On Sat, Apr 02, 2011 at 06:59:42PM +0100, Mark Alan wrote:
> > > /etc/postfix/master.cf
> > > slow unix - - - - - smtp
> > > -o syslog_name=postfix-slow
> > > -o smtp_connection_reuse_time_limit=30s
> > > EOT
> > >
> > > /etc/postfix/main.cf
> > > slow_initial_destination_concurrency = 2
> > > slow_destination_concurrency_limit = 15
> > > slow_destination_concurrency_failed_cohort_limit = 5
> > > slow_destination_concurrency_positive_feedback = 1/5
> > > slow_destination_concurrency_negative_feedback = 1/8
>
> > (...) You can certainly try, and report your findings.
>
> Tried the above setup. It does not help.
> We have much more 421 with this approach than we had with our former
> setup.
The number of 421 responses is not a good metric for success. The real
metric is the resulting average time in the queue for messages that are
finally delivered.
A dymamic feedback mechanism that is able to go faster, will necessarily
elicit 421 responses at a stead rate, to keep it from reaching peak
capacity. Naturally, a statically hand-tuned configuration that stays
under the remote caps will not elicit 421 responses, but it is likely
to stay below the optimal throughput.
With sites whose 421 responses are NOT "sticky", the feedback controls
will in a variety of cases lead to a nearly optimal throughput with
some messages retried at a second MX or deferred, but the overall
delay can be smaller than conservative upper-bounds on concurrency
and message rates.
--
Viktor.
