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
