This statement controls how long it takes until the DBH becomes available in the event of a restart.
Definition
A lower-level DBH option of SYSTEM-STRATEGIES
Function
You can use the DBH option RESTART-CONTROL to optimize the time it takes for the DBH to become available again after a system failure. You will find general information on restarting in the section “Restarting a DBH session”.
You can influence how long a restart takes in two different ways:
The BUFFER-LIMIT and TALOG-LIMIT operands affect the frequency of write accesses to the database. The lower the values for BUFFER-LIMIT and TALOG-LIMIT, the more afterimages are written to the database during operation. This reduces the time required for physical repair in the event of a restart. However, if very low values are selected, the increased number of read and write accesses during normal operation lead to loss of performance. It therefore makes sense to monitor the number of accesses in the “I/O” DBH form of the SESAM/SQL utility SESMON (see "DBH form I/O"). You can adjust the values of the operands during the DBH session by means of the MODIFY-RESTART-CONTROL administration statement (see "MODIFY-RESTART-CONTROL").
The LOGICAL-ROLLBACK operand allows you to delay the logical reset until the start of normal operation. The transactions that were open when the system failed are thus not reset until after the restart. This reduces the time required until the system is available again.
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Operands
RESTART-CONTROL = *STD
The default values are used. The default values are described under *PARAMETERS.
RESTART-CONTROL = *PARAMETERS(...)
You can change the values set for BUFFER-LIMIT and TALOG-LIMIT to suit requirements during the DBH session by means of the MODIFY-RESTART-CONTROL administration statement (see "MODIFY-RESTART-CONTROL"). You must specify the BUFFER-LIMIT and TALOG-LIMIT operands before a system crash. Specifying them at the time of the external restart has no effect. The LOGICAL-ROLLBACK operand, on the other hand, is evaluated at the restart. It can therefore also be set immediately before an external restart.
BUFFER-LIMIT = *PARAMETERS(...)
These operands specify the limit as a percentage for the occupation of the two buffers by writable blocks. As of this limit, the buffers are written to the database.
SYSTEM-DATA-BUFFER = 50 / <integer 1..90>
Specifies a percentage limit for the writing of afterimages to the database for system access data. If the share of the total buffer space occupied by the buffers to be written exceeds the percentage specified for SYSTEM-DATA-BUFFER, afterimages are written to the database.
USER-DATA-BUFFER = 50 / <integer 1..90>
Specifies a percentage limit for the writing of afterimages to the database for user data. If the share of the total buffer space occupied by the buffers to be written exceeds the percentage specified for USER-DATA-BUFFER, afterimages are written to the database.
TALOG-LIMIT = 512 / <integer 128..524288>
Specifies the limit for how much of the TA-LOG file can be occupied by afterimages. In the event of physical repair, this part of the TA-LOG file must be read. If its size exceeds the specified limit, the afterimages are written to the database. TALOG-LIMIT is specified in units of 4 Kbytes.
LOGICAL-ROLLBACK =
Influences the time at which transactions are logically reset in the event of a restart.
LOGICAL-ROLLBACK = *IMMEDIATELY
The transactions are reset before the resumption of normal operation.
LOGICAL-ROLLBACK = *DELAYED
Delays transaction resetting until the resumption of normal operation. This reduces the time required until the system becomes available again. However, if too many updates have to be performed at the beginning of normal operation, this can result in loss of performance and the growth in size of the TA-LOG file. In certain cases, delaying the logical reset is not effective (see the section “Controlling the duration of the restart”).
The LOGICAL-ROLLBACK operand cannot be changed during DBH operation.