Re: [PATCH v10 4/5] NFSD: Implement NFSD_IO_DIRECT for NFS WRITE

[Christoph Hellwig <hch@infradead.org>]

On Fri, Nov 07, 2025 at 09:38:43AM -0500, Chuck Lever wrote:
> On 11/7/25 8:24 AM, Christoph Hellwig wrote:
> > On Thu, Nov 06, 2025 at 02:02:22PM -0500, Mike Snitzer wrote:
> >>>> Selectively pushing the flag twiddling out to nfsd_direct_write()
> >>>> ignores that we don't want to use DONTCACHE for the unaligned
> >>>> prefix/suffix. Chuck and I discussed this a fair bit 1-2 days ago.
> > 
> > Please document why.  Because honestly it makes zero sense to do that.
> 
> There is a slow-down. But it's not like using DONTCACHE on the unaligned
> ends results in a regression, technically, since this is a brand new
> feature.
> 
> So for an UNSTABLE unaligned WRITE in NFSD_IO_DIRECT mode, the unaligned
> ends would go through the page cache, and would not be durable until
> the subsequent COMMIT. Using DONTCACHE for the end segments adds a
> penalty, and no durability; but what is the value we get for that?

Less cache pollution, and especially less conflicts with other direct
I/O writes.  Do you have a link to results with the slowdown?  How much
is it?