Hello, Paolo.

So, I'm currently verifying iocost in the FB fleet.  Around three
thousand machines running v5.2 (+ some backports) with btrfs on a
handful of different models of consumer grade SSDs.  I haven't seen
complete loss of control as you're reporting.  Given that you're
reporting the same thing on io.latency, which is deployed on multiple
orders of magnitude more machines at this point, it's likely that
there's something common affecting your test setup.  Can you please
describe your test configuration and if you aren't already try testing
on btrfs?

Thanks.

-- 
tejun