The I in the graph symbolize space that is used to hold the pointers to actual user data in the data component. This consists of some characters for control information, the pointers to the data control intervals, and most importantly the key value (in compressed format) of the highest record in each control interval. This key is used by VSAM in keyed access to determine if a particular data control interval has the record it is looking for, without having to read each data control interval itself.
There are two column headings that relate directly to the index entries in the graph. The first one is labeled AVG ENTL, which stands for the average entry length of an index entry. This value is the average number of characters required to reference a single data control interval, which includes control information overhead and the compressed key of the data. This value is important when looking at the issue of unreferenceable control intervals, which we will look at in Unreferenceable Data Control Intervals.
The second column applicable to index entries is AVG KEYL. This value represents the average number of characters required to hold each compressed key. What this value tells you is how well VSAM is compressing the keys used to reference the data. This information can be used when deciding upon an explicit CI size for the index if you want to reorganize the cluster with CA Disk (using ARCHIVE and RESTORE). The new CI size can be explicitly stated on the RESTORE command.
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