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

Damien Le Moal
2019-11-06T22:13:30Z

On 2019/11/07 1:04, Alan Stern wrote:
> On Tue, 5 Nov 2019, Jens Axboe wrote:
>
>> On 11/5/19 11:31 AM, Alan Stern wrote:
>>> On Tue, 5 Nov 2019, Andrea Vai wrote:
>>>
>>>> Il giorno lun, 04/11/2019 alle 13.20 -0500, Alan Stern ha scritto:
>>>
>>>>> You should be able to do something like this:
>>>>>
>>>>> cd linux
>>>>> patch -p1 </path/to/patch2
>>>>>
>>>>> and that should work with no errors. You don't need to use git to
>>>>> apply a patch.
>>>>>
>>>>> In case that patch2 file was mangled somewhere along the way, I
>>>>> have
>>>>> attached a copy to this message.
>>>>
>>>> Ok, so the "patch" command worked, the kernel compiled and ran, but
>>>> the test still failed (273, 108, 104, 260, 177, 236, 179, 1123, 289,
>>>> 873 seconds to copy a 500MB file, vs. ~30 seconds with the "good"
>>>> kernel).
>>>>
>>>> Let me know what else could I do,
>>>
>>> I'm out of suggestions. If anyone else knows how to make a kernel with
>>> no legacy queuing support -- only multiqueue -- issue I/O requests
>>> sequentially, please speak up.
>>
>> Do we know for a fact that the device needs strictly serialized requests
>> to not stall?
>
> Not exactly, but that is far and away the most likely explanation for
> the device's behavior. We tried making a bunch of changes, some of
> which helped a little bit, but all of them left a very large
> performance gap. I/O monitoring showed that the only noticeable
> difference in the kernel-device interaction caused by the $SUBJECT
> commit was the non-sequential access pattern.
>
>> And writes in particular?
>
> Andrea has tested only the write behavior. Possibly reading will be
> affected too, but my guess is that it won't be.
>
>> I won't comment on how broken
>> that is, just trying to establish this as the problem that's making this
>> particular device be slow?
>
> It seems reasonable that the access pattern could make a significant
> difference. The device's behavior suggests that it buffers incoming
> data and pauses from time to time to write the accumulated data into
> non-volatile storage. If its algorithm for allocating, erasing, and
> writing data blocks is optimized for the sequential case, you can
> easily imagine that non-sequential accesses would cause it to pause
> more often and for longer times -- which is exactly what we observed.
> These extra pauses are what resulted in the overall performance
> decrease.
>
> So far we have had no way to perform a direct test. That is, we don't
> know of any setting that would change a single kernel between
> sequential and non-sequential access. If you can suggest a simple way
> to force a kernel without the $SUBJECT commit to do non-sequential
> writes, I'm sure Andrea will be happy to try it out and see if it
> causes a slowdown.
>
>> I've lost track of this thread, but has mq-deadline been tried as the
>> IO scheduler? We do have support for strictly serialized (writes)
>> since that's required for zoned device, wouldn't be hard at all to make
>> this cover a blacklisted device like this one.
>
> Please spell out the exact procedure in detail so that Andrea can try
> it. He's not a kernel hacker, and I know very little about the block
> layer.

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

>
> Alan Stern
>
>

--
Damien Le Moal
Western Digital Research