Re: [PATCH v20 02/12] Add infrastructure for copy offload in block and request layer.

[Christoph Hellwig <hch@lst.de>]

On Tue, Jun 04, 2024 at 04:44:34AM -0700, Bart Van Assche wrote:
> On 6/3/24 21:40, Christoph Hellwig wrote:
>> There is no requirement to process them synchronously, there is just
>> a requirement to preserve the order.  Note that my suggestion a few
>> arounds ago also included a copy id to match them up.  If we don't
>> need that I'm happy to leave it away.  If need it it to make stacking
>> drivers' lifes easier that suggestion still stands.
>
> Including an ID in REQ_OP_COPY_DST and REQ_OP_COPY_SRC operations sounds
> much better to me than abusing the merge infrastructure for combining
> these two operations into a single request. With the ID-based approach
> stacking drivers are allowed to process copy bios asynchronously and it
> is no longer necessary to activate merging for copy operations if
> merging is disabled (QUEUE_FLAG_NOMERGES).

Again, we can decided on QUEUE_FLAG_NOMERGES per request type.  In fact
I think we should not use it for discards as that just like copy
is a very different kind of "merge".

I'm in fact much more happy about avoiding the copy_id IFF we can.  It
it a fair amout of extra overhead, so we should only add it if there
is a real need for it