Cross-System Sharing

These share options allow you to specify SHAREOPTION 1 or 2 sharing rules with SHAREOPTION 3 or 4 record management processing. Use either of the following share options when you define a data set that must be accessed or updated by more than one operating system simultaneously:

Cross-system SHAREOPTION 3. The data set can be fully shared. With this option, the access method uses the control block update facility (CBUF) to help. With this option, as in cross-region SHAREOPTIONS 3, each user is responsible for maintaining both read and write integrity for the data the program accesses. User programs that ignore write integrity guidelines can cause VSAM program checks, uncorrectable data set failures, and other unpredictable results. This option places heavy responsibility on each user sharing the data set. The RESERVE and DEQ macros are required with this option to maintain data set integrity.
Cross-system SHAREOPTION 4. The data set can be fully shared, and buffers used for direct processing are refreshed for each request.
This option requires that you use the RESERVE and DEQ macros to maintain data integrity while sharing the data set. Output processing is limited to update and/or add processing that does not change either the high-used RBA or the RBA of the high key data control interval if DISP=SHR is specified. For information about using RESERVE and DEQ, see z/OS MVS Programming: Authorized Assembler Services Reference ALE-DYN and z/OS MVS Programming: Authorized Assembler Services Reference LLA-SDU.

System-managed volumes and catalogs that contain system-managed data sets must not be shared with non-system-managed systems. When sharing data sets in a cross-region or cross-system environment, run the VSAM VERIFY macro before you open a data set. VERIFY locates the current end of the data set and updates internal control blocks. When the data set closes successfully, the system updates the catalog with the information that VERIFY determined. This information and its effects cannot be evident to all systems sharing the data set. If run as the first step of a job stream, VERIFY causes an update to the end-of-file information in the catalog.

To ensure data integrity in a shared environment, VSAM provides users of SHAREOPTIONS 4 (both cross-region and cross-system) with the following assistance:

Each PUT request results in the appropriate buffer being written immediately into the VSAM object's direct access device space. VSAM writes out the buffer in the user's address space that contains the new or updated data record.
Each GET request results in all the user's input buffers being refreshed. The contents of each data and index buffer used by the user's program is retrieved from the VSAM object's direct access device.

When the data set is shared under cross-system SHAREOPTIONS 4, regardless of cross-region requests, VSAM does not allow changes to high-used and high-key RBAs. In addition, VSAM provides assistance to the application to aid in preserving the integrity of the data:

Control area splits and the addition of a new high-key record for a new control interval that results from a control interval split are not allowed; VSAM returns a logical error to the user's program if this condition should occur.
The data and sequence-set control interval buffers are marked nonvalid following I/O operation to a direct access storage device.

Job steps of two or more systems can gain access to the same data set regardless of the disposition specified in each step's JCL. To get exclusive control of a volume, a task in one system must issue a RESERVE macro. For other methods of obtaining exclusive control using global resource serialization (GRS) see z/OS MVS Planning: Global Resource Serialization.