Re: Strange delays on NFS server

[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 ??

I'm using the default on Fedora 1, ordered data.

> 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.

I'm using FC1 which has 2.4.22 as its base.  I think the 
/proc/sys/vm/dirty_expire_centisecs and the above scriptlet are
for 2.6 only.  The nfs stats work, but the /proc/diskstats file
is missing.

Do you have any suggestions for /proc/sys/vm/bdflush instead ? Here
are the current settings:

30      500     0       0       500     3000    60      20      0

> 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.  

I'll start keeping a note of the time of these events.

> Also getting these sets of number for different numbers of nfsd
> threads could turn your subjective impression into objective data.
> 
> NeilBrown

Thanks

Ian
-- 
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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