Re: Slow I/O on USB media

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

Il giorno mar, 04/06/2019 alle 07.43 +0200, Greg KH ha scritto:
> On Mon, Jun 03, 2019 at 01:13:48PM +0200, Andrea Vai wrote:
> > Il giorno gio, 30/05/2019 alle 06.25 -0700, Greg KH ha scritto:
> > > [...]
> > Hi,
> > 
> > > Any chance you can use 'git bisect' to find the offending
> commit?
> > Yes, I am doing it as I managed to build the kernel from source
> 
> Great!  What did you find?

so far, something in between 
2ac5e38ea4203852d6e99edd3cf11f044b0a409f (good) and
b3cc2bfe7244e848f5e8caa77bbdc72c04abd17c (bad)... (about 8 steps left)

> 
> > > And did you accidentally turn on "sync" for the filesystem? 
> > Sorry, I don't think so but actually I don't know exactly what it
> is
> > nor how to check it...
> > 
> > >  How do you
> > > know the old kernel really flushed the buffers out in 1 minute? 
> > 
> > I used to try to unmount the usb media (e.g. "eject" using
> Nautilus
> > file manager), and got a message stating the filesystem was in use
> and
> > could not be mounted, so always answered to not eject it until it
> was
> > unmounted without any warning... does it make sense?
> 
> That does not mean that the data is not flushed to the device yet,
> that
> just means that some userspace program is still accessing the
> device.
> You need to run some other type of test to validate how long it taks
> for
> the data to get to the device.

I understand, I actually omitted here what I found out by using "top",
"ps" and "iotop" to catch the moment when the data write finish. I
found a process named "kworker/u8:0+flush-8:16", or similar, which is
alive while the cp process is in D state (and as long as I can't
"eject" the device), and disappears when the media can be ejected, so
I assumed it to be a good indicator for the data write. But I admit I
am really poorly skilled on this matter, so thanks for pointing it
out, and for any other explanation (or links to deepen it
furthermore).

> 
> > >  But 12
> > > minutes is really long, did anything else change in your
> userspace
> > > between the kernel changes as well?
> > I am not sure if I understand correctly the "userspace" you
> mention:
> > if you mean my home directory and contents, settings etc, then
> yes,
> > maybe... but while I am doing the tests I am quite sure I didn't
> > change anything, and double-checked many times that the 4.20
> kernel is
> > always working (I usually boot up with it when I need to do the
> usual
> > day work).
> 
> I mean, did any other programs on your machine change between the
> upgrade of your kernel?  Maybe some gnome-tracker is going off and
> indexing all of the data on that device after you mount it, and it
> wasn't previously doing that before.  As it is still busy, something
> has
> some open files on that device.

Thank you, now I understand what you mean. Yes, this is definitively
possible (may a "lsof | grep mount_point_of_the_device" or similar
could give some clue?)

Thanks, and bye,
Andrea