Move mysql/postgresql from /var/lib using systemd on CentOS 7.2

What's the proper way to move /var/lib/mysql and /var/lib/postgresql using systemd and symlinks on CentOS 7.2, ideally without editing conf files?

I found a way to start both that doesn't use systemd, but can't figure out why systemd fails (I realize this is technically two separate questions (mysql and postgresql), but I think the root cause is the same here).

After installing CentOS 7.2 and several packages, including mariadb (mysql) and postgresql, I moved the directories (and initialized the databases) as follows:

: the services are already stopped but just to make sure
systemctl stop mariadb postgresql

: move the directories over
mv /var/lib/pgsql /var/lib/mysql /mnt/kemptown/

: symlink back
ln -s /mnt/kemptown/mysql /mnt/kemptown/pgsql /var/lib

: initialize postgresql and mysql
postgresql-setup initdb
mysql_install_db --user=mysql

: this is a VM, so I shut it down and snapshot it here
shutdown -h now

First, the way that works:

# mysqld_safe 
161221 13:16:22 mysqld_safe Logging to '/var/log/mariadb/mariadb.log'.
161221 13:16:22 mysqld_safe Starting mysqld daemon with databases from /var/lib/mysql

# sudo -u postgres pg_ctl -D /var/lib/pgsql/data start

server starting

Testing with mysql and sudo -u postgres psql connects to the databases properly.

This convinces me there's nothing fundamentally wrong with what I'm doing, since it does work. I can even leave symbolic-links=0 in /etc/my.cnf since it apparently only cares about symlinked databases, not whether its home directory is symlinked.

Now, restoring the snapshot, here's the way that fails:

# systemctl start mariadb postgresql

Job for postgresql.service failed because the control process exited with error code. See "systemctl status postgresql.service" and "journalctl -xe" for details.

Job for mariadb.service failed because the control process exited with error code. See "systemctl status mariadb.service" and "journalctl -xe" for details.

# systemctl -l status postgresql.service

* postgresql.service - PostgreSQL database server
   Loaded: loaded (/usr/lib/systemd/system/postgresql.service; disabled; vendor preset: disabled)
   Active: failed (Result: exit-code) since Wed 2016-12-21 13:23:06 EST; 2min 32s ago
  Process: 2395 ExecStart=/usr/bin/pg_ctl start -D ${PGDATA} -s -o -p ${PGPORT} -w -t 300 (code=exited, status=1/FAILURE)
  Process: 2385 ExecStartPre=/usr/bin/postgresql-check-db-dir ${PGDATA} (code=exited, status=0/SUCCESS)

Dec 21 13:23:06 babybrighton systemd[1]: Starting PostgreSQL database server...
Dec 21 13:23:06 babybrighton pg_ctl[2395]: pg_ctl: could not open PID file "/var/lib/pgsql/data/postmaster.pid": Permission denied
Dec 21 13:23:06 babybrighton systemd[1]: postgresql.service: control process exited, code=exited status=1
Dec 21 13:23:06 babybrighton systemd[1]: Failed to start PostgreSQL database server.
Dec 21 13:23:06 babybrighton systemd[1]: Unit postgresql.service entered failed state.
Dec 21 13:23:06 babybrighton systemd[1]: postgresql.service failed.

# systemctl -l status mariadb.service

* mariadb.service - MariaDB database server
   Loaded: loaded (/usr/lib/systemd/system/mariadb.service; disabled; vendor preset: disabled)
   Active: failed (Result: exit-code) since Wed 2016-12-21 13:23:08 EST; 2min 48s ago
  Process: 2423 ExecStartPost=/usr/libexec/mariadb-wait-ready $MAINPID (code=exited, status=1/FAILURE)
  Process: 2422 ExecStart=/usr/bin/mysqld_safe --basedir=/usr (code=exited, status=1/FAILURE)
  Process: 2384 ExecStartPre=/usr/libexec/mariadb-prepare-db-dir %n (code=exited, status=0/SUCCESS)
 Main PID: 2422 (code=exited, status=1/FAILURE)

Dec 21 13:23:06 babybrighton systemd[1]: Starting MariaDB database server...
Dec 21 13:23:07 babybrighton mysqld_safe[2422]: 161221 13:23:07 mysqld_safe Logging to '/var/log/mariadb/mariadb.log'.
Dec 21 13:23:07 babybrighton mysqld_safe[2422]: mkdir: cannot create directory ~/var/lib/mysql~: File exists
Dec 21 13:23:07 babybrighton mysqld_safe[2422]: Fatal error Can't create database directory '/var/lib/mysql/mysql.sock'
Dec 21 13:23:07 babybrighton systemd[1]: mariadb.service: main process exited, code=exited, status=1/FAILURE
Dec 21 13:23:08 babybrighton systemd[1]: mariadb.service: control process exited, code=exited status=1
Dec 21 13:23:08 babybrighton systemd[1]: Failed to start MariaDB database server.
Dec 21 13:23:08 babybrighton systemd[1]: Unit mariadb.service entered failed state.
Dec 21 13:23:08 babybrighton systemd[1]: mariadb.service failed.

Based on the messages above (and looking at /lib/systemd/system/mariadb.service and /lib/systemd/system/postgresql.service), my thoughts:

• ExecStartPre=/usr/libexec/mariadb-prepare-db-dir %n and ExecStartPre=/usr/bin/postgresql-check-db-dir ${PGDATA} appear to be doing some unnecessary checking/changing of mysql's and postgresql's home directories.

• It almost seems like the systemd scripts are trying to run some form of initialization, which is unnecessary, since I already did postgresql-setup initdb and mysql_install_db --user=mysql earlier.

• The error message cannot create directory ~/var/lib/mysql~: File exists also suggests the systemd script is trying to do some unnecessary initialization.

• The error message pg_ctl: could not open PID file "/var/lib/pgsql/data/postmaster.pid": Permission denied confuses me and I'm not sure what to make of it.

How can I fix this without putting mysqld_safe and sudo -u postgres pg_ctl -D /var/lib/pgsql/data start into /etc/rc.local or something (and then disable/mask these services in systemctl). I'd still like systemctl to see these services if possible.

One possibility is creating my own /etc/systemd/system/mariadb.service and /etc/systemd/system/postgresql.service since you're not supposed to edit the /lib/systemd/system/ version, but I'm unsure how to create such a file properly.

NOTE: I have not included the contents of the systemd scripts here, since they are the ones that ship with mariadb and postgresql, but can do so on request.