Thank you very much for reply and suggestions, more comments below.
Still, is there a definite answer on root question: are different btrfs
volumes independent in terms of CPU, or are there some shared workers
that can be point of contention?
What would have been interesting would have been if you had any reports
from for instance htop during that time, showing wait percentage on the
various cores and status (probably D, disk-wait) of the innocent
process. iotop output would of course have been even better, but also
rather more special-case so less commonly installed.
Curiously, I have had iotop but not htop running. [btrfs-transacti] had
some low-level activity in iotop (I still assume it was CPU-limited),
the innocent process did not have any activity anywhere. Next time I'll
also take notice of process state in ps (sadly, my omission).
I believe you will find that the problem isn't btrfs, but rather, I/O
contention
This possibility did not come to my mind. Can USB drivers be still that
bad in 4.4? Is there any way to discriminate these two situations (btrfs
vs usb load)?
BTW, USB adapter used is this one (though storage array only supports
USB 3.0): https://www.asus.com/Motherboard-Accessory/USB_31_TYPEA_CARD/
and that if you try the same thing with one of the
filesystems being for instance ext4, you'll see the same problem there as
well
Not sure if it's possible to reproduce the problem with ext4, since it's
not possible to perform such extensive metadata operations there, and
simply moving large amount of data never created any problems for me
regardless of filesystem.
--
With Best Regards,
Marat Khalili
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