qmail delivery slowdown under high load

[Andras Tudos - Computronic, C3]

Hi,

I have an operational theory question: when the load in a qmail setup 
reaches certain level the queue starts to grow and after a short time the 
number of unprocessed messages starts to grow as well. This is OK, but: at 
the same time the speed of local-deliveries slows down tenfold and the 
queue is filling up more and more. The only way to stop this is to stop the 
incoming flow of messages, then the local deliveries are fast again and the 
server recovers. Of course the  whole story is about an overloaded i/o 
subsystem which has to be upgraded, but still I want to know why do the 
local deliveries slow down so much in a race condition to give an exact 
answer for the collegues who blame qmail and say this would not occur in a 
sendmail or other MTA based system.

Andras Tudos
C3, Budapest