Tuning pop3-login client_limit

[Martin Olsen]

Good day / evening / morning good dovecot people:

Overview:

We are seeing the following issue on one of our servers:

pop3-login: Error: master(pop3): net_connect_unix(pop3) failed: Resource temporarily unavailable - http://wiki2.dovecot.org/SocketUnavailable (client-pid=5418, client-id=1, rip=__REMOVED__, created 534 msecs ago, received 0/4 bytes)

We tried tuning the pop3-login -> client_limit parameter. However, as soon as we increased this parameter (and after restarting dovecot), we saw an onslaught of lmtp connection errors in postfix (unable to deliver via lmtp to dovecot). This led us to set the pop3-login -> client_limit back to 1000 again and the lmtp errors disappeared.

This leads me to think that we should tune some other parameters as well. Increase overall limits or such. It seems like when increasing the client_limit for pop3-logins that somehow affected the lmtp deliverability.

If anyone has any input on this that would be much appreciated.

Cheers,
Martin

--

Version and config:

# dovecot --version
2.2.36 (1f10bfa63)

# dovecot -n
# 2.2.36 (1f10bfa63):
# OS: Centos 7.7
# Hostname:
auth_mechanisms =
auth_verbose = yes
base_dir = __REMOVED__
default_client_limit = 2500
disable_plaintext_auth = no
doveadm_password =
first_valid_uid = 89
last_valid_uid = 89
lmtp_rcpt_check_quota = yes
login_greeting =
login_trusted_networks =
mail_gid = 89
mail_plugins = " notify replication quota"
mail_uid = 89
mbox_write_locks = fcntl
namespace inbox {
  inbox = yes
  location =
  prefix =
}
passdb {
  args =
  driver = ldap
}
plugin {
  mail_replica = tcps:
  quota = maildir:User quota
  quota_rule =
}
protocols = pop3 lmtp
replication_full_sync_interval = 3 hours
replication_max_conns = 20
service aggregator {
  fifo_listener replication-notify-fifo {
    group = postfix
    mode = 0640
    user = postfix
  }
  unix_listener replication-notify {
    group = postfix
    mode = 0640
    user = postfix
  }
}
service auth {
  unix_listener auth {
    mode = 0660
  }
}
service doveadm {
  inet_listener {
    port = 12345
    ssl = yes
  }
}
service lmtp {
  process_min_avail = 10
  unix_listener lmtp {
    group = postfix
    mode = 0640
    user = postfix
  }
}
service pop3-login {
  client_limit = 1000
  process_limit = 2048
  process_min_avail = 10
  service_count = 1
}
service pop3 {
  client_limit = 1
  process_limit = 2048
  process_min_avail = 0
  service_count = 1
}


-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://dovecot.org/pipermail/dovecot/attachments/20210413/4e7def79/attachment-0001.html>