Searching 30 GB mailbox

[Einar Bjarni Halldórsson]

Hi,

We're migrating our mailboxes from Zimbra to dovecot, running on FreeBSD 
12.2. We're using mdbox and solr for fts. dovecot version 2.3.17.

One of our mailboxes gets a bcc of every outgoing mail from our system 
and it currently holds ~4.5 million messages taking 30 GB.

One of the ways we use the mailbox is that when outgoing mail is sent, 
we store the Message-ID in our database, and in our admin interface we 
can open the mail and it's fetched from IMAP by doing a 'SEARCH HEADER 
Message-ID <MSG_ID>'.

In Zimbra this was reasonably fast, but after migrating the mailbox to 
dovecot it's 30 seconds which I think we can reduce.

After turning on mail_debug I see this in our log 
https://pastebin.com/kP9H7gSB

Looks like solr finishes searching instantly, then there's ~30 seconds 
of silence in the logs, and then more log lines about fetching data from 
mail cache.

The same thing happens if I use telnet and just issue a 'SEARCH 
HEADER...' command.

How can I find out what dovecot is doing in those 30 seconds?

.einar