dovecot - May 2015 - Different mdbox_rotate_size for primary and alternate storage

Hello.
In order to speed up backups of very very old messages I would like to set
two different limits for mdbox_rotate_size. Like, 50M for primary storage
and 100M or larger for alternate storage.

There is no mention in docs or such a possibility, so I assume it is not
possible. Is that correct?


While I am at it, is it possible to configure primary storage as maildir
(sturdy indexes) and altstorage as mdbox (more delicate indexes)?

Thanks,
Paolo