[Dovecot] Connection queue full on my directors

[Jan-Frode Myklebust]

I finally upgraded to v1.2.16 on my backend servers, with ldap instead of 
previously mysql, and put a couple of dovecot v2.0.8 directors in front 
of them (used haproxy previously). It seemed to work OK initially, but 
after a few hours we got bit by "Connection queue full" problems on the 
directors.

So I now set default_process_limit=512 and also:

	service anvil { 
		client_limit=1027 
	}

since dovecot said that was now required. Things seems to be working, 
but I'm afraid default_process_limit=512 is too low:

	$ ps -ef|grep dovecot/pop3-login|wc -l
	192
	$ ps -ef|grep dovecot/imap-login|wc -l
	472

so looks like I'm quite close to hitting this limit for my imap logins.

We will eventually put up some webmail servers (using imapproxy to keep
connections alive), that will be connecting over these directors. Then
we will expect to have a lot more active imap sessions over these
directors.  Should we just keep increasing default_process_limit, or per
service process_limit, and follow dovecots warnings about client_limits for
auth/anvil.. or will we be getting into other problems like running out
of file descriptors? Are there any max'es we should expect to run into ? 

Current director config:
###################################################################
$ ../../sbin/dovecot -n
# 2.0.8: /usr/local/dovecot-2.0.8/etc/dovecot/dovecot.conf
# OS: Linux 2.6.18-194.32.1.el5 x86_64 Red Hat Enterprise Linux Server release 5.5 (Tikanga) 
default_process_limit = 256
director_mail_servers = 192.168.42.7 192.168.42.8 192.168.42.9 192.168.42.10 192.168.42.11 192.168.42.28 192.168.42.29
director_servers = 192.168.42.15 192.168.42.17
disable_plaintext_auth = no
listen = *
passdb {
  args = proxy=y nopassword=y
  driver = static
}
service director {
  fifo_listener login/proxy-notify {
    mode = 0666
  }
  inet_listener {
    port = 5515
  }
  unix_listener director-userdb {
    mode = 0600
  }
  unix_listener login/director {
    mode = 0666
  }
}
service imap-login {
  executable = imap-login director
}
service pop3-login {
  executable = pop3-login director
}
ssl_cert = </etc/pki/tls/certs/mail.example.net.crt
ssl_key = </etc/pki/tls/private/mail.example.net.key
###################################################################

$ sudo /usr/local/dovecot/bin/doveadm director status
mail server ip                           vhosts users
192.168.42.7                                 10   315
192.168.42.8                                 10   292
192.168.42.9                                 10   311
192.168.42.10                                10   316
192.168.42.11                                10   292
192.168.42.28                               100  1790
192.168.42.29                               100  1787



  -jf