>> UUID=AA1234) on /mnt/fs-b -- _provided_ that /dev/sda and /dev/sdb are > >>> both part of the same filesystem. So you can't simply prevent mounting > >>> based on the device that the mount's being done with. > >> > >> Okay. The check should rely on a list of known block devices > >> for a given filesystem uuid. > > > > And this is where we fail currently -- that list is held by the > > btrfs module in the kernel, and is constructed on the basis of what > > "btrfs dev scan" finds by looking at superblocks on block devices. > > Currently, there's no method implemented for determining whether a > > block device with a legitimate btrfs superblock on it is a duplicate > > of another device, or whether it's a newly-discovered device which is > > part of an as-yet incompletely specified multi-device FS. > > > > I think it should be possible to look up the device ID as well, and > > complain (loudly, to the user, and in the kernel) at btrfs dev scan > > time if we see duplicates. That would deal with the problem at the > > earliest point of confusion. > > > > Hugo. > > -- === Hugo Mills: hugo@... carfax.org.uk | darksatanic.net | lug.org.uk === PGP key: 65E74AC0 from wwwkeys.eu.pgp.net or http://www.carfax.org.uk --- Computer Science is not about computers, any more than --- astronomy is about telescopes.
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