Defining, Generating, and Punching a DMCL › Adding Segments to the DMCL › Area Status

Area Status

Type of Access

When a DC/UCF system first accesses an area, the type of access is determined by the area status specifications within the DMCL. The choices for area status are:

• Update—indicating that database transactions executing under the central version can retrieve and update data within the area; local mode transactions and other central versions can retrieve from but not update the area.
• Retrieval—indicating that database transactions executing under the central version can retrieve but not update data in the area; a local mode transaction or another central version can update the area.
• Transient retrieval—similar to retrieval except that record (row) locks are not maintained for transactions executing within the central version.
• Offline—indicating that database transactions executing under the central version can neither retrieve nor update data in the area.

The status of an area can be changed dynamically using DCMT VARY AREA and VARY SEGMENT commands.

Retrieval Versus Transient Retrieval

Because locks are not maintained for records or rows in areas whose status is transient retrieval, less CPU (and potentially less storage) may be consumed by a transaction than if the area status were retrieval. (SQL transactions using an isolation level of transient retrieval and non-SQL transactions in a system with a sysgen specification of no retrieval locking are the exceptions.) However, an area whose status is transient retrieval must be varied offline before it can be varied to another status such as update.

To vary an area offline, all concurrently executing transactions must be terminated and all notify locks released. During the time it takes to achieve this quiesce point, new transactions will not be allowed to access the area. If this causes unacceptable processing delays the use of transient retrieval should be avoided.

Permanent Area Status

The status of an area can be changed at run time using a DCMT VARY AREA or VARY SEGMENT command. In addition to establishing a new area status, that status can also be declared as "permanent." A permanent area status remains in effect until changed by a subsequent DCMT command or until the DC/UCF system's SYSTRK or journal files are initialized. A permanent area status survives system shutdowns and abnormal terminations.

Status After System Termination

Unless a permanent area status has been established through a DCMT command, the ON STARTUP and ON WARMSTART parameters determine the status of an area when a DC/UCF system starts up. The first time a DC/UCF system is started or whenever it is restarted after a normal shutdown, the status of an area is established from the ON STARTUP specification. If the system is restarted following an abnormal termination, the status of an area is established from the ON WARMSTART specification. If the warmstart option is MAINTAIN CURRENT STATUS, the area status is set to what it was at the time of the abnormal termination.