[Alpine-info] Very slow IMAP CHECK
markrcrispin at panda.com
Mon Mar 16 09:56:39 PDT 2009
On Mon, 16 Mar 2009, Chris Wilson wrote:
> I believe that this is due at least in part to the IMAP CHECK command,
> which Alpine seems to run quite regularly. My IMAP server, Dovecot, takes
> 40 seconds to checkpoint my mailbox.
The purpose of IMAP checkpoints is to allow a sever to defer necessary,
but potentially time-consuming, maintenance tasks to a checkpoint, instead
of per command. The SELECT, EXAMINE, EXPUNGE, LOGOUT, CLOSE, and CHECK
commands all do a checkpoint. In the case of CHECK, that is all that is
Note that word "necessary". Checkpoint activity includes such "optional"
behavior as saving flag changes in traditional UNIX format. The VERY SAME
people who complain about checkpoints in traditional UNIX format (and
demand that checkpoints be removed) are the ones who scream bloody murder
when their work isn't saved.
The whole point of the IMAP CHECK command is to give the client some say
as to when in-session checkpoints happen. Otherwise, one of two bad
things will happen with a long-lived session:
 checkpoints never happen, and at some point a LOT of work is lost.
 the server unilaterally decides to checkpoint, regardless of whether
or not it is a good time for the client or user.
In the case of maildir, an endemic weakness in maildir (especially when
the mailbox becomes very large) is that messages can go missing.
Dovecot's rescan finds the lost messages and brings them back into the
fold. If you don't mind messages vanishing, perhaps forever, then you may
want to disable checkpoints. Otherwise, it's a BAD idea.
The church of maildir called me numerous vicious names when I said that
maildir had scalability problems. Supposedly, I was ignorant and stupid
and evil and etc. Your problem reminds me of H.G. Wells' epitaph:
"I told you so. You DAMNED fools."
In this case, it applies both to using maildir and to disabling
If you modify Alpine in this area at all, I recommend that your
modification take the form of making the decision to checkpoint be
smarter. That is, instead of removing checkpoints or making them happen
less often, Alpine should look for better opportunities to get in a
checkpoint. Extended user inactivity points to a very obvious time to do
Also, now that Alpine has threading, it would be an excellent idea if the
composer would do the various mailbox background tasks for the mailbox in
a separate thread rather than blocking the user interface in the composer,
and then just have a mutex over any non-composer command.
-- Mark --
Democracy is two wolves and a sheep deciding what to eat for lunch.
Liberty is a well-armed sheep contesting the vote.
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