Is the size of the Root Addressable Area appropriate?

This topic discusses whether the size of the Root Addressable Area is appropriate.

As a reasonable rule of thumb, if you are using a standard DFSHDC40 randomizer or the Sequential Subset Randomizer, the percentage of space used for segments in an RAA (shown as OTHER KEY INDICATORS in the HDAM RAP Chain I/O Summary report of the IPR Unload utility, and referred to as packing density) must be as follows:

75% (for databases with an average database record length smaller than one tenth of the CI/block size)
70% (for databases with a larger average database record length)

Notes:

Aiming for higher packing densities saves DASD space, but often decreases the performance of HDAM database accesses. Each installation will have probably its own idea of what is the ideal trade-off between DASD space and performance and regarding the ideal packing density. For example, some installations might want packing densities of 75% and 80% (instead of the target values of 70% and 75%).
For small or medium-sized HDAM databases (when saving DASD space is not important), it is often reasonable to aim for a packing density below 70%.
If the total size of all database records will eventually grow, it is then reasonable to oversize the root addressable area at database reload time, in order to provide enough space for the future data growth.

You can change the packing density of the root addressable area by varying the number of blocks or CIs in the root addressable area and by varying the size of a block or CI. Increasing the number of blocks or CIs or increasing the block or CI size increases the size of the root addressable area and lower its packing density; this typically increases the performance of random accesses to the database.

Varying the bytes limit might also change the packing density of the root addressable area (reducing the bytes limit will tend to store more information in the overflow area and less information in the root addressable area; this will hence tend to reduce the packing density in the root addressable area).

Discussion of the example

Packing Density is printed under OTHER KEY INDICATORS in the HDAM RAP Chain I/O Summary report of the IPR Unload utility.

In the following figure, the value of PACKING DENSITY OF ROOT ADDRESSABLE AREA is too high. To lower the value of packing density, you can increase the number of blocks or the number of CIs in the RAA, or you can decrease the total bytes of the segments in the RAA, and so on. Make appropriate specifications, taking into account the pros and cons previously discussed.

Figure 1. Database tuning of the primary data set group in an HDAM database: sample 8

IMS HIGH PERFORMANCE UNLOAD                                        PAGE:       1
5655-E06                                                    10/01/2021  13.10.59

                         "HDAM RAP CHAIN I/O SUMMARY"

KEY INDICATORS FOR QUALITY OF RANDOMIZING FROM A VIEWPOINT OF DATABASE I/O

  AVG NBR OF I/O PER DB RECORD............................      2.91
  AVG NBR OF I/O ON RAP CHAIN PER ROOT....................      1.66
  AVG NBR OF I/O ON RAP CHAIN PER RAP CHAIN...............      2.00
  AVG NBR OF I/O IN ROOT ADDRESSABLE AREA PER DB RECORD...      1.50
  AVG NBR OF I/O IN OVERFLOW AREA PER DB RECORD...........      1.41
  AVG POSITION OF ROOT SEGMENTS ON RAP CHAINS.............      1.33


OTHER KEY INDICATORS

  PACKING DENSITY OF ROOT ADDRESSABLE AREA................     46.09


NBR OF I/O'S REQUIRED TO READ A ROOT SEGMENT RANDOMLY
(SIMULATED WITH     4 PCB-BUFFERS)

   NBR I/O     NBR ROOTS       PCT   CUM PCT
  --------   -----------   -------   -------
      1                4    33.33%    33.33%
      2                8    66.66%   100.00%


POSITION OF ROOT SEGMENTS ON RAP CHAINS

  POSITION     NBR ROOTS       PCT   CUM PCT
  --------   -----------   -------   -------
      1                8    66.66%    66.66%
      2                4    33.33%   100.00%

You can check the block size or the CI size and the number of blocks or the number of CIs of the current database data sets in the Data Set Statistics report. You can check the average size of database records in AVERAGE DATABASE RECORD SIZE of the Distribution of Database Record Sizes report.

The value of the bytes limit (MAX BYTES) that is specified is printed in the top part of the HDAM Root Addressable Area Statistics report (and also in the DBD Summary report for a non-HALDB).

Figure 1 shows that MAX BYTES= is blank, which means the bytes limit is not specified. For details, see Is the HDAM bytes limit appropriate for the database record lengths in this database?.