Q:

How are Websphere MQ messages stored to allow persistence?

Are Websphere MQ messages stored in a database or filesystem?
Can I configure it to use MySQL? 

A:

WebSphere MQ persistence is always a local filesystem for distributed platforms.  On the mainframe there is the possibility of using a DB2 database in a Sysplex with a Coupling Facility but for everything else it is a filesystem.
Also, there is no local persistence for WebSphere MQ clients.  Only the queue manager will persist messages.
In the WebSphere family, WebSphere Application Server's System Integration Bus has the ability to use a database for it's message persistence.  SIBus is a JMS implementation provided with WAS for exchanging messages in the WAS cell and between WAS instances.

