Database buffer pool tuning
Allocate sufficient buffers in the IMS database buffer pools to prevent I/O that results from an application program having to reread data previously brought into the pool. Careful choice of subpool sizes and matching against database block sizes and frequency of database references minimizes unnecessary I/O.
For Fast Path buffer pools, you can either explicitly specify the buffer pool size as part of the system definition process (DBBF parameter), or you can use the Fast Path 64-bit buffer manager to control the number and size of the Fast Path database buffer subpools. To enable the Fast Path buffer manager, specify FPBP64=Y in the FASTPATH section of the DFSDFxxx member of the PROCLIB data set. When the Fast Path buffer manager is used, DEDB buffer pools are placed in 64-bit storage, while buffer pools for main storage databases (MSDBs), sequential dependent (SDEP) segments, system services, and buffer headers are managed in 31-bit storage.
Multiple OSAM subpools can be defined as having the same buffer size, and specific data sets can then be directed to specific subpools. If you have many OSAM data sets with similar or equal block sizes, you might be able to obtain some performance advantage by replacing a single large pool with separate subpools. Multiple smaller subpools reduce pool scanning to locate a segment. This method can also be used to prevent low-priority transactions (or BMPs) that access many database segments from monopolizing pool space.
You can define multiple VSAM local shared resource pools. If you have many VSAM data sets with similar or equal control interval sizes, you might get a performance advantage by replacing a single large subpool with separate subpools of identically sized buffers. Creating separate subpools of the same size for VSAM data sets offers benefits like those offered by OSAM multiple subpool support.
Use multiple local shared resource pools to specify multiple VSAM subpools of the same size. Create multiple shared resource pools and then place a VSAM subpool in each one that is the same size as other VSAM subpools in other local shared resource pools. You can then assign a specific database data set to a specific subpool by assigning the data set to a shared resource pool. The data set is directed to a specific subpool within the assigned shared resource pool based on the control interval size of the data set.
You can also create separate subpools for VSAM KSDS index and data components within a VSAM local shared resource pool. Using these subpools can be advantageous, because index and data components do not need to share buffers or compete for buffers in the same subpool.
Multiple VSAM local shared resource pools enhance the benefits provided by Hiperspace buffering. Use Hiperspace buffering to allocate more buffers of 4 KB and multiples of 4 KB by using expanded storage in addition to virtual storage. Using multiple local shared resource pools and Hiperspace buffering allows data sets with certain reference patterns (for example, a primary index data set) to be isolated to a subpool backed by Hiperspace, which reduces the VSAM read I/O activity needed for database processing.
If you are using High Availability Large Databases (HALDBs), you can also direct HALDB partition data sets to specific OSAM or VSAM subpools.