On Thu, Jun 20, 2013 at 11:56:07AM +0100, Hugo Mills 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. Incidentally, there's a conversation on IRC right now about how to deal with this issue. A solution to prevent the data loss cases may be arriving some time Real Soon Now™... 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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