We recently observed something strange: After doubling the number of smtp processes in master.cf and restarting, the size of the active queue started fluctuating over time in almost exact inverse of the incoming queue. I'm using Ganglia to track the sizes of the queues on a graph, and the queue graph showed active and incoming as almost exact mirror images of each other: incoming would drain into active and then fairly quickly a lot of active would move to incoming and the cycle would repeat.
We discovered that we were using too much memory and the servers were swapping, so reduced the number of smtpd processes, and this behavior stopped. It's not a problem, exactly, but a curiosity: What mechanism would cause postfix to move messages back and forth between incoming and active in a situation where memory is scarce? Version: postfix 2.2 on CentOS 4 -- Cos
