linux-kernel - Re: [PATCH] Allow cross subvolume reflinks (2nd attempt)

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Date:	Wed, 20 Jun 2012 14:07:24 -0400
From:	Calvin Walton <calvin.walton@...stin.ca>
To:	Alexander Block <ablock84@...glemail.com>
Cc:	kreijack@...ind.it, linux-btrfs@...r.kernel.org, dave@...os.cz,
	hch@...radead.org, linux-fsdevel@...r.kernel.org,
	chris.mason@...ionio.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Allow cross subvolume reflinks (2nd attempt)

On Wed, 2012-06-20 at 19:27 +0200, Alexander Block wrote:
> On Wed, Jun 20, 2012 at 7:18 PM, Goffredo Baroncelli <kreijack@...ero.it> wrote:
> > Hi Alexander,
> >
> > On 06/20/2012 12:35 PM, Alexander Block wrote:
> >> The patch also does proper vfs mount checks, so cross mount
> >> point reflinks are not possible with this patch. It only allows cross
> >> reflinks between two subvolumes which are in the same mount point.
> >
> > Thanks for working on that. What happens if two subvolumes of the same
> > filesystem are mounted on two different places ?
> >
> > I usually do:
> >  mount -o subvol=__active /dev/sdX /
> >  mount -o subvol=.        /dev/sdX /var/btrfs
> >
> > So two different subvolumes of the same filesystem (the one on /dev/sdX)
> > are mounted on two different places.
> >
> > Is it possible to do
> >  cp --reflink /tmp/foo /var/btrfs/tmp/foo2
> This will fail with -EXDEV. The patch explicitly forbids reflinks crossing vfs
> mount boundaries.

Which means that if I want to do a reflink copy between two separately
mounted subvolumes, I will have to go and do a third mount with a common
subvolume above those two, then do the reflink operation within this
extra mount? This seems slightly impractical.

Is it any extra code to enable doing cross-mount reflinks? Why was the
decision made not to allow them?

Is there some particular opposition to supporting cross-mount operations
on multiple mounts of the same filesystem in general? (I'd love to have
rename() work across bind mounts, for example...)

-- 
Calvin Walton <calvin.walton@...stin.ca>

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