Re: A comparison of the new nfsd iomodes (and an experimental one)

[Jeff Layton <jlayton@kernel.org>]

On Thu, 2026-03-26 at 11:30 -0400, Chuck Lever wrote:
> On 3/26/26 11:23 AM, Jeff Layton wrote:
> > I've been doing some benchmarking of the new nfsd iomodes, using
> > different fio-based workloads.
> > 
> > The results have been interesting, but one thing that stands out is
> > that RWF_DONTCACHE is absolutely terrible for streaming write
> > workloads. That prompted me to experiment with a new iomode that added
> > some optimizations (DONTCACHE_LAZY).
> > 
> > The results along with Claude's analysis are here:
> > 
> >     https://markdownpastebin.com/?id=387375d00b5443b3a2e37d58a062331f
> > 
> > He gets a bit out over his skis on the upstream plan, but tl;dr is that
> > DONTCACHE_LAZY (which is DONTCACHE with some optimizations) outperforms
> > the other write iomodes.
> 
> The analysis of the write modes seems plausible. I'm interested to hear
> what Mike and Jens have to say about that.
> 
> One thing I'd like to hear more about is why Claude felt that disabling
> splice read was beneficial. My own benchmarking in that area has shown
> that splice read is always a win over not using splice.
> 

Good catch. That turns out to be a mistake in Claude's writeup.

The test scripts left splice reads enabled for buffered reads, and the
results in the analysis reflect that. I (and it) have no idea why it
would recommend disabling them, when the testing all left them enabled
for buffered reads.
-- 
Jeff Layton <jlayton@kernel.org>