The block size of each database Index Area (IXX) is determined during IXX initialization. When you initialize the IXX, you can override the default index block size by using the optional BLKSIZE= keyword. BLKSIZE= can be used to specify an index block size of from 4096 bytes through 8192 bytes. The BLKSIZE= value remains in effect until the next initialization of the IXX. You must pick an optimal size for your device type, in a manner similar to how the data area block size is selected.
Important! We do not recommend using a block size other than the default. Changing the block size might help in the utilization of some parts of index storage and processing, but other parts could be hurt by the change. Therefore, changing the block size should only be done after first experimenting to discover whether the change causes better or worse results. Be aware that the index can double in size using 8K blocks when OLDEP= is not specified as NO.
The index is complex and proprietary, typically involving a variety of key lengths, degrees of compression, and the possibility of from one to many duplicate values. Different combinations could therefore increase or decrease DASD usage, and could also, independently, increase or decrease CPU usage. This is why we caution you that, if you want to try to improve resource utilization by changing the index block size from the default, experimentation is needed to find out what results a change in block size has with regard to the mix of applications, CPU availability, and DASD availability.
Note: An OPEN error occurs if an attempt is made to open a database that has an index block size larger than the index buffer size. See the information about the ixxbfrsize parameter of the SYSPOOL Multi-User startup option in the CA Datacom/DB Database and System Administration Guide.
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