nbdview is a small Tcl/Tk program that lets you visualize reads and
writes happening to an nbdkit server in real time.

To use it you will need to start your nbdkit server with the log
filter.  For example, using the memory plugin (but any plugin may be
used):

  size=$((64 * 1024 * 1024))
  nbdkit --filter=log memory size=$size logfile=/tmp/log

Then start nbdview.tcl specifying the log file and virtual size:

  ./nbdview.tcl /tmp/log $size &

Note that only small disk sizes really work (larger sizes would
require too many pixels!)  With the defaults compiled into
nbdview.tcl: 4K block size, 128 x 128 pixel canvas, you can display at
most 64MB.

Then you can write to the NBD server using ordinary tools such as
qemu, qemu-io, guestfish, etc. and observe the reads and writes to the
disk.