Re: Strange delays on NFS server

[Ian Thurlbeck <ian@stams.strath.ac.uk>]

Olaf Kirch wrote:
> 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.

Olaf

I'm confused - you're talking about 2.6 but I'm running 2.4.22 (FC1). 
Does your analysis apply to both 2.4 and 2.6 kernels?

Subjectively, the problem seems to be getting worse the longer the 
server is running (uptime 80 days currently).  This could be a 
red-herring of course.

Ian

PS Red-herring = misleading or incorrect clue or idea!

> 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


-- 
Ian Thurlbeck                http://www.stams.strath.ac.uk/
Statistics and Modelling Science, University of Strathclyde
Livingstone Tower, 26 Richmond Street, Glasgow, UK,  G1 1XH
Tel: +44 (0)141 548 3667           Fax: +44 (0)141 552 2079



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