linux-kernel - Re: If not readdir() then what?

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Date:	Mon, 9 Apr 2007 09:19:18 -0400
From:	Theodore Tso <tytso@....edu>
To:	Trond Myklebust <trond.myklebust@....uio.no>
Cc:	Jörn Engel <joern@...ybastard.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Christoph Hellwig <hch@...radead.org>,
	Ulrich Drepper <drepper@...il.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Neil Brown <neilb@...e.de>
Subject: Re: If not readdir() then what?

On Mon, Apr 09, 2007 at 08:31:37AM -0400, Trond Myklebust wrote:
> On Mon, 2007-04-09 at 13:09 +0200, Jörn Engel wrote:
> > That surely doesn't make life any easier for filesystem developers, I
> > agree.  From that point of view, all telldir cookies should end their
> > life at closedir time.  For "rm -r" it would be sufficient if the nfs
> > client simply didn't seekdir at all.  For "ls -lR", this would return
> > duplicate dentries.
> 
> Please go read the NFS spec. The only thing an NFS client has in order
> to read a directory is a READDIR operation that in essence takes a
> filehandle and a cookie as its arguments. Unless the server is able to
> return the entire rest of the directory in one RPC reply, the client
> needs to send a second READDIR operation with a cookie from the previous
> READDIR operation. The server is expected to return cookies for _each_
> entry in the directory.
> 
> That is a protocol limitation, not a client limitation.

<Groan>

And after quickly checking RFC 3010, I see this limitation hasn't been
lifted in NFSv4.

Speaking of which, right now ext3 doesn't know whether it's talking to
an NFSv2 or NFS v3/v4 server, so it's always passing a 32-bit cookie.
If NFSv3/v4 could use an explicit interface to request a 64-bit
cookie, instead of just relying on the f_pos field in the file handle,
we can reduce the chance of hash collisions when reading an ext3
directory significantly.   

If there are 2 or 3 directory entries that have a hash collision,
would the NFS protocol allow the server to juggle things so that those
2-3 directory entries with the hash collision are sent back in a
single readdir RPC reply?  Is it aceptable/legal to have multiple
entries in the same READDIR reply packet have the same cookie value?

						- Ted
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