VSAM buffers

The PROCLIB member DFSVSMxx defines single or multiple VSAM buffer pools.

DFSVSMxx includes the following parameters:

  • Reserve pools
  • VSAM local shared resource pools
  • VSAM shared pools and subpools, linked by Pool ID
  • Allocation of data sets to pools, linked by Pool ID
  • VSAM performance options such as background write, maximum concurrent requests, fixed blocks

IMS Performance Analyzer IRUR: VSAM Buffer Pool Statistics

One of the Internal Resource Utilization reports (IRUR) is the VSAM Buffer Pool Statistics report. This report provides information related to the activity in each VSAM subpool. Subpools with no activity in the reporting period are not reported on. The final report contains cumulative statistics for all of the VSAM buffer pools. This report is similar to the VSAM Buffer Pool report produced by the IMS DB Monitor. See the IMS Utilities Reference: Database Manager for a description of the meaning of the fields and the uses for this report.

Figure 1. IMS Performance Analyzer IRUR: VSAM Buffer Pool Statistics
Enhanced VSAM Buffer Pool Statistics                                                         Interval :    1.08.59 (HHHH.MM.SS)
                                                              Count /Transact   /Second
          Shared resource pool ID/type              VS8K/D
          Fix option: index/block/data              N/Y/Y
          Buffer size                                         8,192
          Buffers in subpool                                     15
          HS buffers in subpool                                  75
          Write errors                                            0
          Largest number of write errors                          0       .00       .00
          Retrieve by RBA calls                              43,821      4.92     10.59     63.08% of retrieve calls
          Retrieve by Key calls                              25,653      2.88      6.20     36.92% of retrieve calls
    Total retrieve calls                                     69,474      7.81     16.78
          Logical records inserted into ESDS                      1       .00       .00       .07% of update requests
          Logical records inserted into KSDS                      7       .00       .00       .46% of update requests
          Logical records altered in this subpool             1,524       .17       .37     99.48% of update requests
    Total number of updates                                   1,532       .17       .37
          Nbr of background write requests                        9       .00       .00       .01% of calls to VSAM
          Nbr of Synch calls                                    624       .07       .15       .94% of calls to VSAM
          Nbr of VSAM get calls                              65,581      7.37     15.84     99.03% of calls to VSAM
          Nbr of VSAM search buffer calls                         7       .00       .00       .01% of calls to VSAM
    Total Nbr of VSAM calls                                  66,221      7.44     16.00
          Nbr of times VSAM found CI in pool                 54,853      6.16     13.25     83.63% of VSAM buffer requests added
          Nbr of times VSAM read CI from DASD                10,738      1.21      2.59     94.19% of VSAM I/O operation
          Nbr of writes initiated by IMS                        606       .07       .15      5.32% of VSAM I/O operation
          Nbr of writes initiated by VSAM                        56       .01       .01       .49% of VSAM I/O operation
    Total VSAM I/O operations                                11,400      1.28      2.75
          Nbr of successful VSAM reads frm HS                 4,218       .47      1.02
          Nbr of successful VSAM writes to HS                14,957      1.68      3.61
          Nbr of failed VSAM reads from HS                        0       .00       .00
          Nbr of failed VSAM writes to HS                         0       .00       .00
          Nbr of PLH waits                                        0       .00       .00

A key performance indicator for a VSAM Buffer Pool is the number of times VSAM found the CI in the pool, rather than reading the CI from DASD.

When data sharing in IMS sysplex database environments, there needs to be a balance between maximizing the requests satisfied in the pool and minimizing the occurrence of buffer invalidation.