My disk ran full while akonadi and kmail were running. The symptoms I noticed were that kmail wasn't able to display mail content anymore - I'd guess because it tried to write them to a tempfile on the full disk first. After freeing some disk space again, akonadi refuses to start. The debug output shows that it tries to start a mysql instance (which fails), however I had it configured to use the system-wide mysql. In other words: akonadi destroyed its config file when the disk was full! Reproducible: Didn't try Steps to Reproduce: 1. start akonadi and kmail, ideally with akonadi configured to use the system-wide mysql 2. let another process fill up your disk (if you don't have a better idea, dd if=/dev/zero of=~/fill-my-disk) 3. quit kmail and akonadi 4. (try to) restart akonadi Expected Results: The config file should not be destroyed when the disk is full.
Just FYI: restoring ~/.config/akonadi/ from the backup fixes this issue. The "broken" ~/.config/akonadi has lots of agent_config_akonadi*.dat files with 0 bytes. I'd guess akonadiserverrc was also 0 bytes, but can't verify it because it was (probably) overwritten by akonadiconsole.
This bug has only been reported for versions older than KDEPIM 4.14 (at most akonadi-1.3). Can anyone tell if this bug still present? If noone confirms this bug for a recent version of akonadi (part of KDE Applications 15.08 or later), it gets closed in about three months.
Just as announced in my last comment, I close this bug. If you encounter it again in a recent version (at least 5.0 aka 15.08), please open a new one unless it already exists. Thank you for all your input.