Re: Strange delays on NFS server

[Olaf Kirch <okir@suse.de>]

On Wed, Aug 11, 2004 at 05:08:45PM +0100, Ian Thurlbeck wrote:
> OK, I've been running "top -d 1 -i" and trying to see what comes up when
> the server freezes. I caught one instance where a delay coincided
> with about 15 nfsd + 1 kjournald process appearing in the top
> display. I'm simultaneously looking at a graphical network tool to try
> and see the traffic going to the server - anyone got a better suggestion?

This sounds exactly like the COMMIT stall problem for which I submitted
the early-writeout patch to this list about a week ago.

I've been thinking about this a little more. It may be that one reason
the problem is more pronounced in in 2.6 than in 2.4 is the new io
barrier code. In 2.6 ext3 uses barriers by default; Suse's 2.6 has reiserfs
patches that add barriers (and enables them by default). We've reports of
this problem on both file systems.

JFS does i/o barriers while XFS does not; and this also fits the pattern
of what Ian reports. I dimly remember there's a kernel command line
option to turn off barriers at the block io level. Can you try if
that helps, Ian?

The more I think about this, the more I believe the early-writeout patch
is the right way to address this problem (short of turning off barriers).
When data hits the NFS server, it is supposed to go to disk rather
soonishly. This also covers most of the rewrite case, at least as long
as you have just one application writing to the file - all rewriting
happens in the client cache.

The crucial question is, what is a good heursitic to choose when to
initiate a write-out. Sequential writes to the end of file are easy
enough to detect.

I have a somewhat updated version of my patch that covers just this
case, and exports a sysctl to let you tune how often it initiates
an early write-out.

Olaf
-- 
Olaf Kirch     |  The Hardware Gods hate me.
okir@suse.de   |
---------------+ 


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