Re: [PATCH v3 01/18] block: introduce duration-limits priority class

[Damien Le Moal <damien.lemoal@opensource.wdc.com>]

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