Opening and preopening DEDB areas
By default, IMS does not open a DEDB area until an eligible application accesses the area.
Although this prevents unneeded areas from being opened at startup, the first application that accesses a DEDB area incurs some additional processing overhead. Multiple calls to multiple areas immediately following a startup process can increase this burden significantly.
You can limit the overhead of opening areas by preopening your DEDB areas. You can also distribute this overhead between the startup process and online operation by preopening only those areas that applications use the most and by leaving all other areas closed until an application first accesses them.
You specify the preopen status of an area
using the PREOPEN and NOPREO parameters of the DBRC commands INIT.DBDS or CHANGE.DBDS.
By default IMS preopens all DEDB areas that have been assigned preopen status during the startup process; however, preopening a large number of DEDB areas during the startup process can delay data processing. To avoid this delay, you can have IMS preopen DEDB areas after the startup process and asynchronously to the execution of your application programs. In this case, if IMS has not preopened a DEDB area when an application program attempts to access the area, IMS opens the DEDB area at that time. You can specify this behavior by using the FPOPN= keyword in the IMS and DBC startup procedures. Specifically, FPOPN=P causes IMS to preopen DEDB areas after startup and asynchronous to application program execution.
The FPOPN= keyword determines
how IMS reopens DEDB areas for
both normal restarts (/NRE) and emergency restarts
(/ERE).
DEDB areas can also be opened by issuing either of the following type-2 commands with the OPTION(OPEN) keyword:
- UPDATE AREA NAME(areaname) START(ACCESS) OPTION(OPEN)
- UPDATE DB NAME(dedbname) AREA(*) START(ACCESS) OPTION(OPEN)