Re: Slow I/O on USB media after commit f664a3cc17b7d0a2bc3b3ab96181e1029b0ec0e6

[Jens Axboe <axboe@kernel.dk>]

On 11/8/19 1:42 AM, Damien Le Moal wrote:
> On 2019/11/08 4:00, Andrea Vai wrote:
>> [Sorry for the duplicate message, it didn't reach the lists due to
>> html formatting]
>> Il giorno gio 7 nov 2019 alle ore 08:54 Damien Le Moal
>> <Damien.LeMoal@wdc.com> ha scritto:
>>>
>>> On 2019/11/07 16:04, Andrea Vai wrote:
>>>> Il giorno mer, 06/11/2019 alle 22.13 +0000, Damien Le Moal ha scritto:
>>>>>
>>>>>
>>>>> Please simply try your write tests after doing this:
>>>>>
>>>>> echo mq-deadline > /sys/block/<name of your USB
>>>>> disk>/queue/scheduler
>>>>>
>>>>> And confirm that mq-deadline is selected with:
>>>>>
>>>>> cat /sys/block/<name of your USB disk>/queue/scheduler
>>>>> [mq-deadline] kyber bfq none
>>>>
>>>> ok, which kernel should I test with this: the fresh git cloned, or the
>>>> one just patched with Alan's patch, or doesn't matter which one?
>>>
>>> Probably all of them to see if there are any differences.
>>
>> with both kernels, the output of
>> cat /sys/block/sdh/queue/schedule
>>
>> already contains [mq-deadline]: is it correct to assume that the echo
>> command and the subsequent testing is useless? What to do now?
> 
> Probably, yes. Have you obtained a blktrace of the workload during these
> tests ? Any significant difference in the IO pattern (IO size and
> randomness) and IO timing (any device idle time where the device has no
> command to process) ? Asking because the problem may be above the block
> layer, with the file system for instance.

blktrace would indeed be super useful, especially if you can do that
with a kernel that's fast for you, and one with the current kernel
where it's slow.

Given that your device is sdh, you simply do:

# blktrace /dev/sdh

and then run the test, then ctrl-c the blktrace. Then do:

# blkparse sdh > output

and save that output file. Do both runs, and bzip2 them up. The shorter
the run you can reproduce with the better, to cut down on the size of
the traces.

-- 
Jens Axboe