Preopen areas and VSO areas in a data sharing environment

A VSO can be registered with different share levels.

A VSO can be registered with any of the following share levels:

SHARELVL(0)

Exclusive access: in a data sharing environment, any SHARELVL(0) area with the PREOPEN option (including VSO PREOPEN and VSO PRELOAD) is opened by the first IMS system to complete its control region initialization. IMS will not attempt to preopen the area for any other IMS.

SHARELVL(1)

One updater, many readers: in a data sharing environment, a SHARELVL(1) area with the PREOPEN option is preopened by all sharing IMS systems. The first IMS system to complete its control region initialization has update authorization; all others have read authorization.

If the SHARELVL(1) area is a VSO area, it is allocated to a data space by any IMS that opens the area. If the area is defined as VSO PREOPEN or VSO PRELOAD, it is allocated to a data space by all sharing IMS systems.

If the area is defined as VSO NOPREO NOPREL, it is allocated to a data space by all IMS systems, as each opens the area. The first IMS to access the area has update authorization; all others have read authorization.

SHARELVL(2)

Block-level sharing: a SHARELVL(2) area with at least one coupling facility structure name (CFSTR1) defined is shared at the block or control interval (CI) level within the scope of a single IRLM. Multiple IMS systems can be authorized for update or read processing if they are using the same IRLM.

SHARELVL(3)

Block-level sharing: a SHARELVL(3) area with at least one coupling facility structure name (CFSTR1) defined is shared at the block or control interval (CI) level within the scope of multiple IRLMs. Multiple IMS systems can be authorized for nonexclusive access.

Attention: Be careful when registering a VSO area as SHARELVL(1). Those systems that receive read-only authorization never see the updates made by the read/write system because all reads come from the data space (not from DASD, where updates are eventually written).