Look at mbox_read_locks and mbox_write_locks parameters. From http://wiki2.dovecot.org/MailboxFormat/mbox:

There are at least four different ways to lock a mbox: • dotlock: mailboxname.lock file created by almost all software when writing to mboxes. This grants the writer an exclusive lock over the mbox, so it's usually not used while reading the mbox so that other processes can also read it at the same time. So while using a dotlock typically prevents actual mailbox corruption, it doesn't protect against read errors if mailbox is modified while a process is reading.

• flock: flock() system call is quite commonly used for both read and write locking. The read lock allows multiple processes to obtain a read lock for the mbox, so it works well for reading as well. The one downside to it is that it doesn't work if mailboxes are stored in NFS.

• fcntl: Very similar to flock, also commonly used by software. In some systems this fcntl() system call is compatible with flock(), but in other systems it's not, so you shouldn't rely on it. fcntl works with NFS if you're using lockd daemon in both NFS server and client.

• lockf: POSIX lockf() locking. Because it allows creating only exclusive locks, it's somewhat useless so Dovecot doesn't support it. With Linux lockf() is internally compatible with fcntl() locks, but again you shouldn't rely on this.

but:

• lockf is unsupported by dovecot
• flock doesn't work over NFS
• fcntl works over NFS only if lockd is running on both sides (useful only if you can setup lockd on your storage appliance)
• dotlock is not so performing, and sometimes could give read errors, but should work (and protect you from mailbox corruption)

Anyway take a look also to maildir format (almost entirely lockless).

Hope this helps.

Bye, gc

Il 11/07/2011 15.56, satish patel ha scritto:

Anyone ?

...

From: satish_lx@hotmail.com To: dovecot@dovecot.org Date: Fri, 8 Jul 2011 17:08:40 +0000 Subject: [Dovecot] Dovecot NFS locking issue

Hi All,

We had 15 imap/pop UW-imap servers behind load balancer and mail storage was NFS for almost around ~30,000 users on it. Currently we migrate UW-imap to dovecot with fcntl NFS locking. After migration on dovecot it started error related locking see following error

Error: fcntl() failed with mbox file /home/w/a/wahmed/.inbox: No locks available Error: fcntl() failed with mbox file /home/e/d/edurand/.inbox: No locks available

Look like my NFS locking getting overload or something its (netapps NFS server)

what you suggest to fix this issue ? how uw-imap was working with NFS before ?

Appreciate your help.

-S

-- Giulio Casella giulio at dsi.unimi.it System and network manager Computer Science Dept. - University of Milano