Typically, on a single machine (be it a physical or a virtual one) only a single MySQL instance (process) is running. This is perfectly ok for all those situations where a single instance is sufficient, like for storing small amounts of data (RedHat using MySQL for postfix, KDE using it for akonadi, ...), as well as those where a dedicated machine per MySQL instance is appropriate (high CPU load, memory fully loaded, availability requirements).
But there are also those users who want to (or would like to) have multiple instances which would still fit into a single machine. Even among them, a single instance per machine is typical. For this, there are good reasons:
- MySQL comes with defaults for files (config file, error log, ...) and directories (data directory, binlogs, ...) which would cause conflicts between multiple instances (unless they are changed).
- The scripts coming with MySQL, especially the automated …