Reducing network search overhead using search reduction

When a resource is unreachable in a network, futile attempts to reach it can still occur. Excessive searching for unreachable resources can adversely affect network performance. Therefore, VTAM® provides search reduction support, which limits requests for unreachable resources.

If search reduction support is enabled, an APPN or subarea search reduction entry is created when a resource discovery search (RDS) fails to locate a target resource. An APPN search reduction entry is saved as a directory entry in the directory services database. A subarea search reduction entry is saved as a CDRSC in the subarea resource definition table (RDT). VTAM then reduces searching for the unreachable resource in the appropriate part of the network during a designated amount of time or until a designated number of requests have been received for the resource.

If a subarea search reduction entry exists for a destination resource and VTAM is the SSCP of the originating LU (including instances where VTAM is an interchange node for requests entering subarea from APPN), search reduction optimizes adjacent SSCP table searching for that resource. VTAM limits attempts during the designated period to locate the unreachable resource in the subarea network.

If an APPN search reduction entry exists for a destination resource, the scope of optimization varies depending on the node type of VTAM.

If VTAM is the central directory server or network node server for the originating LU, including instances where VTAM is an interchange node for requests entering APPN from a subarea, search reduction optimizes APPN searching in VTAM's subnetwork. VTAM limits attempts during the designated period to locate the unreachable resource in its APPN subnetwork. For information about of APPN subnetworks, see APPN multiple network connectivity.
If VTAM is a border node for requests received across a subnetwork boundary, search reduction optimizes APPN searching in the VTAM subnetwork. VTAM limits attempts during the designated period to locate the unreachable resource in its APPN subnetwork.
If VTAM is a border node for the originating LU, search reduction optimizes APPN searching throughout the APPN network. VTAM limits attempts during the designated period to locate the unreachable resource in the APPN network.

To use VTAM search reduction support, code the SRCHRED=ON start option or enter the MODIFY VTAMOPTS command with the SRCHRED=ON operand. Use the SRTIMER start option, or MODIFY VTAMOPTS with the SRTIMER operand, to designate the amount of time that VTAM should limit requests for a resource. Use the SRCOUNT start option, or MODIFY VTAMOPTS with the SRCOUNT operand, to designate the number of requests for a resource that VTAM should limit before attempting to locate the resource again.

In determining a proper value for SRTIMER or SRCOUNT, keep in mind that the value should be large enough to limit searching for an appropriate amount of time, but small enough so that a resource that becomes available again does not remain unreachable because of waiting for SRTIMER or SRCOUNT to expire. Also, keep in mind that the SRCLEAR=YES operand can be used on the MODIFY RESOURCE command to clear search reduction entries. That is, you can use this when a resource becomes available to clear search reduction entries, so that the specified resource can now be located.

Note that SRCLEAR=YES only clears search reduction entries; it does not turn off search reduction support. If a subsequent RDS fails to locate the target resource, VTAM creates a new search reduction entry and limits subsequent requests for the unreachable resource during the designated amount of time or until the designated number of requests have been made.

Notes:

The SRTIMER and SRCOUNT operands can also be specified for resources on the CDRSC definition statement or the GROUP definition statement in a CDRSC major node. These values can be dynamically modified using the MODIFY RESOURCE command.
The DISPLAY DIRECTRY command displays information about search reduction entries in the directory services database. The DISPLAY ID command displays information about search reduction entries for a particular CDRSC, and entries in the directory services database (if the IDTYPE=RESOURCE operand is used).
When a resource has search reduction information associated with it, these commands display the defined values for SRTIMER and SRCOUNT, and the remaining amount of time and remaining number of searches before VTAM discards the search reduction information.
Use DISPLAY VTAMOPTS for information about the settings of the search reduction start options.
When using SRTIMER, SRCOUNT, or both, along with AUTOTI for autologon sessions, an unsuccessful autologon-session search for a controlling PLU can cause the creation of a search reduction entry for it. In this situation, be aware of the following situations:
- If AUTOTI=x and SRTIMER=y, where y is greater than x, when the AUTOTI timer expires, subsequent requests for the PLU are limited by the search reduction entry until SRTIMER also expires.
- If AUTOTI=x and SRCOUNT=z, when the AUTOTI timer expires, subsequent requests for the PLU are limited by the search reduction entry for (x * z) seconds. At that time, the search reduction entry expires.
See Controlling searches for additional information.