linux-kernel - Re: [RFC 0/8] Aufs2 documents

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Date:	Thu, 26 Feb 2009 13:54:22 +0900
From:	hooanon05@...oo.co.jp
To:	"David P. Quigley" <dpquigl@...ho.nsa.gov>
Cc:	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC 0/8] Aufs2 documents 


"David P. Quigley":
> I think it would be useful to see the source code for AUFS2 posted to
> LKML. One of the questions I have not which doesn't seem to be addressed
> in these documents is how robust is your xattr support and are you
> making the appropriate LSM calls to make this usable with SELinux and
> Smack. Also from a labeling perspective you have a very interesting
> question of which label do you select when unifying directories. If you
> have a/foo and b/foo each with different labels which do you choose.
> Based on the history of Union type file systems I would suspect the
> answer is whichever branch is listed first. 

Aufs doesn't support xattr curretnly because I don't decide how to
support it yet.
As far as I know, the implementation of xattr and its key/name pairs are
filesystem dependent. For instance,
- there are two branches (rw and ro) in aufs and their filesystem type
  differs from each other.
- an application issues getxattr() or listxattr() and makes sure
  "key.brabra" exists (or set).
- and then it issues setxattr() for "key.brabra".
- aufs will copies-up the file and tries setxattr() for the upper one.
- I am afraid there may happen "key.brabra" is not supported by the
  upper filesystem and aufs returns an error.
- from the users' point of view, this behaviour must be very strange.

Finally I am considering to make some levels to support xattr.
- support minimum common set of key only (if such set exists)
  Here "minimum common set" means a group of key which are surely
  supported by all filesystems. Aufs will filter-out other keys.
- create a new internal status flag
  This flag is set when the type of all branches are same. When the flag
  is set, aufs will handle xattr by simply redirecting.
- create a new aufs mount option
  the option will select two behaviours (above).

Unfortunately I could not understand what label means.
Is it a volume label at mounting like UUID?


J. R. Okajima
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