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[46.116.236.159]) by smtp.gmail.com with ESMTPSA id e12-20020adfdbcc000000b002258235bda3sm2981299wrj.61.2022.10.25.08.58.20 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 25 Oct 2022 08:58:21 -0700 (PDT) Message-ID: Date: Tue, 25 Oct 2022 18:58:19 +0300 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.2.2 Subject: Re: [PATCH rfc] nvme: support io stats on the mpath device Content-Language: en-US To: Christoph Hellwig Cc: Keith Busch , Max Gurtovoy , linux-nvme@lists.infradead.org, Chaitanya Kulkarni , linux-block@vger.kernel.org, Jens Axboe , Hannes Reinecke References: <20220928195510.165062-1-sagi@grimberg.me> <20220928195510.165062-2-sagi@grimberg.me> <760a7129-945c-35fa-6bd6-aa315d717bc5@nvidia.com> <1b7feff8-48a4-6cd2-5a44-28a499630132@grimberg.me> <414f04b6-aeac-5492-c175-9624b91d21c9@grimberg.me> <20221025153017.GA24137@lst.de> From: Sagi Grimberg In-Reply-To: <20221025153017.GA24137@lst.de> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20221025_085827_112921_E41089A1 X-CRM114-Status: GOOD ( 19.17 ) 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 >>> make up the multipath device. Only the low-level driver can do that right now, >>> so perhaps either call into the driver to get all the block_device parts, or >>> the gendisk needs to maintain a list of those parts itself. >> >> I definitely don't think we want to propagate the device relationship to >> blk-mq. But a callback to the driver also seems very niche to nvme >> multipath and is also kinda messy to combine calculations like >> iops/bw/latency accurately which depends on the submission distribution >> to the bottom devices which we would need to track now. >> >> I'm leaning towards just moving forward with this, take the relatively >> small hit, and if people absolutely care about the extra latency, then >> they can disable it altogether (upper and/or bottom devices). > > So looking at the patches I'm really not a big fan of the extra > accounting calls, and especially the start_time field in the > nvme_request Don't love it either. > and even more so the special start/end calls in all > the transport drivers. The end is centralized and the start part has not sprinkled to the drivers. I don't think its bad. > the stats sysfs attributes already have the entirely separate > blk-mq vs bio based code pathes. So I think having a block_device > operation that replaces part_stat_read_all which allows nvme to > iterate over all pathes and collect the numbers would seem > a lot nicer. There might be some caveats like having to stash > away the numbers for disappearing paths, though. You think this is better? really? I don't agree with you, I think its better to pay a small cost than doing this very specialized thing that will only ever be used for nvme-mpath.