CI size and block size

The CI size or the block size can be found in the Randomizing Statistics report.

The current CI size/block size is shown in the top portion of Figure 1.

A general rule of thumb is a 4 KB CI/block size for HDAM.

However, if the average database record length is higher than 800 bytes (that is, larger than 1/5th of the block or CI size), then an increase of the block or CI size from 4 KB to 8 KB might improve the performance of HDAM database accesses (especially if the database administrator wants a high packing density (more than 70%) of the root addressable area). In this case, an increase of the CI size/block size from 4 KB to 8 KB can often reduce the number of I/O operations (but will increase slightly the time of an I/O operation).

CI/block sizes larger than 8 KB should be an exception (for example, for very large average database record sizes).

Discussion of the example

In the example in Figure 1, the average database record size is 682. This is approximately 1/6th of the block or CI size. 1/6th of the block or CI size is not very large and seems acceptable. However, it could be reasonable to experiment with an 8 KB block or CI size.

A decision to increase the block or CI size from 4 KB to 8 KB for this database is probably a matter of the personal taste of the database administrator.