I was actually exploring inotify-tools for something else, but they can also be handy for seeing what goes on below a mysqld process. inotify hooks into the filesystem handlers, and sees which files are accessed. You can then set triggers, or just display a tally over a certain period.
It has been a standard Linux kernel module since 2.6.13
(2005, wow that’s a long time ago already) and can be
used through calls or the inotify-tools (commandline). So with
the instrumentation already in the kernel,
inotify-tools is all you need to get started.