Slow I/O on USB media after commit f664a3cc17b7d0a2bc3b3ab96181e1029b0ec0e6

[Andrea Vai <andrea.vai@unipv.it>]

Hi,
  I have a problem writing data to a USB pendrive, and it seems
kernel-related. With the help of Greg an Alan (thanks) and some
bisect, I found out the offending commit being

commit f664a3cc17b7d0a2bc3b3ab96181e1029b0ec0e6
Author: Jens Axboe <axboe@kernel.dk>
Date:   Thu Nov 1 16:36:27 2018 -0600

    scsi: kill off the legacy IO path
    
    This removes the legacy (non-mq) IO path for SCSI.
    
So, here I am to notify you about the problem and ask you if I can
help in any way to work it out and fix it.

The problem is that if I copy a file from the internal SATA HD to the
pendrive, it takes ~10 times to complete (in respect of the time
needed with the patch reverted).

The test script, which I use to detect if the problem triggers or not,
is:

#!/bin/bash
logfile=...
uname -a | tee -a $logfile
echo -n "Begin: " | tee -a $logfile
date | tee -a $logfile
touch inizio
SECONDS=0
mount UUID="05141239-4ea5-494d-aa91-acd67db89ce5" /mnt/pendrive
cp /NoBackup/buttare/testfile /mnt/pendrive
umount /mnt/pendrive
tempo=$SECONDS
touch fine
echo -n "...end: " | tee -a $logfile
date | tee -a $logfile
echo "It took $tempo seconds!" | tee -a $logfile

If I run the test with a 512MB file it takes >10min vs. half a minute.

The problem is still present in last tested git (cloned today in the
morning).

You can see the previous discussion that lead to these results at

https://marc.info/?t=155922230700001&r=1&w=2

Thanks, and bye
Andrea