[debian-mysql] my.cnf problems

[Kristian Nielsen]

I think this discussion is missing the point here.

The my.cnf is a config file, it is owned by the user. We should not change it
in most cases. In MariaDB .deb packaging we have seen many times that users
would during upgrade install the new config to replace their old one, breaking
their setup. Those users then have no clue what went wrong or how to change
it.

The two typical ways that changing my.cnf breaks are:

 - Changes related to data directory (eg. the path to it, the innodb log size,
   etc). Then mysqld cannot start because the config and the data directory
   are not matched.

 - Changes related to other applications (eg. changing the default sql mode to
   be more strict, breaking wordpress).

(Note that _any_ change in package my.cnf can cause this; users may have
changed the critical setting locally, and then upgrade will ask to replace
their (correct) config with the new (incorrect) config. We have seen many
times that users mess up and select to replace the config).

So the problem is not my.cnf compatibility between MySQL, MariaDB, and Percona
Server. The problem is compatibility with the users data directory and with
installed applications.

The solution is to not change /etc/mysql/my.cnf. If needed, we can drop
distro-specific options into /etc/mysql/conf.d/. Eg. in the 3rd-party MariaDB
packages we have /etc/mysql/conf.d/mariadb.cnf in package mariadb-common,
which is included by the my.cnf in upstream package mysql-common (however
mariadb.cnf is actually empty, we do not have any default MariaDB specific
options enabled, nor should we, I think).

Note that _if_ we were to use separate my.cnf configuration files for MySQL,
MariaDB, and Percona Server, we would also need to separate the data
directories. So that installing eg. Percona Server would no longer
automatically take over the data directory of a previous installed MySQL
server. Because in general the data directory cannot be correctly/safely used
without the corresponding my.cnf. However, this is not how things have been
done traditionally. Eg. packages mysql-server-5.1 and mysql-server-5.5 have
shared configuration and data directory, and the situation with MariaDB /
Percona Server is no different from the 5.1/5.5 distinction.

 - Kristian.