From: Wietse Venema (no email)
Date: Tue May 01 2007 - 15:37:15 EDT
Peter Rabbitson:
> Wietse Venema wrote:
> > Peter Rabbitson:
> >> Hi,
> >>
> >> I am experiencing 421 errors between my secondary and primary MXes, and
> >> it seems it is cause by lack of connection caching.
> >
> > What is the error message?
>
> There is no error message as such, see below.
Any reasonable SMTP server sends 421 followed by some text that
explains why it hangs up.
Having looked at the text below, I think your problem is that
you are making an insane number of SIMULTANEOUS connections
to the primary MX host.
> >> http://www.postfix.org/postconf.5.html#smtp_connection_cache_on_demand
> >> misses to explain what is "high volume of mail in the active queue".
> >> When is exactly connection caching activated?
> >
> > Roughly, it is activated when the active queue contains another
> > message before the current delivery is completed.
>
> If the primary MX is down for an extended period of time and a large
> queue accumulates on the backup, all messages are rushed to the primary
> MX in what it seems separate smtp connections. At least I was able to
> count as many smtp processes in `ps` as 2/3 of the number of queued
> messages, right after I issue `postfix flush`. If I specify explicit
> caching for the particular mx host, things work as expected. I guess
> there is not enough time for the caching on demand to activate when
> doing a flush or having enough queued messages to simulate one.
This is not a surprise.
If the number of SIMULTANEOUS connections is 2/3 the number of
queued messages, then most connections will never be reused because
the mail is already delivered.
I suggest that you revert to no more than 10-20 SIMULTANEOUS
connections to the primary MX (or to any machine).
/etc/postfix/main.cf:
smtp_destination_concurrency_limit=20
relay_destination_concurrency_limit=20
If you do that, not only will the primary MX perform better, you
will also see connection reuse happen automatically.
Wietse
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