[rwmj]

> I can't conduct the experiment anymore obviously

Why not?

  $ virt-make-fs --type=msdos /usr/share/doc /var/tmp/disk.img
  $ export file=/var/tmp/disk.img
  $ nbdkit eval \
       after_fork=' echo 0 > $tmpdir/read ' \
       get_size=' stat -Lc %s $file ' \
       pread='
           diff=$(($4 - `cat $tmpdir/read`))
           echo $4 > $tmpdir/read
           if [ $diff -lt -10000 ] || [ $diff -gt 10000 ]; then
               # simulate a seek
               sleep 0.1
           fi
           dd if=$file skip=$4 count=$3 iflag=count_bytes,skip_bytes
       '
  $ sudo nbd-client localhost /dev/nbd0
  $ sudo mount /dev/nbd0 /tmp/mnt

The tricky thing is probably making a deliberately fragmented disk image for testing using modern tools.