1. A method for obtaining I/O measurement data, the method comprising: in response to executing a start subchannel instruction, starting an I/O operation process; obtaining I/O measurement data associated with the performance of the I/O operation process during the I/O operation process; saving the obtained I/O measurement data; generating an I/O interrupt at the completion of the I/O operation process; and in response to executing a test subchannel instruction, storing the obtained I/O measurement data in an IRB memory location specified by the test subchannel instruction.