linux-kernel - Re: Union mounts comparison with overlay file system prototype?

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Message-ID: <4D8C86B2.2060902@redhat.com>
Date:	Fri, 25 Mar 2011 08:12:34 -0400
From:	Ric Wheeler <rwheeler@...hat.com>
To:	Szeredi Miklos <miklos@...redi.hu>
CC:	Valerie Aurora <valerie.aurora@...il.com>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	viro@...iv.linux.org.uk, Christoph Hellwig <hch@...radead.org>
Subject: Re: Union mounts comparison with overlay file system prototype?

On 03/25/2011 07:38 AM, Szeredi Miklos wrote:
> On Thu, Mar 24, 2011 at 2:43 PM, Ric Wheeler<rwheeler@...hat.com>  wrote:
>> Can one or both of you summarize what we union mounts and overlay do better
>> or worse? Do we need both or just one?
> The semantics are very similar, the differences are in the implementation.
>
> Union mounts:
>
>   - whiteout/opaque/fallthrough support in filesystems
>   - whiteout operation is atomic
>   - no dentry and inode duplication
>   - copy up on lookup and readdir
>   - does not support union of two read-only trees
>   - merged directory stored in upper tree
>
> Overlayfs
>
> - whiteout/opaque as xattrs
> - whiteout operation is not atomic
> - dentry and inode duplication(*)
> - only copy up on modification
> - supports union of two read-only trees
> - merged directory not cached(**)
>
> (*) it's possible to eliminate inode duplication of non-directories
> with some VFS modifications
> (**) caching should be possible to do

Thanks for the high level overview!

Ric

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