The following are demonstrations of the rfsio.d script.



Here we trace file read() activity that has fully missed the cache and 
must be read from disk,

   # ./rfsio.d
   
   Read IOPS (count)
   /                                   logical          7
   /extra1                            physical        162
   /boot                               logical        235
   /boot                              physical        410
   /extra1                             logical       9514
   
   Read Bandwidth (bytes)
   /                                   logical        533
   /boot                               logical    1502386
   /boot                              physical    1512960
   /extra1                            physical   97153024
   /extra1                             logical   97228668
   
   Total File System miss-rate: 100%
   ^C

The miss rate of 100% means that all of the file system activity missed
the cache, and had to read from disk.



The following demonstrates file read() activity to the root filesystem
that mostly returned from the file system cache.

   # ./rfsio.d
   
   Read IOPS (count)
   /                                  physical          1
   /extra1                            physical          9
   /devices                            logical          9
   /                                   logical         15
   /extra1                             logical       4096
   
   Read Bandwidth (bytes)
   /devices                            logical          9
   /                                   logical        949
   /                                  physical       8192
   /extra1                            physical     917504
   /extra1                             logical    4194304
   
   Total File System miss-rate: 22%
   ^C

The total miss-rate was 22%, which is based on the bytes transferred that
missed the cache.




Now for an unusual demonstration,

   # ./rfsio.d
   
   Read IOPS (count)
   /devices                            logical          1
   /                                   logical         10
   /extra1                            physical        106
   /extra1                             logical       6337
   
   Read Bandwidth (bytes)
   /devices                            logical          2
   /                                   logical        961
   /extra1                             logical   64846450
   /extra1                            physical   66151424
   
   Total File System miss-rate: 102%
   ^C

Here the miss-rate is 102%, which indicates that more data was read from 
disk than was requested; this can occur due to UFS read-ahead, which
assists the performance of sequential disk activity at the small risk of
reading too much data.