Re: Two identical copies of an image mounted result in changes to both images if only one is modified

[Hugo Mills <hugo@carfax.org.uk>]

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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 ===
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     --- Computer Science is not about computers,  any more than ---     
                     astronomy is about telescopes.                      

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