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

[Bernd Schubert]

>> 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

 - Bernd