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[76.210.143.223]) by smtp.gmail.com with ESMTPSA id w27sm17486740pfq.104.2020.12.22.12.31.39 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 22 Dec 2020 12:31:39 -0800 (PST) Date: Tue, 22 Dec 2020 12:31:39 -0800 (PST) X-Google-Original-Date: Tue, 22 Dec 2020 12:31:37 PST (-0800) Subject: Re: [dm-devel] [PATCH v1 0/5] dm: dm-user: New target that proxies BIOs to userspace In-Reply-To: <20201222133246.GA5099@infradead.org> CC: josef@toxicpanda.com, bvanassche@acm.org, Christoph Hellwig , snitzer@redhat.com, corbet@lwn.net, kernel-team@android.com, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-raid@vger.kernel.org, song@kernel.org, dm-devel@redhat.com, linux-kselftest@vger.kernel.org, shuah@kernel.org, agk@redhat.com, michael.christie@oracle.com From: Palmer Dabbelt To: Christoph Hellwig Message-ID: Mime-Version: 1.0 (MHng) Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, 22 Dec 2020 05:32:46 PST (-0800), Christoph Hellwig wrote: > On Mon, Dec 14, 2020 at 07:00:57PM -0800, Palmer Dabbelt wrote: >> I haven't gotten a whole lot of feedback, so I'm inclined to at least have some >> reasonable performance numbers before bothering with a v2. > > FYI, my other main worry beside duplicating nbd is that device mapper > really is a stacked interface that sits on top of other block device. > Turning this into something else that just pipes data to userspace > seems very strange. Agreed. It certainly doesn't fit the DM model. We'd considered doing a non-DM version of this (maybe "ubd"), but decided to stick with dm-user because we didn't want to duplicate all the device creation stuff that DM provides. A simple version of that wouldn't be that hard to do, but the DM version has a lot of features and we get that all for free. We essentially decided to run with DM until it gets in the way, and the only sticking point we ended up with was that REQUEUE stuff (though not sure how that would show up with a bare block device) and that scheduler question. I'm going to stick with DM for now, unless it gets in the way, to avoid coming up with a device creation scheme myself. In the long term it's probably best to have this be a standalone thing, but I don't want to dump a bunch of time into putting that stuff together only to find that this isn't interesting enough from a performance perspective to stick around.