Process states inside the linux kernel. [Especially about the STATE D]

["vamsi krishna" <vamsi.krishnak@gmail.com>]

Hello All,

 I have been debugging a program which takes huge memory around 12Gb
(On a 64-bit machine). I was trying to monitor the VmSize and RSS
sizes using top command for this process. I frequently see that the
process STATUS changes its state from 'D' to 'R' most of the time its
in the state 'D'.

 I looked at the manual it says D  uninterruptable sleep state (I have
googled on this but could'nt find much information about this stage).
As far as my text book knowledge, the process is either 1). Running 
'R'  2.) Ready 'Re' 3.) Wait (for I/O) W .

 So what is 'uninterruptable sleep state' D ? when does the process is
put in this state?

 Can someone releate me the states cycle of the process in linux kernel.

 Really appreciate your time and effort.

 Thank you,
 Vamsi.