Re: [PATCH v4 2/3] NFSD: add new NFSD_IO_DIRECT variants that may override stable_how

[Chuck Lever <chuck.lever@oracle.com>]

On 11/6/25 3:17 PM, Mike Snitzer wrote:
>> I asked for the use of a file_sync export option because we need to test
>> the BUFFERED cache mode as well as DIRECT. So, continue to experiment
>> with this one, but I don't plan to merge it for now.
> Doesn't the client have the ability to control NFS_UNSTABLE,
> NFS_DATA_SYNC and NFS_FILE_SYNC already?  What experiment are you
> looking to run?
> 
> If just looking to compare NFS_FILE_SYNC performance of
> NFSD_IO_BUFFERED versus NFSD_IO_DIRECT then using the client control
> is fine right?

Not necessarily. You can mount with "sync" but for large application
write requests, that might still generate UNSTABLE + immediate COMMIT.


> Anyway, maybe I'm just being overly concerned about the permanence of
> an export option.  I thought it best to avoid export for now given we
> do seem to have adequate controls for a NFS_FILE_SYNC performance
> bakeoff.

I agree on not rushing another administrative API change. I was thinking
you would be prototyping some of this stuff first and playing with it
for a little while before it goes upstream.

It feels like it would be much more straightforward to implement an
export option that applies to all cache modes rather than gluing it to
DIRECT.

And as I said above, "no plan to merge it for now," meaning it's still
on the table for sometime down the road. I have some other ideas I'm
cooking up, such as using BDI congestion to control NFS WRITE
throttling.

But let's get the base direct WRITE stuff finished.


-- 
Chuck Lever