linux-kernel - Re: Unionmount status?

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Message-ID: <BANLkTinmWtL0U2L7+DFeHW0R9Kpy-JevcA@mail.gmail.com>
Date:	Fri, 15 Apr 2011 14:48:05 +0200
From:	Miklos Szeredi <miklos@...redi.hu>
To:	Michal Suchanek <hramrach@...trum.cz>
Cc:	Jiri Kosina <jkosina@...e.cz>, Ric Wheeler <ricwheeler@...il.com>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	David Howells <dhowells@...hat.com>,
	Ian Kent <ikent@...hat.com>, Jeff Moyer <jmoyer@...hat.com>,
	Christoph Hellwig <hch@...radead.org>
Subject: Re: Unionmount status?

On Fri, Apr 15, 2011 at 2:34 PM, Michal Suchanek <hramrach@...trum.cz> wrote:
> On 15 April 2011 14:29, Miklos Szeredi <miklos@...redi.hu> wrote:
>> 2) take whiteout/opaque support from union mounts and use that
>
> How far from importing full unionmount is that?

Whiteout/opaque support is quite separate from the rest of union
mounts, and could be reusable for overlayfs.

There are several reasons why I didn't want to go that way with:

- lots of filesystems would have to be updated
- it introduces incompatibility in the filesystem format, which can be
a real pain (not for tmpfs, obviously, since tmpfs doesn't have a
persistent backing)

There *are* advantages to doing whiteouts in the filesystem, for
example it makes file removal atomic.  But atomicity is something that
needs to be addressed in lots of other places (e.g. copy up) not just
during whiteout, and there are other ways to do that than push the
responsibility into each and every filesystem.

Thanks,
Miklos
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