Re: [PATCH 1/6] NFSD: add the ability to enable use of RWF_DONTCACHE for all IO

Chuck Lever
2025-06-12T13:15:42Z

On 6/12/25 3:13 AM, Christoph Hellwig wrote:
> On Wed, Jun 11, 2025 at 04:29:58PM -0400, Jeff Layton wrote:
>> I think if we can crack the problem of receiving WRITE payloads into an
>> already-aligned buffer, then that becomes much more feasible. I think
>> that's a solveable problem.
>
> It's called RDMA :)
>
> To place write payloads into page aligned buffer, the NIC needs to split
> the various headers from the payload. The data placement part of RDMA
> naturally takes care of that. If you want to do it without TCP, you need
> hardware that is aware of the protocol headers up to the XDR level. I
> know and the days where NFS was a big thing there were NICs that could do
> this offload with the right firmware, and I wouldn't be surprised if
> that's still the case.
>
>

Agreed: RDMA is the long-standing solution to this problem.

For TCP:

- For low workload intensity, handling unaligned payloads is adequate.

- For moderate intensity workloads, software RXE, or better, software
iWARP is the right answer. It's just a matter of making those drivers
work efficiently.

- For high intensity workloads, hardware RDMA is the right answer.

--
Chuck Lever