Region size requirements for DEDB Reload

The DEDB Reload utility operates in a virtual storage region as z/OS® batch job steps. FABCUR3, FABCUR3W, and FABC3ICT run in 31-bit addressing mode.

Region size for the reload program depends on the size of a UOW, the number of IOVF buffers allocated. Typically, database maintenance is a relatively important function. To increase the performance of the reload program, it would be advisable to run it in a region that has a relatively high dispatching priority (that is, at least equal to the dispatching priority of a message processing region).

Subsections:

Estimating the region size for reload

Region size for the reload program must satisfy the requirements for the program, and requires access method modules and buffers—that is, input sequential file BUFNO= and VSAM BUFND=. Elapsed time could be further reduced by increasing the BUFND value.

The following worksheet is for estimating the region size that is required for a typical reload job. The actual region size depends on the keywords that are specified for the job. Therefore, when you specify the region size in your JCL, use a higher value than the value calculated from this worksheet.

Figure 1. Worksheet for estimating the region size for Reload
Reload programs, internal tables, control blocks        180,000

1024 + BLKSIZE * BUFNO                                  -------

(IOVFBUF+UOW2) * max CISZ in DBD                        -------

Number of areas defined in DBD * 1300                   -------  (If ACBLIB DD and DBDNAME keyword are specified.)

Number of areas defined in DBD * 72                     -------  

(Root key length+388) * TBLENTRY                        -------

max(CISZ * UOW1)                                        -------

Total bytes of reload storage                           -------

Reducing region size constraints

The reload program (FABCUR3) is implemented using various techniques aimed at reloading DEDB areas from the segment data in the shortest possible time. The trade-off is increased storage requirements. The following list contains a series of actions that may be taken to reduce the region size requirements for a given reload request. They are listed in increasing order of impact on the performance of FABCUR3.
  1. If the maximum block size for the input file devices is large, specify the DCB=BLKSIZE=nnnnn parameter in the DD statement. Typically, reduce the block size to 15-20K.
    • This reduces the storage required for the QSAM buffers for each of the files.
  2. Specify a value for the IOVFBUF parameter. This reduces the storage required for the set of IOVF buffers.
  3. Specify an AMP=('BUFND=...' override on the DD statements for the areas being reloaded to reduce the number of buffers below the "2 X CIs-per-Track + 1" value. This should be considered only as a last resort. FABCUR3 automatically calculates an optimum value for this parameter when opening the VSAM Sequential ACBs. The value calculated by FABCUR3 is designed to maximize the number of CIs read per EXCP. A JCL override takes precedence over the calculated value and can have a drastic effect on the number of EXCPs issued to extract the segment data in the areas being unloaded.
    • This reduces the storage required for the buffers acquired by VSAM.
    • It results in increased elapsed processing time as the EXCP count increases, and more channel contention and I/O wait time incurs.