Victor Duchovni put forth on 9/20/2010 6:01 PM: > 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.
Maybe worth reading the relevant documentation on connection caching: http://www.postfix.org/scache.8.html -- Stan
