VSAM cluster definitions

The HCL Workload Automation for Z databases and plans are stored in VSAM KSDS clusters. Default cluster definitions provided with HCL Workload Automation for Z do not consider the I/O performance to the clusters. While changes to the cluster definitions can provide significant performance improvements, such changes cost either in additional storage usage by the controller address space or by increased DASD space.

Consider these performance recommendations:
  • Use the IMBED and REPLICATE options if the clusters are not behind CACHE.

    IMBED specifies that the sequence-set record for each control area is written as many times as it fits on the first track adjacent to the control area. If the allocation is less than a cylinder, one track is added to the primary and secondary allocation quantities.

    REPLICATE specifies that each index record is to be written on a track as many times as it fits. With REPLICATE, rotational delay is reduced and performance is improved. But the cluster's index usually requires more direct access device space.

  • Define some free space on clusters that have significant insert activity, particularly the JCL repository (EQQJS1DS and EQQJS2DS), which requires at least FREESPACE(20, 20). Consider a free space allocation for the current plan (EQQCP1DS, EQQCP2DS, EQQCXDS, EQQNCPDS, and EQQSCPDS), extended data (EQQXD1DS, EQQXD2DS, and EQQNXDDS), application descriptions (EQQADDS), resource descriptions (EQQRDDS), and operator instructions (EQQOIDS).

    FREESPACE(CI-percent, CA-percent) specifies the percentage of each control interval and control area that is to be set aside as free space when the cluster is initially loaded, during a mass insert, and after any split of control intervals (CI-percent) and control areas (CA-percent). Empty space in the control interval and control area is available for data records that are updated and inserted after the cluster is initially loaded. CI-percent translates into a number of bytes that is equal to or slightly less than the percentage value of CI-percent. CA-percent translates into a number of control intervals that is equal to or less than the percentage value of CA-percent.

    CI-percent and CA-percent must be equal to or less than 100. When you specify FREESPACE(100 100), one data record is placed in each control interval used for data and one control interval in each control area is used for data (that is, one data record is stored in each control area when the data set is loaded). When no FREESPACE value is coded, the default specifies that no free space be reserved when the data set is loaded.

    When you define the cluster using the RECORDS parameter, the amount of free space specified is not taken into consideration in the calculations to determine primary allocation.

  • Allocate more buffers on clusters that do not have LSR buffering.

    BUFFERSPACE specifies the minimum space to be provided for buffers. The buffer space size you specify helps VSAM determine the data component's and index component's control interval size. If BUFFERSPACE is not coded VSAM attempts to get enough space to contain two data component control intervals and one index component control interval. Try to have one buffer for each index CA, plus one extra.

  • If you do not require SPANNED support, do not allocate the cluster with the spanned attribute. By default, EQQADDS, EQQLTDS, and EQQLDDS are spanned clusters.

    SPANNED specifies that if the maximum length of a data record (as specified with RECORDSIZE) is larger than a control interval, the record is contained on more than one control interval. This allows VSAM to select a control interval size that is optimum for the direct access device.

    When a data record that is larger than a control interval is put into a cluster that allows spanned records, the first part of the record completely fills a control interval. Subsequent control intervals are filled until the record is written into the cluster. Unused space in the record's last control interval is not available to contain other data records.

    Spanning a cluster causes the number of physical I/Os to be dramatically increased. If you must define a cluster with the spanned attribute and performance is important, consider using CACHE and DASD-FAST-WRITE for these clusters.

  • Specify at least AMP=('BUFND=5,BUFNI=5') in the HCL Workload Automation for Z JCL procedure for the controller on the EQQCPxDS and EQQJSxDS ddnames. CP and JS copies are NSR, they run faster with additional buffers.
  • This is true for CP only during CP copy process which uses NSR. For other processing where the CP uses LSR buffering, this does not make a difference. The selection of the optimal values to use for BUFND and BUFNI, can be progressively tuned by observing the time interval between the following two messages:
    • EQQN056I: A current plan copy process has started.
    • EQQN057I: A current plan data set was successfully copied assuming that the same number of CP records is involved.