'''D''' state occurs then the process is in uninterruptible disk sleep. This state is bad, because you can't do anything with the process in D state. Fortunately, process normally remains in such state not for so long. But if you have a heap of D state processes then some logic in system is disrupt. If that is happening, the very important thing is to determine where this unlucky sleep occurs. It is easy to do with ps command with ''l'' option. WCHAN column shows the name of the kernel function where the process is sleeping: