Percona server not starting in jail after upgrading FreeBSD host from 9.2 to 9.3

How to fix Percona server not starting in a jail after upgrading the FreeBSD host from 9.2 to 9.3

1. service mysql-server start returned, but service mysql-server status showed that it was not started successfully.

2. The log file /var/db/mysql/mysql-error.log warned that writing a file to /var/db/mysql failed.

3. Checking the permissions on /var/db/mysql showed that existing files in the folder were owned by user mysql and group mysql, but the directory itself was owned by user root and group wheel.

4. After granting user mysql and group mysql ownership of the folder (chown mysql:mysql /var/db/mysql) the server could be started successfully with service mysql-server start.