Increasing mail_max_userip_connections and sys resources

[Joseph Tam]

> In our low-traffic server we have always kept the default value for IMAP
> mail_max_userip_connections (10).
>
> The server has been working fine! (Thank you Timo for this!)
>
> However, recently we have been having:
>
>    Maximum number of connections from user+IP exceeded
>
> errors both for IMAP and webmail users.
>
> Thus, we have now changed the value to 100.
>
> The questions:
>
> 1.Might this lead to overloading the server?
> 2. Could another dovecot setting prevent the above setting from being
> applied (e.g. due to connections exhaustion), i.e. should we also change
> other setting(s)?
> 3. What should we take into account when deciding the value for
> mail_max_userip_connections?

It depends on how many users you have, the number of simultaneous clients
they use, and the #connections each client opens.  Potentially, you could
exhaust connection limits --  they usually don't max out memory or CPU
or disk since most of the connections are idle.  (There is a new IDLE
hibernate feature that might help to minimize memory use by idle imap
worker processes.)

I haven't found a value that gets rid of users running into connection
limits.  If I set it to n, the next day someone will choke on n+1.  This is
a snapshot connections  (count : #connections) which shows the
typical decay:

      25 1
      14 2
      14 3
       7 4
       9 5
       1 6
       2 7
       1 8
       1 10

Occasionally, some user's connection demand will spike and exceed the limit because

 	- they started yat another client
 	- they started global operation like searching
 	- they're using a network that support roaming IP
 	(I've tracked some users across the city on their bus ride.)

Pragmatically, I set it high enough so that it meets the need of most
clients, then deal with problems on a case by case basis (e.g. get
them to lower their mail client's idle connection setting, or asking
them to reduce the number of active clients by logging out).  It also
prevents what this setting was designed for: connection starvation by
busy/buggy clients.  Usually, hitting the limit doesn't produce problems
as mail clients are smart enough to close idle connections or reuse them.

If you only got a dozen users, setting it to 100 is feasible.  If you
got 1000 users, probably not.

> 4. How can we set a different value to this directive for webmail
> connections (coming from 127.0.0.1, ::1)?

I don't know if there is another method, but at the very least, you
can start another dovecot instance with another config file that does
specific things for 127.0.0.1.

Joseph Tam <jtam.home at gmail.com>