I am setting up a web server using Exim 4 on Debian 8 and, as usual, I was going to use MySQL as backend to store the settings related to the domains and users (like quota, emails, aliases, forwards, enabled/disabled account/domain, antispam/antivirus on/off, whitelist, blacklist and so on). At some point I stopped, using MySQL as backend for Exim 4 would simplify my life a lot but I think that it is not the best solution for performance.
There are consistent differences in terms of performance and/or resource consumption between using MySQL, SQLite, DBM or plain text files? For example, would be a good solution put a CDB or a DBM in a in-memory filesystem?
The load on the server will be very low, there will be more or less 50 websites with not more than 150/200 email accounts in total, so it doesn't really matter but I want to do some experiments and tests to find out the best possibility. For that reason I don't want to put the spool in memory, there would be very few emails.
I would like to do some tests but I have very little time right now, so any direct/indirect experience would be really appreciated!