On Tue, Sep 21, 2010 at 12:56:14AM +0200, Jeroen Geilman wrote: >> Yes, when traffic to the destination is light (message deliveries >> are spaced multiple seconds or more apart) or is very heavy (message >> deliveries are many in each interval equal to the delivery of a single >> message). When traffic is moderate, demand connection caching may >> introduce some short-term bias towards recently used IPs. As the load >> rises multiple connections are cached in parallel, and these will tend >> to use all the available IPs. > > Am I correct in thinking that this adaptive behaviour is designed to > prevent overloading the nameservers postfix talks to ?
No, not at all, DNS lookups are cached and therefore cheap. It is SMTP connection setup that is expensive for heavily loaded destinations with multiple MX hosts behind load-balancers, where some MX hosts may be slow to respond and initial connections are subjected to various DNS tests, ... that don't apply to a second message for the same connection. Connection caching is especially attractive when some MX hosts are down and non-responsive, incurring high connection setup latency. -- Viktor.