linux-kernel - Re: [ANNOUNCE] Btrfs: a copy on write, snapshotting FS

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Date:	Wed, 13 Jun 2007 06:17:39 -0400
From:	Chris Mason <chris.mason@...cle.com>
To:	Christoph Hellwig <hch@...radead.org>,
	Mike Snitzer <snitzer@...il.com>, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org
Subject: Re: [ANNOUNCE] Btrfs: a copy on write, snapshotting FS

On Wed, Jun 13, 2007 at 04:08:30AM +0100, Christoph Hellwig wrote:
> On Tue, Jun 12, 2007 at 04:14:39PM -0400, Chris Mason wrote:
> > > Aside from folding snapshot history into the origin's namespace... It
> > > could be possible to have a mount.btrfs that allows subvolumes and/or
> > > snapshot volumes to be mounted as unique roots?  I'd imagine a bind
> > > mount _could_ provide this too?  Anyway, I'm just interested in
> > > understanding the vision for managing the potentially complex nature
> > > of a Btrfs namespace.
> > 
> > One option is to put the real btrfs root into some directory in
> > (/sys/fs/btrfs/$device?) and then use tools in userland to mount -o bind
> > outside of that.  I wanted to wait to get fancy until I had a better
> > idea of how people would use the feature.
> 
> We already support mounting into subdirectories of a filesystem for
> nfs connection sharing.  The patch below makes use of this to allow
> mounting any subdirectory of a btrfs filesystem by specifying it in
> the form of /dev/somedevice:directory and when no subdirectory
> is specified uses 'default'.

Neat, thanks Christoph, this will be much nicer longer term.  I'll
integrate it after I finish off -enospc.

> To make this more useful btrfs directories
> should grow some way to be marked head of a subvolume,

They are already different in the btree, but maybe I'm not 100% sure
what you mean by marked as the head of a subvolume?

> and we'd need
> a more useful way to actually create subvolumes and snapshots without
> fugly ioctls.

One way I can think of that doesn't involve an ioctl is to have a
special subdir at the root of the subvolume:

cd
/mnt/default/.snaps
mkdir new_snapshot
rmdir old_snapshot

cd /mnt
mkdir new_subvol
rmdir old_subvol

-chris

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