Munin shows "MySQL InnoDB free tablespace" critical alert

Symptoms

Munin service monitor can show a critical alert, "MySQL InnoDB free tablespace".

Description 

This warning should be disregarded. It is due to how Munin handles tablespace for InnoDB compared to how it is handled by MySQL. By default, Munin throws a warning at 2GB free tablespace and a critical alert at 1GB free tablespace. However, MySQL defaults to creating a 12MB auto-expanding tablespace. The tablespace can be verified with the following:

# mysql -e "SHOW VARIABLES LIKE 'innodb_data_file_path'"
+-----------------------+------------------------+
| Variable_name | Value |
+-----------------------+------------------------+
| innodb_data_file_path | ibdata1:12M:autoextend |
+-----------------------+------------------------+

Note how it is configured to auto-extend. This means InnoDB will perpetually show <12MB of space (resulting in a permanent Munin warning) yet will always seamlessly auto-extend as needed. The only "limit" in such a configuration is the partition that/var/lib/myqlis stored on. This render's the InnoDB free tablespace useless in an "auto-extend" configuration.

Workaround

The way to resolve this is to disable the InnoDB monitoring plugin. Remove the /etc/munin/plugins/mysql_innodbsymlink with the following:

rm -i /etc/munin/plugins/mysql_innodb

It will prompt you to remove the symlink, ensure that it mentions the file is a symbolic link:

# rm -i /etc/munin/plugins/mysql_innodb
rm: remove symbolic link ‘/etc/munin/plugins/mysql_innodb’? y

At that point, run the following commands to apply the new configuration:

systemctl restart munin-node
sudo -u munin /usr/local/cpanel/3rdparty/perl/532/bin/munin-cron