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

Andrea Vai
2019-12-02T07:01:47Z

On 29/11/19 12:44:53, Bernd Schubert wrote:
> >> Trace attached. Produced by: start the trace script
> >> (with the pendrive already plugged), wait some seconds, run the test
> >> (1 trial, 1 GB), wait for the test to finish, stop the trace.
> >>
> >> The copy took 73 seconds, roughly as already seen before with the fast
> >> old kernel.
> >
> > This trace shows a good write IO order because the writeback IOs are
> > queued to block layer serially from the 'cp' task and writeback wq.
> >
> > However, writeback IO order is changed in current linus tree because
> > the IOs are queued to block layer concurrently from the 'cp' task
> > and writeback wq. It might be related with killing queue_congestion
> > by blk-mq.
>
> What about using direct-io to ensure order is guaranteed? Pity that 'cp'
> doesn't seem to have an option for it. But dd should do the trick.
> Andrea, can you replace cp with a dd command (on the slow kernel)?
>
> dd if=<path-to-src-file> of=<path-to-copy-on-flash-device> bs=1M
> oflag=direct

On the "new bad patched" kernel, this command take 68 seconds to complete (mean on 100 trials, with a narrow standard deviation), so perfectly
aligned with the cp command on the old fast good kernel.

Thanks, and bye
Andrea