Postfix Overview - Introduction - SCN Research

Postfix Overview - Queue Management
Page 1 of 2
Postfix Overview - Queue Management
Up one level | Introduction | Goals and features | Global architecture | Queue Management | Security
Postfix mail queues
Postfix has four different queues: maildrop, incoming, active and deferred (click the upper left-hand icon for
the big picture). Locally-posted mail is deposited into the maildrop, and is copied to the incoming queue after
some cleaning up. The incoming queue is for mail that is still arriving or that the queue manager hasn't looked
at yet. The active queue is a limited-size queue for mail that the queue manager has opened for delivery. Mail
that can't be delivered goes to the deferred queue, so that it does not get in the way of other deliveries.
The queue manager keeps information in memory about the active queue only. The active queue size is limited
on purpose: the queue manager should never run out of working memory because of a peak message workload.
Whenever there is space in the active queue, the queue manager lets in one message from the incoming queue
and one from the deferred queue. This guarantees that new mail will get through even when there is a large
backlog.
No thundering herd
Implementing a high-performance mail system is one thing. However, no-one would be pleased when Postfix
connects to their site and overwhelms it with lots of simultaneous deliveries. This is an issue especially when a
site has been down and mail is backed up elsewhere in the network.
Postfix tries to be a good network neighbor. When delivering mail to a site, Postfix will initially make no more
than two simultaneous connections. As long as deliveries succeed, the concurrency slowly increases up to some
configurable limit (or until the host or network is unable to handle the load); concurrency is decreased in the
face of trouble. For those familiar with TCP/IP implementation details, Postfix implements its own analog of
the TCP slow start algorithm
Fairness
Apart from the thundering herd controls, the Postfix delivery strategy is based on round-robin selection. The
queue manager sorts message recipients in the active queue by destination, and makes round-robin walks along
all destination queues.
On the average, Postfix will do simultaneous deliveries to the same domain only when there is not enough work
to keep all outbound SMTP channels busy. So, when AOL goes off-line and comes back, it should not stop the
system from delivering to other sites.
When mail arrives faster than Postfix can deliver it, Postfix will favor new mail over delayed mail. The idea is
that new mail should be delivered with as little delay as possible; delayed mail can be delivered while the
system would otherwise be idle.
Exponential backoff
Postfix implements per-message exponential backoff. When a message cannot be delivered upon the first
http://www.porcupine.org/postfix-mirror/queuing.html
6/26/01