Re: nfsd write throughput

[Greg Banks <gnb@sgi.com>]

On Tue, Aug 03, 2004 at 08:02:13AM +0200, Olaf Kirch wrote:
> On Tue, Aug 03, 2004 at 12:10:18PM +1000, Greg Banks wrote:
> 
> > First, the way the v3 server is supposed to work is that normal page
> > cache pressure pushes pages from unstable writes to disk before the
> > COMMIT call arrives from the client.  The best way to achieve this
> > for a dedicated NFS server box is tuning the pdflush parameters
> > to be more aggressive about writing back dirty pages, e.g. bumping
> > down the following in /proc/vm: dirty_background_ratio, dirty_ratio,
> > dirty_writeback_centisecs, and dirty_expire_centisecs.  I have to
> 
> Yes and no. Can we expect every user to fiddle with the pdflush
> tunables to get an NFS server that performs reasonably well?

No, of course not, hence the next idea.

> > I think another useful approach would be to writeback pages which
> > have been written by NFS unstable writes at a faster rate than pages
> > written by local applications, i.e. add a new /proc/vm/ sysctl like
> > nfs_dirty_writeback_centisecs and a per-page flag.
> 
> That may be a useful solution, too. My patch basically does what
> fadvise(WONTNEED) does.

Sure, the key question is when and for how many pages.  You don't
really have enough information in nfsd_write() to tell that safely.

> > For example, imagine the disk backend is a hardware RAID5 with a
> > stripe size of 128K or greater and the client is doing streaming
> > 32K WRITE calls.  With your patch, every second WRITE call will now
> > try to write half a RAID stripe unit,
> 
> No, it doesn't. If you look at the if() expression, you'll see it
> writes every 64 client-size pages.

> > > +		if ((cnt & 1023) == 0
> > > +		 && ((offset / cnt) & 63) == 0

It writes every time `offset' is a multiple of 64 times `cnt' and
`cnt' is a multiple of 1024.  At this point `cnt' is the length
of the data received in the WRITE call, which has only a vague
relationship to the client page size.

> In the worst case that's every 64K,
> but for Linux clients that's every 256K which is a reasonable size
> for IDE DMA, as well as most RAID configurations

We have configurations with hardware RAID stripe sizes up to 4MB.

Greg.
-- 
Greg Banks, R&D Software Engineer, SGI Australian Software Group.
I don't speak for SGI.


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