Re: Strange delays on NFS server (with piccies)

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

Neil Brown wrote:
> On Monday August 16, ian@stams.strath.ac.uk wrote:
> 
>>I bumped the nfsd's up to 64 (from 32) and subjectively the problem gets 
>>worse.  I then reduced them to 16 and things are a bit better...
> 
> 
> Odd.
> 
> 
>>Would changing some of the bdflush settings help at all?
> 
> 
> Maybe.  I would start with
>   echo 200 > /proc/sys/vm/dirty_expire_centisecs
> 
> You said you are using ext3.  Are you using journal=data or the
> default journal=ordered ??
> 
> Also, it would be interesting to compare nfs ops per second against
> disk i/os per second over time.
> Something like..
> 
>   while :
>   do
>     perl -ne 'if (/^proc3/)  { @a=split ; shift @a; shift @a; print eval(join("+", @a))."  ";}' /proc/net/rpc/nfsd
>     perl -ne 'if (/hda /) { @a=split; print $a[9]."\n";}' /proc/diskstats  
>     sleep 1
>   done | perl -ne '@_=split; print( ($_[0]-$a[0])." ".($_[1]-$a[1])."\n"); @a=@_;'
> 
> If the pauses correspond to periods with very low nfs ops/sec and very
> high writes per second, then it confirms that it is a disk flushing
> problem.
> 
> It would also be interesting to see if there was a pattern in the
> timing, particular how long the interval was between one pause and the
> next.  
> Also getting these sets of number for different numbers of nfsd
> threads could turn your subjective impression into objective data.
> 
> NeilBrown

Neil, and others

I've gathered some useful data (I hope) on the problem. I ran a variant 
of Neil's script for 2.4 kernel for most of a day (9.30-15.00).

It's all here:

http://www.stams.strath.ac.uk/~ian/nfs/

Files:
data.all.raw	raw data, 3 columns: HH:MM:SS nfsops diskwrites
data.all.plot	massaged data, 3 columns: SECONDS nfsops diskwrites
data.all.eps	postscript plot of massaged data
data.all.gif	gif plot of massaged data

(The massaged data has had missing data filled in. Sometimes the
seconds field jumps 2 seconds. HH:MM:SS changed to seconds from start)

I think this shows no correlation between NFS ops and disk writes
with respect to these big slowdowns (the big peaks in lower graph, there
are 5). Something with a periodicity of 600 seconds is also writing to
the disk. (This was done with 32 nfsd threads, BTW)

There is another similar set of files zooming in on the first 2
events (data.zoom.*). You can see from this graph that in the disk 
writing event lasts about 50 seconds, and the client machines hang on 
NFS ops for this period (pretty annoying, I can tell you!).

Also in the directory is the output of "vmstat 1" showing one of these
events (vmstat.log).

Can anyone deduce anything from this?

Many thanks

Ian

PS: How big are these "wsect" counts in /proc/partitions in terms of bytes ?

-- 
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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