disabling cleanup?

[Casey Connor]

[sorry if this is a repeat -- I thought I submitted this message but it 
has not appeared after ~24 hours]

We have a VM running postfix that receives all emails, analyzes the 
stream, and "discard"'s 100% of messages via a custom milter in smtpd 
(i.e. a pre-queue milter).

In other words, it never enqueues any email. Thus, we were surprised to 
see 'cleanup' processes running.

Is it an expected part of the postfix architecture that cleanup must run 
even for messages that are discarded?

If so, what should we do to ensure that cleanup is doing as little work 
as possible? (E.g. no point in checking headers, etc.)

In this scenario, would cleanup be doing any disk access? If so, any way 
to avoid that?

I have read some threads that imply that cleanup can't be disabled; if 
that's correct, we just need to limit it's resource usage as much as 
possible, especially in terms of disk usage.

Thanks!
-Casey