Options for improving VSAM buffer performance

You can make a number of adjustments to improve the performance of VSAM buffers.

• If background write is turned on and the number in the NUMBER OF VSAM WRITES TO MAKE SPACE IN THE POOL field is not zero, you probably do not have enough buffers allocated in the subpool. Try allocating more buffers to decrease the number or reduce it to zero.
• If you need to improve performance for a specific application, you can reserve subpools for certain data sets by:
  • Defining multiple local shared resource pools.
  • Dedicating subpools to a specific data set.
  • Defining separate subpools for index and data components of VSAM data sets.
• If VSAM sequential mode processing is not used, the number of VSAM buffers specified in the DFSVSAMP DD statement can dramatically affect performance. This problem occurs when the number of VSAM KSDS indexes that must be read, plus one for the data portion, is equal to or greater than the number of VSAM buffers allocated. This problem can be alleviated either by increasing the number of buffers or by using VSAM sequential mode. With VSAM sequential mode, the need to read indexes above the sequence set is reduced. However, sequential mode can only be obtained in a batch environment with a DBD referenced by a single PCB and with a processing option of LOAD or RETRIEVE only. VSAM sequential mode is not available in data sharing.
• VSAM buffers can take advantage of z/OS® Hiperspace buffering.