From: Gabriel de Perthuis <g2p.code@gmail.com>
To: linux-btrfs@vger.kernel.org
Subject: Re: Two identical copies of an image mounted result in changes to both images if only one is modified
Date: Thu, 20 Jun 2013 10:22:07 +0000 (UTC) [thread overview]
Message-ID: <kpul4f$ei7$1@ger.gmane.org> (raw)
In-Reply-To: 20130620091622.GL11290@carfax.org.uk
On Thu, 20 Jun 2013 10:16:22 +0100, Hugo Mills wrote:
> On Thu, Jun 20, 2013 at 10:47:53AM +0200, Clemens Eisserer wrote:
>> Hi,
>>
>> I've observed a rather strange behaviour while trying to mount two
>> identical copies of the same image to different mount points.
>> Each modification to one image is also performed in the second one.
>> touch m2/hello
>> ls -la m1 //will now also include a file calles "hello"
>>
>> Is this behaviour intentional and known or should I create a bug-report?
>
> It's known, and not desired behaviour. The problem is that you've
> ended up with two filesystems with the same UUID, and the FS code gets
> rather confused about that. The same problem exists with LVM snapshots
> (or other block-device-layer copies).
>
> The solution is a combination of a tool to scan an image and change
> the UUID (offline), and of some code in the kernel that detects when
> it's being told about a duplicate image (rather than an additional
> device in the same FS). Neither of these has been written yet, I'm
> afraid.
To clarify, the loop devices are properly distinct, but the first
device ends up mounted twice.
I've had a look at the vfs code, and it doesn't seem to be uuid-aware,
which makes sense because the uuid is a property of the superblock and
the fs structure doesn't expose it. It's a Btrfs problem.
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.
>> I've deleted quite a bunch of files on my production system because of this...
>
> Oops. I'm sorry to hear that. :(
>
> Hugo.
next prev parent reply other threads:[~2013-06-20 10:22 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-06-20 8:47 Two identical copies of an image mounted result in changes to both images if only one is modified Clemens Eisserer
2013-06-20 9:11 ` Fajar A. Nugraha
2013-06-20 9:16 ` Hugo Mills
2013-06-20 10:22 ` Gabriel de Perthuis [this message]
2013-06-20 10:34 ` Hugo Mills
2013-06-20 10:41 ` Gabriel de Perthuis
2013-06-20 10:56 ` Hugo Mills
2013-06-20 13:22 ` Kevin O'Kelley
2013-06-20 13:28 ` Hugo Mills
2013-06-20 13:29 ` Gabriel
2013-06-20 13:32 ` Hugo Mills
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