Multiple instances of GTF can be active in a system at the same time. When you activate GTF, each instance operates as a system task, in its own address space. The only way to activate GTF is to enter a START GTF command from a console with master authority. Using this command, select either IBM's procedure or your cataloged procedure for GTF. The cataloged procedure defines GTF operation; you can accept the defaults that the procedure establishes, or change the defaults by specifying certain parameters on the START GTF command.
Because GTF sends messages to a console with master authority, enter the command only on a console that is eligible to be a console with master authority. Otherwise, you cannot view the messages from GTF that verify trace options and other operating information.
Each instance of GTF can be assigned a unique identifier that is specified on the START GTF command after the GTF keyword. This will allow you to recognize and control specific instances of GTF. If a unique identifier is not specified, the operating system assigns the default, which is the device number of the device where the trace data set resides. See the example in the topic Starting GTF with trace output to an existing data set on tape for an instance of GTF with the default identifier.