Re: async vs. sync

[Bernd Schubert <bernd-schubert@web.de>]

Hello Olaf,

> the way the sync export option affects NFSv3 writes is limited to
> COMMITs, so if you see a slow-down here it must be bottle-necking in
> that part of the code.
>
> Quite possibly, this is a problem of the underlying file system.
> You're using a journaled file system, right? So what seems to happen
> is that on every n-th commit call or so all nfsd processes stall
> as the file system tries to write its journal. Note that the
> VM currently allows dirty data to accumulate for up to 30 seconds
> before it is forcibly written to disk (dirty_expire_centisecs sysctl).
>
> A good way to simulate this is to run several iozone processes on the
> server and tell them to sync() every 1 MB or so.
>
> 	iozone -s 1g -r 1m -o -i 0

I just did some tests:

/home: 10MB/s (mirrored via drbd)
/worka: 60MB/s (not mirrored)

The filesystem is reiserfs in both cases, but it seems drbd has in this case a 
terrible performance problem. I also tested to write to a drbd mirrored ext2 
partition, but it has the same problem, so I think this is filesystem 
independent.

Well, during the testing period of our server, I also tested the drbd 
performance, but unfortunately I did most tests with linux-2.6.7. With 2.6.7 
I got more than 30MB/s over drbd. Well, I also did some tests with 2.4., but 
this was with drbd-0.6.12, now we are using drbd-0.7.0. As far as I remember 
the numbers with drbd-0.6+linux-2.4 were similar or even faster than 
drbd-0.7+linux-2.6.

>
> This takes NFS out of the equation.

It seems you are right, its not a nfs issue.

>
> Maybe it would help to play with the dirty writeback strategy, e.g. by
> lowering /proc/sys/vm/dirty_writeback_centisecs (to e.g. 250), increasing
> dirty_background_ratio or lowering vm_dirty_ratio.

Well, I only see this on 2.6. systems, but not on 2.4., are there similar 
triggers in 2.4.?  With 2.6. we don't have the problem at all.


Thanks a lot for pointing me in the right direction!


Best regards,
	Bernd



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