Comments - What to Do if MariaDB Doesn't Start
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As Leho mentioned it may be also a case, f.e. if you update your Debian 8 installation to Debian 9. I had my datadir symlinked from /var/lib/mysql to my /home/mysql and I was getting errcode 13 Permission denied, even if mysql users had all the chmod's properly set. I had to change ProtectHome to false in /etc/systemd/system/multi-user.target.wants/mariadb.service
Further to the Debian 9 issue where you get Permission Denied if your data dir is located under /home....
The correct way to override this setting is to make a local copy in /etc/systemd/system:
1: sudo cp /lib/systemd/system/mariadb.service /etc/systemd/system
2. Edit /etc/systemd/system/mariadb.service and change ProtectHome to be false (ProtectHome=false). Save the file
3. sudo systemd daemon-reload # refresh the systemd services so it sees your override file
4. sudo systemctl start mysql # and it should finally start
I spent ages searching for AppArmor and SELinux fixes to this problem, when it's just systemd preventing MySQL from writing to /home.
The more correct is to create /etc/systemd/system/mariadb.service.d/homedir.conf
This way the rest of the systemd file is preserved and other changes during upgrade still occurs. See MariaDB systemd documentation: https://mariadb.com/kb/en/library/systemd/. Thanks for contributing, this form of failure sure be documented here too.