[Dovecot] Maildir locking by LDA of dovecot

[Mike Grozak]

Hi all!

First of all, I want ti thank the whole Dovecot community (developers
for developing and fast responses, users for populating and responses
too). Dovecot is a really good and fast IMAP server - it serves near
3000 in our installation.

Now I have a problem and request community's help its resolving;)
Dovecot version: 1.0.13, compiled with vpopmail support.
I have an account spam at domain.com (:-), in which we put all our spam (we
gather statistic later). There can be 10 and more delivery attempts in a
second to maildir of this account during the spam attacks on our server.
The queue of our server can grow up to 20k messages, and all of these
messages are sent to spam at domain.com. I found a lot of such strings in
dovecot.log file during such DoS attack:

dovecot.log:deliver(spam at domain.com): May 16 04:18:48 Info:
msgid=<01c8b6da$a8bacf80$b7af2abe at sale>: save failed to INBOX: Timeout
while waiting for lock
dovecot.log:deliver(spam at domain.com): May 16 04:18:59 Info:
msgid=<6611811842.20080516012444 at poiusadfhnvxzmdiasdasgkhlsdkfg.com>:
save failed to INBOX: Timeout while waiting for lock
dovecot.log:deliver(spam at domain.com): May 16 04:20:06 Info:
msgid=<01c8b6a8$7b552e00$a0a4034c at sale>: save failed to INBOX: Timeout
while waiting for lock
dovecot.log:deliver(spam at domain.com): May 16 04:20:14 Info:
msgid=<360825920.32055246029506 at mirgames.ru>: save failed to INBOX:
Timeout while waiting for lock
dovecot.log:deliver(spam at domain.com): May 16 04:20:50 Info:
msgid=<01c8b6b1$0df9cb00$cc36dfc9 at tek2>: save failed to INBOX: Timeout
while waiting for lock
dovecot.log:deliver(spam at domain.com): May 16 04:21:00 Info:
msgid=<087901c7ca2d$ef272640$1e01a8c0 at telefonia.InterCable.net>: save
failed to INBOX: Timeout while waiting for lock
dovecot.log:deliver(spam at domain.com): May 16 04:21:01 Info:
msgid=<000701c8b6d2$01ff999f$c491aca7 at snoueiij>: save failed to INBOX:
Timeout while waiting for lock
dovecot.log:deliver(spam at domain.com): May 16 04:22:32 Info:
msgid=<000801c8b69b$060c0230$3cd8a4b7 at cpbvlrx>: save failed to INBOX:
Timeout while waiting for lock
dovecot.log:deliver(spam at domain.com): May 16 04:22:45 Info:
msgid=<000801c8b6a5$04ba6412$9910848d at aldldr>: save failed to INBOX:
Timeout while waiting for lock
dovecot.log:deliver(spam at domain.com): May 16 04:22:45 Info:
msgid=<031401c772e2$adc5be40$1e01a8c0 at mtu-net.ru>: save failed to INBOX:
Timeout while waiting for lock
dovecot.log:deliver(spam at domain.com): May 16 04:23:08 Info:
msgid=<01c8b6db$0222fe80$b7af2abe at info>: save failed to INBOX: Timeout
while waiting for lock

What can I do for resolving this locks? The queue of my server becomes 
really big!
May be, while looking into uidlist file, can dovecot LDA lock it? And
other deliver processes can't open it and wait for him?

$ dovecot --version
1.0.13

# dovecot -n
# 1.0.13: /etc/dovecot.conf
log_path: /var/log/dovecot.log
protocols: imap pop3
ssl_disable: yes
login_dir: /var/run/dovecot/login
login_executable(default): /usr/libexec/dovecot/imap-login
login_executable(imap): /usr/libexec/dovecot/imap-login
login_executable(pop3): /usr/libexec/dovecot/pop3-login
login_greeting: Server is ready.
login_process_per_connection: no
login_greeting_capability(default): yes
login_greeting_capability(imap): yes
login_greeting_capability(pop3): no
login_max_connections: 1024
first_valid_uid: 39
last_valid_uid: 39
first_valid_gid: 39
last_valid_gid: 39
fsync_disable: yes
maildir_copy_with_hardlinks: yes
mail_executable(default): /usr/libexec/dovecot/imap
mail_executable(imap): /usr/libexec/dovecot/imap
mail_executable(pop3): /usr/libexec/dovecot/pop3
mail_plugins(default): quota imap_quota
mail_plugins(imap): quota imap_quota
mail_plugins(pop3): quota
mail_plugin_dir(default): /usr/lib/dovecot/imap
mail_plugin_dir(imap): /usr/lib/dovecot/imap
mail_plugin_dir(pop3): /usr/lib/dovecot/pop3
auth default:
   mechanisms: plain login cram-md5
   default_realm: tversu.ru
   cache_size: 10240
   user: vpopmail
   username_format: %Lu
   passdb:
     driver: vpopmail
     args: cache_key=%u dovecot
   userdb:
     driver: vpopmail
   socket:
     type: listen
     client:
       path: /var/spool/postfix/private/auth
       mode: 438
       user: postfix
       group: postfix
     master:
       path: /var/run/dovecot/auth-master
       mode: 438
plugin:
   quota: maildir:ignore=Trash

---------------------------
WBR, Mike Grozak, TvSU IC