To improve the performance of z/OS UNIX System Services applications that use thread level security services such as the pthread_security_np service, RACF® can use the virtual lookaside facility (VLF) to cache user security packets (USPs). Use of VLF to cache USPs is optional. For RACF to use VLF to cache USPs, you must define the IRRSMAP class to VLF, and VLF must be active. For information on how to do this, see VLF considerations for caching user security packets (USPs).