Re: [RFC PATCH] NFSD: Make FILE_SYNC WRITEs comply with spec

[Mike Snitzer <snitzer@kernel.org>]

On Wed, Oct 22, 2025 at 02:25:42PM -0400, Tom Talpey wrote:
> On 10/22/2025 1:46 PM, Mike Snitzer wrote:
> > On Wed, Oct 22, 2025 at 01:22:52PM -0400, Tom Talpey wrote:
> > > There needs to be some statement of the performance consequences of
> > > removing this "optimization". I'm going to predict it's significant
> > > on rotating media, and should not go unmeasured.
> > 
> > I agree that rotational storage will likely see an impact.  But
> > Linux's more recent (now like 14 years ago) FLUSH+FUA rework really
> > helped improve things -- that was a major undertaking where Christoph,
> > Jens and others really did a fantastic job of breathing new life into
> > Linux performance on modern rotational storage.
> 
> I agree, but I think honesty requires it to be measured. Writing
> the metadata has to wait for the data to make it to the same storage,
> this can roughly double the total latency. If it's small (NVMe), great.
> 
> This kind of "optimization" was tried by pretty much every server
> vendor in the early NFSv3 days, I recall many amusing scenes at
> old Connectathons where the schemes, or completely unaware servers
> were exposed. There was no place to hide when operation latencies
> were measured in tens of milliseconds. Same game today, just shifted.
> 
> > Related blast from the past: https://lwn.net/Articles/457667/
> > 
> > My point: may not be as grim as we think...
> > but there is a difference between SATA rotational storage and
> > "enterprise" rotational storage (e.g. NetApp or EMC fronted by
> > elaborate caching that is configured to report wbc=0 because they have
> > battery backed cache)
> 
> Yep. But this is upstream, right? It can't assume.

Correct, but catering to really old and slow rotational storage isn't
(or shouldn't be) a priority.

> To be clear - I totally agree with the change. Only concern is
> stating why it's so important, in the face of performance.

Yes.  In the end, data safety is priority 1, eeking out best
performance is secondary. I think all of us can agree on that (even
those who have really old/slow rotational storage that will get
whiplash from this change).

Mike