On 10/25/2012 11:16 PM, Ben Morrow wrote:
At 1AM +0300 on 26/10/12 you (Robert JR) wrote:
On 2012-10-26 00:15, Ben Morrow wrote:
As Stan said earlier, this is a Postfix question. The rule for
[Looking back at the thread it wasn't Stan, it was Dennis Guhl. Sorry about that.]
I prodded him a second time, might have been off-list, and he finally posted there. So call it a team effort. ;)
Wietse has already replied, and in typical fashion, asked for "concrete" evidence that Postfix was performing fcntl before dotlock, because he obviously knows better than anyone that Postfix applies a dotlock first, which you already explained here.
dotlocking is that you must create the .lock *before* opening the file, in case whoever has it locked will be replacing the file altogether; but with fcntl locking you must acquire the lock *after* opening the file, since that's the way the syscall works. This means that if Postfix is going to use both forms of lock, it has to acquire a dotlock before it can look for a fcntl lock.
In other words: the methods in mailbox_delivery_lock are *not* tried in order, because they can't be. Dotlock is always tried first.
You should have compatible locking settings for all your programs accessing your mboxes. If Postfix is using dotlock, Dovecot should be using dotlock as well. If you don't have any local programs (mail clients, for instance) which require dotlocks, you should probably change Postfix to just use fcntl locks.
<doc stuff snipped>
but if that is the case you might as well configure everything to just use fcntl locks, and forget dotlocks altogether.
Yep. Postfix can use either or both. And, surprise, recommends using maildir to avoid mailbox locking entirely.
Stan's earlier point is fundamentally correct: if you can treat the Dovecot mailstore as a black box, with mail going in through the LDA and LMTP and mail coming out through POP and IMAP, your life will be much easier. Traditional Unix mailbox locking strategies are *completely* insane, and if all you are doing is delivering mail from Postfix and reading it from Dovecot it would be better to avoid them altogether, and switch to dbox if you can. However, if you have any other programs which touch the mail spool (local or NFS mail clients, deliveries through procmail) this may not be possible.
And since this is a POP only server, users' MUAs should be deleting after download, so there shouldn't be much mail in these mbox files at any given time, making migration to maildir or dbox relatively simple.
When using Dovecot LDA you'll eliminate the filesystem level locking problems with mbox. However, you may still have read/write contention within Dovecot, such as in your 20MB download as new mail arrives example, especially if the new message has an xx MB attachment. I don't believe Dovecot is going to start appending a new message while it's still reading out the existing 20MB of emails. Depending on how long this takes Dovecot may still issue a 4xx to Postfix, which will put the new message in the deferred queue. With maildir or dbox, reading existing mail and writing new messages occurs concurrently, as each message is a different file.
-- Stan