fast doveadm search results

[Vu Ngoc VU]

hello,
I've posted same question here months ago.
But obtaine no answer, even not a "you question is stupid".
So I don't know what to think about that.

My goal is to be able to find mails quickly with "doveadm search".
We are using dovecot in a corporate. And some managers, sometimes ask to delete some messages.
These messages are "unwanted" ones: mistakenly sent, defamatory...

So, that situations appear rarely, but when asked, they expected me to deleted quickly these messages from few to thousands of users mailboxes.

Is that possible to change behaviour of Dovecot, some headers like sender, message-id, date are stored in cache for at least 48h? (7 days would be better).

I've read this page : http://wiki2.dovecot.org/Design/Indexes/Cache but cannot understand every parts.


"A specific decision can be forced by ORing it with MAIL_CACHE_DECISION_FORCED."
Is OR in this sentence like AND/OR comparators?
Where to put "MAIL_CACHE_DECISION_FORCED"? 
"mail-cache-decisions.c file contains the rules how Dovecot changes the decisions." <-- is it in this file?

My setup is 2.2.9 on Ubuntu 14.04 running on OpenVZ containers with 2 Director and 2 imap/pop3 backends.
The storage is shared via NFS. Is this the performance problem? If dovecot has metadata I want in its cache, I cannot see how this is a problem.

Can these settings be useful for my case?
mail_always_cache_fields = 
mail_cache_fields = flags
mail_cache_min_mail_count = 0
mail_never_cache_fields = imap.envelope

Thanks.