Date: Thu, 14 Feb 2013 17:01:10 -0500
From: "J. Bruce Fields" <bfields@fieldses.org>
To: Dave Chinner <david@fromorbit.com>
Cc: "Theodore Ts'o" <tytso@mit.edu>, Anand Avati <anand.avati@gmail.com>,
        Bernd Schubert <bernd.schubert@itwm.fraunhofer.de>, sandeen@redhat.com,
        linux-nfs@vger.kernel.org, linux-ext4@vger.kernel.org,
        gluster-devel@nongnu.org
Subject: Re: [Gluster-devel] regressions due to 64-bit ext4 directory cookies
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On Thu, Feb 14, 2013 at 05:10:02PM +1100, Dave Chinner wrote:
> On Wed, Feb 13, 2013 at 05:20:52PM -0500, Theodore Ts'o wrote:
> > Telldir() and seekdir() are basically implementation horrors for any
> > file system that is using anything other than a simple array of
> > directory entries ala the V7 Unix file system or the BSD FFS.  For any
> > file system which is using a more advanced data structure, like
> > b-trees hash trees, etc, there **can't** possibly be a "offset" into a
> > readdir stream. 
> 
> I'll just point you to this:
> 
> http://marc.info/?l=linux-ext4&m=136081996316453&w=2
> 
> so you can see that XFS implements what you say can't possibly be
> done. ;)
> 
> FWIW, that post only talked about the data segment. I didn't mention
> that XFS has 2 other segments in the directory file (both beyond
> EOF) for the directory data indexes. One contains the name-hash btree
> index used for name based lookups and the other contains a freespace
> index for tracking free space in the data segment.

OK, so in some sense that reduces the problem to that of implementing
readdir cookies for directories that are stored in a simple linear
array.

Which I should know how to do but I don't: I guess all you need is a
provision for making holes on remove (so that you aren't required move
existing entries, messing up offsets for concurrent readers)?

Purely out of curiosity: is there a more detailed writeup of XFS's
directory format?  (Or a pointer to a piece of the code a person could
understand without losing a month to it?)

--b.

> 
> IOWs persistent, deterministic, low cost telldir/seekdir behaviour
> was a problem solved in the 1990s. :)