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

[Jens Axboe]

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? And writes in particular? 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?

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.

> In the absence of any responses, after a week or so I will submit a
> patch to revert the f664a3cc17b7 ("scsi: kill off the legacy IO path")
> commit.

That's not going to be feasible.

-- 
Jens Axboe