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 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@carfax.org.uk> 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 ===
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     --- Computer Science is not about computers,  any more than ---     
                     astronomy is about telescopes.                      

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