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.