This tool lets you monitor I/O latency in real time. It shows disk latency in the same way as ping shows network latency.

 Usage: ioping [-LABCDWRq] [-c count] [-w deadline] [-pP period] [-i interval]
               [-s size] [-S wsize] [-o offset] directory|file|device
        ioping -h | -v

      -c <count>      stop after <count> requests
      -w <deadline>   stop after <deadline>
      -p <period>     print raw statistics for every <period> requests
      -P <period>     print raw statistics for every <period> in time
      -i <interval>   interval between requests (1s)
      -s <size>       request size (4k)
      -S <wsize>      working set size (1m)
      -o <offset>     working set offset (0)
      -L              use sequential operations (includes -s 256k)
      -A              use asynchronous I/O
      -C              use cached I/O
      -D              use direct I/O
      -W              use write I/O *DANGEROUS*
      -R              seek rate test (same as -q -i 0 -w 3 -S 64m)
      -B              print final statistics in raw format
      -q              suppress human-readable output
      -h              display this message and exit
      -v              display version and exit