Prior to z/OS® Release 2, automatic tape switching was accomplished by using the IEFAUTOS coupling facility structure. Systems in a sysplex stored the status of online automatically switchable tape devices in IEFAUTOS.

With z/OS R2, the ATS STAR design improved the availability and system management characteristics of the prior automatic tape switching function. The ATS STAR design dropped the use of the IEFAUTOS structure and instead used global resource serialization and XCF services to maintain serialization when allocating shared tape devices. To maximize the performance of the ATS STAR function, it is strongly recommended that you use the global resource serialization Star configuration, rather than the Ring configuration.