z/OS MVS JCL Reference
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Subparameter definition

z/OS MVS JCL Reference

Specifies "no read integrity" (NRI). The application can read all records. Use this subparameter if the application can read uncommitted changes made to a data set by another application. NRI provides better performance than the CR subparameter because it avoids the overhead of obtaining a lock when reading a record from the data set.
Specifies "consistent read" (CR). This subparameter requests VSAM to obtain a SHARE lock on each record the application reads. This ensures the application will not read uncommitted changes made to a data set by another application. VSAM obtains the lock while processing a GET NUP request, and releases the lock before completing the GET request. An application that processes a data set allocated with RLS=CR may require modification if it tries to read changes to the data set.
Specifies "consistent read explicit" (CRE). This subparameter requests serialization of the record access with update or erase of the record by another unit of recovery. CRE read provides the application a means of ensuring that records read by a unit of recovery are not changed by other units of recovery until the reading unit of recovery issues a syncpoint.

For VSAM record-level sharing (RLS), CRE is valid only for an application that supports commit and backout. With DFSMS Transactional VSAM Services (DFSMStvs), if a batch (non-CICS) application specifies CRE as the value of the JCL RLS keyword or specifies CRE through the ACB, the data set is opened for DFSMStvs access.

After a batch application opens a data set for RLS access with the NRI or CR subparameter, it is an error for the application to request CRE through the RPL.

CRE does not inhibit update or erase of the record by the unit of recovery that issued the CRE request. The consistent-read explicit GET request obtains a SHARE lock on the record. The SHARE lock remains held until a commit or backout request is processed.

CRE readers should issue frequent commits to reduce contention with updaters. CRE read does not affect update locking rules. Deadlocks are possible when a mixture of CRE read and update is used.

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