Mysql "D" processes

[yacenty]

Hello,
Do You know what are and what is the source of "D" processes in the htop application?

Recently I had notticed that in the peak time from time to I get about 10 to 30 mysql processes marked as "D" - then load goes from 20 to 40 and when D processes are runing only 4 processes are active and CPU usage is about 50-60%
then just after the ammount of D processes goes to 0 then I see about 90-120 active processes and CPU usage is 100% on every core of QUAD processor.

What could be done to avoid these "D" processes.
Have You notticed such problem?

I read that it could be caused by huge disk access needs by mysql process, but during this time - mysql disk access is very low - I mean iostat logs very minor ammount of blocks that are written to the hard drive.
What is going on?