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Mon, 11 Sep 2023 07:45:19 +0000 (UTC) Received: from dovecot-director2.suse.de ([192.168.254.65]) by imap2.suse-dmz.suse.de with ESMTPSA id mrbHNY/F/mTKbAAAMHmgww (envelope-from ); Mon, 11 Sep 2023 07:45:19 +0000 Message-ID: <19be4f1f-dc2a-47e7-a7d0-94f3a18778d3@suse.de> Date: Mon, 11 Sep 2023 09:45:19 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v15 09/12] dm: Add support for copy offload Content-Language: en-US To: Nitesh Shetty Cc: Jens Axboe , Jonathan Corbet , Alasdair Kergon , Mike Snitzer , dm-devel@redhat.com, Keith Busch , Christoph Hellwig , Sagi Grimberg , Chaitanya Kulkarni , Alexander Viro , Christian Brauner , martin.petersen@oracle.com, mcgrof@kernel.org, gost.dev@samsung.com, linux-block@vger.kernel.org, linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, linux-nvme@lists.infradead.org, linux-fsdevel@vger.kernel.org References: <20230906163844.18754-1-nj.shetty@samsung.com> <20230906163844.18754-10-nj.shetty@samsung.com> <20230911070724.GA28177@green245> From: Hannes Reinecke In-Reply-To: <20230911070724.GA28177@green245> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20230911_004521_877476_BD6F849B X-CRM114-Status: GOOD ( 18.47 ) X-BeenThere: linux-nvme@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "Linux-nvme" Errors-To: linux-nvme-bounces+linux-nvme=archiver.kernel.org@lists.infradead.org On 9/11/23 09:07, Nitesh Shetty wrote: > On Fri, Sep 08, 2023 at 08:13:37AM +0200, Hannes Reinecke wrote: >> On 9/6/23 18:38, Nitesh Shetty wrote: >>> Before enabling copy for dm target, check if underlying devices and >>> dm target support copy. Avoid split happening inside dm target. >>> Fail early if the request needs split, currently splitting copy >>> request is not supported. >>> >> And here is where I would have expected the emulation to take place; >> didn't you have it in one of the earlier iterations? > > No, but it was the other way round. > In dm-kcopyd we used device offload, if that was possible, before using default > dm-mapper copy. It was dropped in the current series, > to streamline the patches and make the series easier to review. > >> After all, device-mapper already has the infrastructure for copying >> data between devices, so adding a copy-offload emulation for device-mapper >> should be trivial. > I did not understand this, can you please elaborate ? > Please see my comments to patch 04. We should only implement copy-offload if there is a dedicated infrastructure in place. But we should not have a 'generic' copy-offload emulation. Problem is that 'real' copy-offload functionalities (ie for NVMe or SCSI) are riddled with corner-cases where copy-offload does _not_ work, and where commands might fail if particular conditions are not met. Falling back to a generic implementation will cause applications to assume that copy-offload worked, and that it gained performance as the application just had to issue a single command. Whereas in fact the opposite is true; it wasn't a single command, and the application might have performed better by issuing the commands itself. Returning -EOPNOTSUPP in these cases will inform the application that the attempt didn't work, and that it will have to fall back to the 'normal' copy. Cheers, Hannes -- Dr. Hannes Reinecke Kernel Storage Architect hare@suse.de +49 911 74053 688 SUSE Software Solutions GmbH, Maxfeldstr. 5, 90409 Nürnberg HRB 36809 (AG Nürnberg), Geschäftsführer: Ivo Totev, Andrew Myers, Andrew McDonald, Martje Boudien Moerman