GT.M's design allows the primary and secondary systems to operate on the same machine as separate instances.

Different instances of an application are distinguished by the value of the environment variable gtmgbldir which defines a GT.M Global Directory. All processes of an instance must have the same value of gtmgbldir.

A process accessing a database file in an instance must use the Global Directory of that instance. GT.M does not enforce this rule; however, violating this rule may lead to unpredictable results.