[Mimedefang] ALLOW_NEW_CONNECTIONS_TO_QUEUE behavior

[Amit Gupta]

Hi, I wanted to get a better understanding of of the nuances of
ALLOW_NEW_CONNECTIONS_TO_QUEUE in a high volume burst scenario.

MX_MAXIMUM=20
MX_QUEUE_TIMEOUT=300

Assume a burst of emails comes in and all 20 md processes are busy working.
1) What should the 21st and higher connecting clients experience?
2) Is there a configurable limit to the number of queued connections?
3) What's the downside of letting the connections queue other than
holding open more TCP connections?
4) Any other tips on the settings you use in production?

The reason I'm asking is because we recently had a burst of traffic
that caused the IO on our server to go up to a point where everything
became unresponsive for a couple minutes. We had our MX_MAXIMUM set
to 110.
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[Dianne Skoll]

On Sat, 26 Aug 2017 22:52:15 -0700

Post by Amit Gupta
Assume a burst of emails comes in and all 20 md processes are busy working.
1) What should the 21st and higher connecting clients
experience?

Sendmail will appear to respond more slowly than usual.

Post by Amit Gupta
2) Is there a configurable limit to the number of queued
connections?

Yep. The "-q" option to mimedefang-multiplexor. See the -q and -Q
command-line options in the mimedefang-multiplexor(8) man page.

Post by Amit Gupta
3) What's the downside of letting the connections queue
other than holding open more TCP connections?

Holding open more connections and using more milter threads.

Post by Amit Gupta
4) Any other tips on the settings you use in production?

If you actually need to queue requests more than occasionally, you
need a more powerful server. The queueing feature is supposed to handle
transient bursts of traffic. It isn't meant to squeeze more steady-state
performance out of a server.

Post by Amit Gupta
The reason I'm asking is because we recently had a burst of traffic
that caused the IO on our server to go up to a point where everything
became unresponsive for a couple minutes. We had our MX_MAXIMUM set
to 110.

How much RAM did you have? You really, really, really don't want a
MIMEDefang scanning server to start swapping.

Regards,

Dianne.
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