Buffer sizes for databases that use OSAM or VSAM

Performance studies often indicate a need to increase DMB or PSB buffer space. You can override the system definition values to implement this change.

Several parameters enable you to redefine the size of buffer pools that hold DMBs and PSBs. Specify a value that represents the size as a number of 1024-byte blocks. For example, a value of 18 represents the size as 18432 bytes (18 x 1024 = 18432). IMS then rounds up this number based on the nearest whole number of pages on which these bytes fit. For example, if one page in z/OS® equals 4096 bytes, the value of 18 blocks would result in the allocation of 20480 bytes. This value overrides the value defined in system definition. The calculation is as follows:

Formula

18 x 24 = 18432/4096 = 4.5 pages (round up to 5)
5 x 4096 = 20480 bytes

If you use the DL/I address space option, two PSB pools exist: one in the z/OS common area and one in the DL/I address space. Use the CSAPSB and DLIPSB parameters on the IMS procedure to specify the size of the PSB pools. If the DL/I address space is not used, specify the size of the single PSB pool using the PSB= parameter on the IMS procedure.

If Fast Path resources are used, an EPCB pool must contain extensions to Fast Path PCBs. For each PCB referencing a DEDB or MSDB database, an EPCB is required. This pool is used only by MPP regions, not by IFP or BMP regions.

Use the parameters of the IMS procedure in the following list to specify buffer pool sizes:

  • DMB
  • PSB
  • PSBW
  • CSAPSB
  • DLIPSB
  • EPCB

The IMS procedure is generated with null value parameters for these storage areas.