Resolving a MySQL startup error Job failed to start, ran as an Ubuntu service systemctl start mysql.service (or service mysql start) configured to have its data folder on another partition.

After moving to a new PC with a smaller SSD partition and a larger one for data, I configured MySQL to use its data folder on a directory on that partition.

TLDR: The solution to this is a misconfigured partition-folder modes. Make sure all upper directories have 0755 mode.

For more information about possible error logs and configuration, see below.

Configuration and Syslog Error⌗

Even with the correct configuration and data-folder ownership, above issue occurred.

Checking the /etc/mysql/mysql.conf.d/mysqld.cnf config file:

datadir = /data/mysql_data

And the folder’s ownership is properly set to:

$ stat -c "%U %G" mysql_data/
mysql mysql

=> Seems good

Syslog Error⌗

Looking at /var/log/syslog I found:

mysqld: Can't change dir to '/data/mysql_data/' (Errcode: 13 - Permission denied)
...
...  0 [Warning] Can't create test file /data/mysql_data/myhost.lower-test

Workaround⌗

In many places there are suggestions resolving this issue with tweaking apparmor, I tried that for quite a while but without success. And since I had to move along I just started MySQL inside as a docker container:

$ docker run --rm -p 3306:3306 --name=mysql57 -v /data/mysql_config/mysql.cnf:/etc/my.cnf -v /data/mysql_data:/var/lib/mysql -d mysql/mysql-server:5.7

which works fine. Just when connecting to it, the connection protocol has to be set to TCP:

$ mysql -u admin -ppwd --protocol=tcp ...

Solution⌗

After a while I came across namei and finally saw that the parent folder required a chmod to make it readable/executable. From the man page: namei - follow a pathname until a terminal point is found and prints the mode and ownership of the recursive directories.

$ namei -l /data/mysql_data
f: /data/mysql_data/
drwxr-xr-x root   root   /
drwx------ tokosh tokosh data
drwxr-xr-x mysql  mysql  mysql_data

And after changing the data folder to chmod 0755 /data, the MySQL service was finally functional. ε-(´・｀) ﾌ