Packing density of the root addressable area
The packing density of the HDAM root addressable area can be found in the Randomizing Statistics report.
Specifically, you can find the packing density of the HDAM root addressable area in Figure 1 to the right of the phrase
PACKING DENSITY OF RAA.
With the standard DFSHDC40 randomizer and with the Sequential Subset Randomizer, a reasonable rule of thumb is a packing density of approximately 75% (for databases with an average database record length smaller than one tenth of the CI/block size) or 70% (for databases with a larger average database record length).
- 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 may 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 will increase the size of the root addressable area and lower its packing density; this will usually increase the performance of random accesses to the database.
Varying the bytes limit may 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).
As a rule of thumb, you should increase the size of the root addressable area, if the Database Tuning Statistics shows that:
- The packing density is higher than the recommended target values of 70% and 75%
- The average number of I/Os in the root addressable area per database record is higher than 1.15.
Discussion of the example
In the example of Figure 1, the packing density of the root addressable area is 82.86%. This is higher than the recommended value of 70% for databases with this type of average database record length. This high packing density provides a probable and partial explanation for the following numbers, which look poor:
- The average number of I/O on RAP chain per root, which is 1.34 (usually this number should not be much higher than 1.10).
- The average number of I/O in RAA per database record, which is 1.46 (usually this number should not be much higher than 1.15).
- The percentage of accessed roots in the overflow area, which is 6.01 (usually this number should not be much higher than 1.0).
In this case, you could decrease the packing density of the root addressable area. Bytes limit shows that the bytes limit is already too low, so a decrease of the packing density should not be attempted through a decrease of the bytes limit. Instead, a decrease of the packing density should be achieved by increasing the number of blocks or CIs in the root addressable area (or by an increase of the block or CI size).