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

Alan Stern
2019-11-06T16:03:57Z

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.

Alan Stern