Specifying free space (HDAM, PHDAM, HIDAM, and PHIDAM only)

Dependent segments inserted after an HD database is loaded are put as close as possible to the segments to which they are related.

However, as the database grows and available space decreases, dependent segments are increasingly put further from their related segments. When this happens, performance decreases, a problem that can only be eliminated by reorganizing the database. (When segments are close to the segments that point to them, the I/O time needed to retrieve a dependent segment is shorter. The I/O time is shorter because the seek time and rotational delay time are shorter.)

To minimize the effect of insert operations after the database is loaded, allocate free space in the database when it is initially loaded. Free space allocation in the database will reduce the performance impact caused by insert operations, and therefore, decrease the frequency with which HD databases must be reorganized.

For OSAM data sets and VSAM ESDS, free space is specified in the FRSPC= keyword of the DATASET statement in the DBD. In the keyword, one or both of the following operands can be specified:

  • Free block frequency factor (fbff). The fbff specifies that every nth block or CI in a data set group be left as free space when the database is loaded (where fbff=n). The range of fbff includes all integer values from 0 to 100, except 1. Avoid specifying fbff for HDAM or PHDAM databases. If you specify fbff for HDAM or PHDAM databases and if at load time the randomizing module generates the relative block or CI number of a block or CI marked as free space, the randomizer must store the root segment in another block.

    If you specify fbff, every nth block or CI will be considered a second-most desirable block or CI by the HD Space Search Algorithm. This is true unless you specify SEARCHA=1 in the DATASET statement of the DBDGEN utility. By specifying SEARCHA=1, you are telling IMS not to search for space in the second-most desirable block or CI.

  • Free space percentage factor (fspf). The fspf specifies the minimum percentage of each block or CI in a data set group to be left as free space when the database is loaded. The range of fspf is from 0 to 99.
Note: This free space applies to VSAM ESDS and OSAM data sets. It does not apply to HIDAM or PHIDAM index databases or to DEDBs.

For VSAM KSDS, free space is specified in the FREESPACE parameter of the DEFINE CLUSTER command. This VSAM parameter is disregarded for a VSAM ESDS data set used for HIDAM, PHIDAM, HDAM, or PHDAM. This command is explained in detail in z/OS® DFSMS Access Method Services for Catalogs.