All members that use the checkpoint data set(s) specified by the CKPTn parameters on the CKPTDEF statement must at least have access to the shared volume on which CKPT1 exists. This shared access ensures adequate workload communication from member to member. JES2 checkpoint access in a MAS configuration depends on whetherthe checkpoint data set(s) reside on a Coupling facility structure or on a DASD volume.

For the checkpoint data set(s) that reside on a structure, the locking capabilities of the Coupling facility prevent JES2 members fromsimultaneously referencing and updating information kept in the checkpoint data set.

For the checkpoint data set(s) that reside on a DASD volume, JES2 processing uses RESERVE and RELEASE logic with a software checkpoint lock to prevent JES2 members from simultaneously referencing and updating information kept in thecheckpoint data set. Because of the use of RESERVE, it is recommended that the checkpoint data set not be placed on a spool volume.

For checkpoint data sets residing on DASD, IBM® suggests that you add the checkpoint data set(s) to the RESERVE conversion resource exclusion name list (RNL) to prevent global resource serialization (GRS) from limiting access to that data set and degrading performance. For more information about specifying the RESERVE conversion RNL list, see z/OS MVS Planning: Global Resource Serialization.