Emails are sent from a machine running Postfix 2.5.0. They are generated
by software as a batch (triggered by certain events from outside), and
injected very quickly into the local Postfix instance, which never sends
out email directly to the Internet, but only through some Postfix
gateways on other machines.
Destinations are very diverse, by domain and by username, but there's
only one destination per message (no mass distribution of same message).
I want to "load balance" the outbound email between two Postfix
gateways, each one running 2.7.0. Each gateway should receive an
approximately equal amount of outbound messages.
I created a fake domain with the two gateways as MX records:
foobar.local. 604800 IN MX 0 thingone.local.
foobar.local. 604800 IN MX 0 thingtwo.local.
Then I created a sender_dependent_relayhost_maps table with the fake
domain as the nexthop:
[email protected] foobar.local
Tested it - seems to work.
But in reality there's no even distribution between the two nexthops.
The local nexthop always receives 3x ... 4x more messages than the
nexthop across VPN in the other datacenter. Local nexthop is also
slightly faster hardware - not sure if that matters.
One way to do equal-volume load balancing would be to tell the initial
Postfix instance to only send, like, 10 or 100 messages through any
given SMTP connection to the nexthops, then hang off and connect again.
Due to the way DNS works, this would ensure a statistically fair
distribution, by volume, between nexthops.
Correct me if I'm wrong, but that doesn't seem possible with Postfix. I
couldn't find any setting that says "cut off delivery after N messages".
Is there another way?
Also, can someone clarify how and why I end up with the 3:1 or 4:1
distribution? What makes one system receive more emails? Is it because
it's more responsive? (closer topologically, also faster hardware)
What's the algorithm?
--
Florin Andrei
http://florin.myip.org/
