We are gradually rolling out Dovecot (IMAP only, no POP3) to our
customer base. We are replicating between a pair of CentOS 7 boxes.
All has been working wonderfully. However, to be sure our rollout
continues to go smoothly, we put together a simple benchmark client
program to fire up X persistent IMAP connections (using hundreds of
mailboxes) that login, list the folders, select the INBOX, periodically
FETCH an email from the INBOX, and otherwise sit in an IDLE loop. As
long as we keep the number of concurrent IMAP connections under 2k,
everything works fine. 2,000 processes generate a minimal load on the
server. However, 2048 is a "hard limit" for number of IMAP processes on
a box the way things are configured right now. The number (2048) is a
bit too magical to be anything but a misconfiguration of either some
kernel limit or a dovecot configuration limit. I have been unable to
figure out where else to look. I have pasted in my dovecot config below
along with the kernel setting for max_user_instances.
Hoping someone can tell me what stupid mistake I have made or what else
to check.
Just an aside, we front-end this with a separate set of proxy servers
that provide an SSL front-end, hence, no SSL in the dovecot config.
/proc/sys/fs/inotify/max_user_instances = 4096
# 2.2.10: /etc/dovecot/dovecot.conf
# OS: Linux 3.10.0-123.13.1.el7.x86_64 x86_64 CentOS Linux release
7.0.1406 (Cor
e)
auth_master_user_separator = *
auth_mechanisms = plain login
default_internal_user = sysadm
default_login_user = sysadm
default_vsz_limit = 512 M
disable_plaintext_auth = no
doveadm_password = **************
doveadm_port = 12345
imap_max_line_length = 128 k
listen = *
mail_gid = 2001
mail_location = maildir:~/Maildir
mail_plugins = notify replication
mail_uid = 2001
managesieve_notify_capability = mailto
managesieve_sieve_capability = fileinto reject envelope
encoded-character vacation subaddress comparator-i;ascii-numeric
relational regex imap4flags copy include variables body enotify
environment mailbox date ihave
mbox_write_locks = fcntl
namespace inbox {
inbox = yes
location =
mailbox Drafts {
special_use = \Drafts
}
mailbox Junk {
special_use = \Junk
}
mailbox Sent {
special_use = \Sent
}
mailbox "Sent Messages" {
special_use = \Sent
}
mailbox Trash {
special_use = \Trash
}
prefix =
}
passdb {
args = scheme=PLAIN /z/imap/usr/master-users
driver = passwd-file
master = yes
pass = yes
}
passdb {
args = scheme=PLAIN username_format=%u /z/imap/usr/users
driver = passwd-file
}
plugin {
mail_replica = tcp:imap0s
sieve = ~/.dovecot.sieve
sieve_dir = ~/sieve
sieve_max_actions = 32
sieve_max_redirects = 4
sieve_max_script_size = 1M
}
protocols = imap pop3 sieve
service aggregator {
fifo_listener replication-notify-fifo {
mode = 0666
user = sysadm
}
unix_listener replication-notify {
mode = 0666
user = sysadm
}
}
service auth {
unix_listener auth-userdb {
mode = 0777
}
}
service doveadm {
inet_listener {
port = 12345
}
user = sysadm
}
service imap {
process_limit = 4096
}
service managesieve-login {
inet_listener sieve {
port = 4190
}
}
service managesieve {
process_limit = 4096
}
service replicator {
process_min_avail = 1
}
ssl = no
userdb {
args = username_format=%u /z/imap/usr/users
driver = passwd-file
}
protocol lda {
mail_plugins = sieve notify replication
}
protocol imap {
mail_max_userip_connections = 40
}
protocol sieve {
managesieve_max_line_length = 65536
}
