dovecot - Sep 2010 - Maybe wrong quota recalculation behaviour on overquota delivery try

Hi all,
we are experencing some, may be strange, slowness in our dovecot/postfix 
mail system.
We have three dovecot 2.0.1 server with LMTP, POP3 and IMAP enabled.
Three shared OCFS2 filesystem are used for mail storage.
We have too a load balancer based on dovecot director in front of these 
three server.
Our SMTP cluster receives mail and send by LMTP to load balancer.

Everything work flowless, but sometime, under heavy load, three server 
slow down a lot.
Our mail gateway logs a lot of:
status=deferred (host 10.194.1.20[10.194.1.20] said: 451 4.4.0 Remote 
server not answering (DATA output timeout)

IP 10.194.1.20 is dovecot director.

Looking at mbox servers log nothing seem to happen, but stracing lmtp 
process we can see stat system calls to all Maildir files, as if dovecot 
was recalculating quota and regenerating maildirsize file.

It happens always when LMTP tries to deliver a mail to an overquota account.

Is this possible and right? How can we look deeper in system to have 
more information?

Best regards

Luca

PS: dovecot configuration for maildir servers is:

# 2.0.1: /etc/dovecot/dovecot.conf
# OS: Linux 2.6.26-2-686-bigmem i686 Debian 5.0.5
auth_master_user_separator = *
auth_verbose = yes
disable_plaintext_auth = no
mail_location = maildir:~/Maildir
mail_plugins = $mail_plugins notify mail_log quota
managesieve_notify_capability = mailto
managesieve_sieve_capability = comparator-i;octet 
comparator-i;ascii-casemap fileinto reject envelope encoded-character 
vacation subaddress comparator-i;ascii-numeric relational regex 
imap4flags copy include variables body enotify environment mailbox date 
spamtest spamtestplus virustest
passdb {
   args = /etc/dovecot/dovecot-sql.conf.ext
   driver = sql
}
plugin {
   mail_log_events = delete undelete expunge copy mailbox_delete 
mailbox_rename
   mail_log_fields = uid box msgid from size
   quota = maildir:User quota
   quota_warning = storage=95%% quota-warning.sh 95 %u
   quota_warning2 = storage=80%% quota-warning.sh 80 %u
   sieve = ~/.dovecot.sieve
   sieve_dir = ~/sieve
   sieve_global_dir = /etc/dovecot/sieve/global/
   sieve_global_path = /etc/dovecot/sieve/default.sieve
}
protocols = imap pop3 lmtp sieve
service auth {
   unix_listener auth-userdb {
     group = dovecot
     mode = 0600
     user = dovecot
   }
}
service director {
   inet_listener {
     port = 8100
   }
}
service lmtp {
   inet_listener lmtp {
     address = 10.194.1.31 127.0.0.1 ::1
     port = 24
   }
}
service managesieve-login {
   inet_listener sieve {
     address = 10.194.1.31 127.0.0.1 ::1
     port = 4190
   }
   process_min_avail = 0
   service_count = 1
}
service quota-warning {
   executable = script /virtual/shared/quota-warning.sh
   unix_listener quota-warning {
     user = vmail
   }
   user = dovecot
}
ssl_cert = </etc/ssl/certs/dovecot.pem
ssl_key = </etc/ssl/private/dovecot.pem
userdb {
   args = /etc/dovecot/dovecot-sql.conf.ext
   driver = sql
}
verbose_proctitle = yes
protocol lmtp {
   mail_plugins = $mail_plugins quota sieve
}
protocol lda {
   mail_plugins = $mail_plugins quota
}
protocol imap {
   mail_plugins = $mail_plugins quota imap_quota
}
protocol pop3 {
   mail_plugins = $mail_plugins quota
}

On Thu, 2010-09-02 at 19:14 +0200, Luca Palazzo wrote:
> 
> Looking at mbox servers log nothing seem to happen, but stracing lmtp 
> process we can see stat system calls to all Maildir files, as if
> dovecot 
> was recalculating quota and regenerating maildirsize file.
> 
> It happens always when LMTP tries to deliver a mail to an overquota
> account.
Yes, that's pretty much how Maildir++ quota works. If maildirsize file
doesn't only contain a single line showing the quota, or if it's older
than 15 minutes, it's recalculated. The maildir filenames really should
have ,S=size in the filenames, otherwise quota recalculation is horribly
slow.

Maybe you should switch to dict quota with file backend.
http://wiki2.dovecot.org/Quota/Dict