Control Block Update Facility (CBUF)

Whenever a data set is opened with DISP=SHR, cross-region SHAREOPTION 3 or 4, and cross-system SHAREOPTION 3, VSAM record management maintains a copy of the critical control block data in common storage. The control block data in the common storage area is available to each program (each memory) sharing the data set. The common storage area is available only to regions within your operating system. Communicating this information to another operating system is your responsibility.

CBUF eliminates the restriction that prohibits control area splits under cross-region SHAREOPTION 4. Therefore, you do not need to restrict code to prevent control area splits, or allow for the control area split error condition. The restriction to prohibit control area splits for cross-systems SHAREOPTION 4 still exists.

CBUF processing is not provided if the data set has cross-system SHAREOPTION 4, but does not reside on shared DASD when it is opened. That is, the data set is still processed as a cross-system SHAREOPTION 4 data set on shared DASD. When a key-sequenced data set or variable-length RRDS has cross-system SHAREOPTION 4, control area splits are prevented. Also, split of the control interval containing the high key of a key range (or data set) is prevented. With control interval access, adding a new control interval is prevented.

Cross-system sharing can be accomplished by sending the VSAM shared information (VSI) blocks to the other host at the conclusion of each output request. Generally, the VSIs will not have changed and only a check occurs.

If you use SHAREOPTION 3, you must continue to provide read/write integrity. Although VSAM ensures that SHAREOPTION 3 and 4 users will have correct control block information if serialization is done correctly, the SHAREOPTION 3 user will not get the buffer invalidation that will occur with SHAREOPTION 4.

When improved control interval processing is specified with SHAREOPTION 3 or 4, the data set can be opened. However, if another control block structure extends the data set, the control block structure using improved control interval processing will not be updated unless it is closed and reopened.

Table 1 shows how the SHAREOPTIONS specified in the catalog and the disposition specified on the DD statement interact to affect the type of processing.

Table 1. Relationship between SHAREOPTIONS and VSAM functions
(CR CS) when DISP=SHR¹	Functions Provided
(3 3)	CBUF
(3 4)	Data and sequence set buffers invalidated. CA split not allowed.
(4 3)	Data and index component buffers invalidated. CBUF.
(4 4)	Data and sequence set buffers invalidated. CA split not allowed.
Legend: CA = Control area CR = Cross-region CS = Cross-system CBUF = Control block update facility Buffer invalidated = Invalidation of buffers is automatic
Note: When DISP=OLD is specified or the data set is in create or reset mode (regardless of the disposition specified), the share options specified in the catalog are ignored. The data set is processed under the rules for SHAREOPTIONS(1 3). OPEN ensures that the user has exclusive control of the data set within a single system. If the data set can be shared between systems, VSAM does nothing to ensure that another system is not accessing the data set concurrently. With cross-system sharing, the user must ensure that another system is not accessing the data set before specifying DISP=OLD.