Is there a way to stop the Ethernet controller from monopolizing the
upstream USB channel, while in high usage, in detriment of other USB
devices on LAN9514 (Raspberry Pi 2 USB-Hub/Ethernet chip)?
Basically, if you have a USB-attached storage device and start writing
data that comes from the Ethernet into this USB disk, the Ethernet chip
will always have priority on the USB bus and the kernel will never have
the chance to use it to write on the storage device.
As a result, the data incoming from the Ethernet gets stored in the
kernel IO buffer until memory runs out, and then the transfer stops for
a while (as the kernel starting dropping TCP packets and the TCP flow
control algorithm kicks in). On the other hand, if you stop the network
transfer before the buffer is filled, the content on it is immediately
flushed to disk.
This would not be a huge problem if the USB-attached storage devices
didn't have a timeout: after not receiving some keepalive packets from
the system it's attached to, it dies. With my USB disk, at least, you
have to unplug its USB cable and plug it again to convince its
controller that the system is back up.
Of course, this is a even bigger issue on Raspberry if the system's root
is installed on this USB disk.
I'm not sure that this is fixed in the Raspberry Pi Foundation's remix
of Debian, but I'm definitely hitting this bug on Fedora 25 beta with
Linux 4.8.0-0.rc7.git0.1.fc25.armv7hl.
How to reproduce, considering /dev/sda is some USB-attached storage on
the rasp2:
desktop$ dd if=some_big_file bs=8M | ssh root@rasp2 dd of=/dev/sda bs=8M
Meanwhile, you may watch the kernel buffer's filling up on the rasp2
with htop or free. You may also notice that if you cancel the SSH
transfer before the rasp2 kernel buffer is filled (and also before your
USB disk gives up waiting on the keepalive packet), the content stored
on the buffer is almost immediately flushed to the disk.
How can we fix this?
--
Bernardo Donadio
DevOps Engineer at Alligo Tecnologia
https://bcdonadio.com/
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