Selecting the Directory-to-Element Ratio

You cannot control directly the number of directory entries or data elements the cache structure will hold. The installation uses the CFRM policy to specify the amount of storage a particular cache structure will occupy. When the cache structure is allocated, its storage is subdivided to reserve space for cache structure components such as data elements and directory entries. The value you specify for the directory-to-element ratio is used by the system to determine the proportion of the cache structure storage to allocate to each component. The ratio, expressed as a pair of whole numbers, such as 1:4, is passed to IXLCONN using the DIRRATIO and ELEMENTRATIO parameters as follows:

The DIRRATIO parameter specifies the part of the ratio for the directory entries (for instance, the 1 in the 1:4 ratio)
The ELEMENTRATIO parameter specifies the part of the ratio for the data elements (for instance, the 4 in the 1:4 ratio).

In general, the directory-to-element ratio should reflect the average number of data elements per cache entry. For example, if your data element size is 4096 bytes, and you estimate that about half of the cache entries will require 1 data element and about half of the cache entries will require 8 data elements, then you would want a ratio of 1:4.5 which you would express in whole numbers as 2:9.

Although you request a particular directory-to-element ratio, the system might use a slightly different ratio. The actual number of entries and elements in the structure, rather than the ratio, is returned to you in the IXLCONN answer area mapped by the IXLYCONA macro. Note that these values in IXLYCONA are not exact values.

If the directory-to-element ratio is incorrect for your use of the structure, you will encounter frequent rejections of IXLCACHE requests because either the cache or cache structure is full.

See Using the IXLCSP Service to Determine Structure Size or Attributes.