VSAM Performance Enhancement › Advanced Topics › VPE Implementation Considerations › Region Computations

Region Computations

VPE's region computations involve the amount of space available below the line at the time of first OPEN plus the following VPE parameters. These parameters are set in the VPE Rules Table and/or by VPE Control DD statements:

• MINRESV—Specifies the minimum amount of memory to reserve below the 16 Mb line for the application to use. The default setting is 300K.
• TGTBUF—Directs VPE to attempt to obtain the specified number of megabytes for the VSAM I/O buffers. This defaults to 9 Mb if the LSR buffers are above the 16 Mb line, and to 4 Mb if they are below it.
• MINBUF—Specifies the minimum VSAM I/O buffer size, which is acceptable. The default value for MINBUF is zero bytes. MINBUF can be used to generate error messages or to prevent jobs from running when sufficient buffer space is not available.

VPE optimizes buffer space on files opened for load or sequential only access (high RBA = 0 or 1 or ACB specifies sequential only access) using space from below the line. VPE uses the following calculation to obtain a value for the upper limit of size of sequential buffers to assign:

Case 1: LSR below the line:

LIMIT = (AVAIL REGION BELOW 16MB - LSR POOL SIZE / 2).

Case 2: LSR above the line:

LIMIT = (AVAIL REGION BELOW 16MB - MINRESRV)/2.

In Case 1, the LSR pool size is set to the smaller of TGTBUF or the available region minus MINRESRV. In Case 2, the entire MINRESRV is available for the application. In Case 1, a minimum of half of the region specified by MINRESRV is available.

Note: This amount of region is not actually taken, but is merely a calculated upper limit. A message is written if the minimum calculated LSR pool is not available or if the sequential limit is reached prior to fully optimizing all files.