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Date: Wed, 5 Jun 2024 10:20:28 +0200
From: Christoph Hellwig <hch@lst.de>
To: Bart Van Assche <bvanassche@acm.org>
Cc: Christoph Hellwig <hch@lst.de>, Nitesh Shetty <nj.shetty@samsung.com>,
	Damien Le Moal <dlemoal@kernel.org>, Jens Axboe <axboe@kernel.dk>,
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	Mike Snitzer <snitzer@kernel.org>,
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Subject: Re: [PATCH v20 02/12] Add infrastructure for copy offload in block
 and request layer.
Message-ID: <20240605082028.GC18688@lst.de>
References: <20240520102033.9361-3-nj.shetty@samsung.com> <eda6c198-3a29-4da4-94db-305cfe28d3d6@acm.org> <9f1ec1c1-e1b8-48ac-b7ff-8efb806a1bc8@kernel.org> <a866d5b5-5b01-44a2-9ccb-63bf30aa8a51@acm.org> <665850bd.050a0220.a5e6b.5b72SMTPIN_ADDED_BROKEN@mx.google.com> <abe8c209-d452-4fb5-90eb-f77b5ec1a2dc@acm.org> <20240601055931.GB5772@lst.de> <d7ae00c8-c038-4bed-937e-222251bc627a@acm.org> <20240604044042.GA29094@lst.de> <4ffad358-a3e6-4a88-9a40-b7e5d05aa53c@acm.org>
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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