blockio is a terminal front-end to whatever the kernel will say about block
I/O: on FreeBSD it drives the DTrace io provider and diskinfo(8). It draws
one pane per device and one cell per slice of the address space, green for
reads, red for writes and amber for TRIM, so a resilver or a scrub shows up
as a bright head marching across the map while scattered metadata writes
speckle it.

Each cell carries an activity layer that fades in milliseconds and a dimmer
trail that fades over a minute, showing both what the disk is doing now and
where the head has been.

It needs root, and on FreeBSD it needs the DTrace modules loaded
(kldload dtraceall). A demo mode draws synthetic devices without root.