linux-ext4 - Re: regressions due to 64-bit ext4 directory cookies

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Date:	Tue, 12 Feb 2013 16:00:54 -0500
From:	"J. Bruce Fields" <bfields@...ldses.org>
To:	Bernd Schubert <bernd.schubert@...m.fraunhofer.de>
Cc:	linux-ext4@...r.kernel.org, sandeen@...hat.com,
	Theodore Ts'o <tytso@....edu>, gluster-devel@...gnu.org,
	Andreas Dilger <adilger.kernel@...ger.ca>
Subject: Re: regressions due to 64-bit ext4 directory cookies

On Tue, Feb 12, 2013 at 09:56:41PM +0100, Bernd Schubert wrote:
> On 02/12/2013 09:28 PM, J. Bruce Fields wrote:
> > 06effdbb49af5f6c "nfsd: vfs_llseek() with 32 or 64 bit offsets (hashes)"
> > and previous patches solved problems with hash collisions in large
> > directories by using 64- instead of 32- bit directory hashes in some
> > cases.  But it caused problems for users who assume directory offsets
> > are "small".  Two cases we've run across:
> > 
> > 	- older NFS clients: 64-bit cookies cause applications on many
> > 	  older clients to fail.
> > 	- gluster: gluster assumed that it could take the top bits of
> > 	  the offset for its own use.
> > 
> > In both cases we could argue we're in the right: the nfs protocol
> > defines cookies to be 64 bits, so clients should be prepared to handle
> > them (remapping to smaller integers if necessary to placate applications
> > using older system interfaces).  And gluster was incorrect to assume
> > that the "offset" was really an "offset" as opposed to just an opaque
> > value.
> > 
> > But in practice things that worked fine for a long time break on a
> > kernel upgrade.
> > 
> > So at a minimum I think we owe people a workaround, and turning off
> > dir_index may not be practical for everyone.
> > 
> > A "no_64bit_cookies" export option would provide a workaround for NFS 
> > servers with older NFS clients, but not for applications like gluster.
> > 
> > For that reason I'd rather have a way to turn this off on a given ext4 
> > filesystem.  Is that practical?
> 
> I think Ted needs to answer if he would accept another mount option. But
> before we are going this way, what is gluster doing if there are hash
> collions?

They probably just haven't tested NFS with large enough directories.
The birthday paradox says you'd need about 2^16 entries to have a 50-50
chance of hitting the problem.

I don't know enough about ext4 directory performance.  But unfortunately
I suspect there's a range of directory sizes that are too small to have
a significant chance of having directory collisions, but still large
enough to need dir_index?

--b.
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