graid3

[Wojciech Puchar]

i read the graid3 manual and http://www.acnc.com/04_01_03.html to make 
sure i know what's RAID3 and i don't understand few things.

1)

"The number of components must be equal to 3, 5, 9, 17, etc.
                 (2^n + 1)."

why it can't be say 5 disks+parity?

2) "-r  Use parity component for reading in round-robin fashion.
"Without this option the parity component is not used at
all for reading operations when the device is in a complete state.
  With this option specified random I/O read operations are even 40% faster
, but sequential reads are slower.  One cannot use this option if the -w option 
is also specified."


how parity disk could speed up random I/O?


is there any description of how graid3 actually works?