I spent a while scratching my head over the collectd 'disk' plugin producing no output whatsoever on a Debian 9 server. No log output (other than successful load), no rrd files, nothing.
Frustratingly, debug logging in collectd is compiled out by default, and compiling collectd from debian source packages requires an insane number of dependencies, so I avoided that route.
Instead through analysing the source and history of the disk plugin I came across this commit. The issue turned out to be simple - in the backported kernel I was using,
/proc/diskstats has a tonne more fields than the default 4.9 kernel, and this was causing the disk plugin to fall over silently.
It's a ballache to do anything about this situation on Debian 9, assuming the backported kernel is essential. If you hit it then best just wait for Buster.