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Date: Sat, 1 Jun 2024 07:59:31 +0200
From: Christoph Hellwig <hch@lst.de>
To: Bart Van Assche <bvanassche@acm.org>
Cc: 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>,
	Mikulas Patocka <mpatocka@redhat.com>,
	Keith Busch <kbusch@kernel.org>, Christoph Hellwig <hch@lst.de>,
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Subject: Re: [PATCH v20 02/12] Add infrastructure for copy offload in block
 and request layer.
Message-ID: <20240601055931.GB5772@lst.de>
References: <20240520102033.9361-1-nj.shetty@samsung.com> <CGME20240520102842epcas5p4949334c2587a15b8adab2c913daa622f@epcas5p4.samsung.com> <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>
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On Thu, May 30, 2024 at 10:11:15AM -0700, Bart Van Assche wrote:
> This new approach has the following two disadvantages:
> * Without plug, REQ_OP_COPY_SRC and REQ_OP_COPY_DST are not combined. These two
>   operation types are the only operation types for which not using a plug causes
>   an I/O failure.

So?  We can clearly document that and even fail submission with a helpful
message trivially to enforce that.

> * A loop is required to combine the REQ_OP_COPY_SRC and REQ_OP_COPY_DST operations.

I don't see why that is a problem.  Especiallly once we allow multiple
sources which is really useful for GC workloads we'll need to do that
anyway.

> Please switch to the approach Hannes suggested, namely bio chaining. Chaining
> REQ_OP_COPY_SRC and REQ_OP_COPY_DST bios before these are submitted eliminates the
> two disadvantages mentioned above.

No.