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Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.6.0 Subject: Re: [PATCH v3 01/18] block: introduce duration-limits priority class Content-Language: en-US To: Bart Van Assche , Niklas Cassel Cc: Paolo Valente , Jens Axboe , Christoph Hellwig , Hannes Reinecke , "linux-scsi@vger.kernel.org" , "linux-ide@vger.kernel.org" , "linux-block@vger.kernel.org" References: <20230124190308.127318-2-niklas.cassel@wdc.com> <731aeacc-74c0-396b-efa0-f9ae950566d8@opensource.wdc.com> <873e0213-94b5-0d81-a8aa-4671241e198c@acm.org> <4c345d8b-7efa-85c9-fe1c-1124ea5d9de6@opensource.wdc.com> <5066441f-e265-ed64-fa39-f77a931ab998@acm.org> <275993f1-f9e8-e7a8-e901-2f7d3a6bb501@opensource.wdc.com> <86de1e78-0ff2-be70-f592-673bce76e5ac@opensource.wdc.com> <7f0a2464-673a-f64a-4ebb-e599c3123a24@acm.org> <29b50dbd-76e9-cdce-4227-a22223850c9a@opensource.wdc.com> <049a7e88-89d1-804f-a0b5-9e5d93d505f7@opensource.wdc.com> From: Damien Le Moal Organization: Western Digital Research In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org On 1/28/23 09:47, Bart Van Assche wrote: > On 1/27/23 16:40, Damien Le Moal wrote: >> On 1/28/23 02:23, Bart Van Assche wrote: >>> I hope that I have it made it clear that I think that the proposed user >>> space API will be very painful to use for application developers. >> >> I completely disagree. Reusing the prio class/level API made it easy to allow >> applications to use the feature. fio support for CDL requires exactly *one line* >> change, to allow for the CDL class number 4. That's it. From there, one can use >> the --cmdprio_class=4 nd --cmdprio=idx options to exercise a drive. The value of >> "idx" here of course depends on how the descriptors are set on the drive. But >> back to the point above. This depends on the application goals and the >> descriptors are set accordingly for that goal. There is no real discovery needed >> by the application. The application expect a certain set of CDL limits for its >> use case, and checking that this set is the one currently defined on the drive >> is easy to do from an application with the sysfs interface we added. >> >> Many users out there have deployed and using applications taking advantage of >> ATA NCQ priority feature, using class RT for high priority IOs. The new CDL >> class does not require many application changes to be enabled for next gen >> drives that will have CDL. > As I mentioned before, the new I/O priority class IOPRIO_CLASS_DL > makes it impossible to use a single I/O priority class across devices > that support CDL and devices that do not support CDL. I'm surprised that > you keep denying that IOPRIO_CLASS_DL is a royal pain for users who have > to support devices that support CDL and devices that do not support CDL. I am not denying anything. I simply keep telling you that CDL is not a generic feature for random applications to use, including those that already use RT/BE/IDLE. It is for applications that know and expect it, and so have a setup suited for CDL use down to the drive CDL descriptors. That includes DM setups. Thinking about CDL in a generic setup for any random application to use is nonsense. And even if that happens and a user not knowing about it still tries it, than as mentioned, nothing bad will happen. Using CDL in a setup that does not support it is a NOP. That would be the same as not using it. > > Bart. -- Damien Le Moal Western Digital Research