linux-ext4 - Re: Ext4 slow on links

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Message-ID: <20120622140818.GA28155@thunk.org>
Date:	Fri, 22 Jun 2012 10:08:18 -0400
From:	Ted Ts'o <tytso@....edu>
To:	Bernd Schubert <bernd.schubert@...tmail.fm>
Cc:	Eric Sandeen <sandeen@...hat.com>,
	Norbert Preining <preining@...ic.at>,
	"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>
Subject: Re: Ext4 slow on links

On Fri, Jun 22, 2012 at 11:53:13AM +0200, Bernd Schubert wrote:
> On 06/21/2012 06:05 AM, Eric Sandeen wrote:
> >On 6/20/12 9:28 PM, Norbert Preining wrote:
> >>Hi Eric,
> >>
> >
> >You could see if you could get this LD_PRELOAD working:
> >
> >http://git.kernel.org/?p=fs/ext2/e2fsprogs.git;a=blob_plain;f=contrib/spd_readdir.c
> >
> 
> Hrmm, I need to look through that commit again, but on a first
> glance I cannot see code doing the sorting for ext3/ext4 only (e.g.
> by checking the fsid). So while I like the general approach, it will
> have the opposite effect for some file systems. I will report that
> back on the coreutils list.
> 
> Thanks a lot for the pointer to the commit!

One warning about spd_readdir.  It's not thread-safe, and I've noted
that some programs crash when they try using spd_readdir.so as a
pre-load.  I've tried to fix some of the causes, and I think
thread-safety is the primary fix which is missing, but it's possible
that program which really care about telldir()/seekdir() behaviour as
it relates to readdir() and when files are added to a directory may
also end up getting surprised.

I wrote it primarily as a demonstration of how sorting by inode number
is a big win.  It is *not* suitable for use in /etc/ld.so.preload!

If people want to try to make it safer, patches are accepted, but
ultimately it's better to fix this in the application; that way you
will get your performance gains no matter what OS you happen to be
running on, whether it's Linux, Solaris, AIX, OS X, etc.

     	       	     	   	    	- Ted
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