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Date:   Tue, 5 Nov 2019 13:31:46 -0500 (EST)
From:   Alan Stern <stern@rowland.harvard.edu>
To:     Andrea Vai <andrea.vai@unipv.it>
cc:     Jens Axboe <axboe@kernel.dk>,
        Damien Le Moal <Damien.LeMoal@wdc.com>,
        Johannes Thumshirn <jthumshirn@suse.de>,
        USB list <linux-usb@vger.kernel.org>,
        SCSI development list <linux-scsi@vger.kernel.org>,
        Himanshu Madhani <himanshu.madhani@cavium.com>,
        Hannes Reinecke <hare@suse.com>,
        Ming Lei <ming.lei@redhat.com>, Omar Sandoval <osandov@fb.com>,
        "Martin K. Petersen" <martin.petersen@oracle.com>,
        Greg KH <gregkh@linuxfoundation.org>,
        Hans Holmberg <Hans.Holmberg@wdc.com>,
        Kernel development list <linux-kernel@vger.kernel.org>
Subject: Re: Slow I/O on USB media after commit f664a3cc17b7d0a2bc3b3ab96181e1029b0ec0e6
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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.

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.

Alan Stern