Hello,

I'm trying to experiment with Dovecot and Solr server.
I have >30k email addresses that I want to index to speed up searching and save IOPS on mail servers.
For now - I'm doing some experiments and I'm testing how it is working. I'm thinking about adding one additional server with Solr and
configure all mail servers to use that server.

I have some questions.
1. I have 15 mail servers. It will be good If I add new server with Solr and use it on all Dovecot servers? Or maybe I should install Solr on all mail servers?

You need to start somewhere. If you've never played with Solr before I suggest you start with one and get it working before you explore "sharding". When you're ready for that you should consult the solr mailing list. The importance of enough RAM for Solr cannot be overstated.

2. I notice - I have mail account with 3GB of mail. Index files in mail dir has 5MB. After indexing mailbox in Solr - index files has 15MB. What changes in those files? FTS indexing adds something to that files - but what?

What mail storage format are you using?  dbox?

Thinking...I believe that Dovecot records which mails have been reported to the FTS.  That may help account for the increased size.