[Dovecot] Maildir "locking"

[Timo Sirainen]

On Thu, 2011-09-15 at 11:37 +0200, Attila Nagy wrote:
> > So you have a proxy that decides what backend server the connections are
> > redirected to? How about you do it completely without locking with
> > dsync? Moving between servers works basically the same as converting a
> > mailbox format, with the difference of "changing mail_location" you
> > "change backend server".
> > http://wiki2.dovecot.org/Tools/Dsync#example_converting
> >
> Yes, there is a proxy in front of the servers. Is dsync usable with 3rd 
> party maildir programs? (not only Dovecot uses these mailboxes)

The problems with 3rd party maildir programs come if during the move
they:

 - Expunge last message(s) from mailbox. (dsync can't know if it should
add or expunge them, so it plays it safe and adds them back)

 - Delete a mailbox. (dsync can't know if it should add or delete it, so
again it just adds it back.)

 - Remove subscriptions. (again pretty much the same reason.)

It's probably quite unlikely that they do any of this during the move.
You could even reduce the window by doing:

1. dsync backup
2. dsync backup
3. switch to new server
4. kill all existing connections
5. dsync mirror

The 3-5 steps probably take only a few seconds. The "dsync backup" then
guarantees that the destination server will look exactly like the source
server. ("dsync mirror" is used in step 5, because between steps 3-4
either server can get changes.)