With DFSMShsm installation exits, you can tailor DFSMShsm functions and associate them with other space management activities. As you write exits, remember that they run as if they were DFSMShsm code. Therefore, it is recommended that you routinely incorporate the following considerations whenever you write installation exits:

• Keep supervisor services such as I/O to a minimum.
• Schedule for another time any processes that extend the elapsed time of automatic primary space management (for example, defragmentation of a DASD volume).
• Write all exits supporting the DFSMShsm basic functions as reentrant, with the exception of the two following exits. Writing exits as reentrant removes the rewrite requirement should the functions calling the exits become capable of concurrent processing by multiple tasks. You don't need to code the following exits as reentrant:
  • ARCINEXT (the initialization exit)
  • ARCSDEXT (the shutdown exit)
• Be aware that the recall exit is called in a system performing JES3 setup. The exit is called for migrated non-SMS-managed data sets during the JES3 converter-interpreter locate routine and is, therefore, in a critical system performance path.

Also always remember that DFSMShsm installation exits:

• Run enabled for interrupts.
• Run in problem program state.
• Run in either the DFSMShsm primary address space or the ABARS secondary address space (ARCTVEXT exit).
• Have pageable storage.
• Are protected by ESTAE.
• Are entered in the standard address space protection key of 8. Control must return in the same key as at entry.
• Execute in an authorized program facility (APF) authorized address space. As with all subroutines, do not link edit installation exits with APF authorization.