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This parameter reserves space in either the CI or CA at load time so that new records or new CIs are usually written without splits taking place. FREESPACE works when file updates are nonclustered in a key range. It can work with clustered updates, but the space is allocated to the entire file to help one CI or CA and thus it is wasted. If many updates/additions take place with similar keys, large amounts of free space are needed to prevent splits. In fact, the maximum value (100,100), which puts one record in a CI and one CI in a CA, cannot be enough.
CI splits occur when not enough free space is available to contain a record, which is being inserted. Without Sequential Insert Strategy (SIS), VSAM must move half the data in the CI to a new CI before continuing with the insert operation. These splits result in CPU and I/O overhead and should be avoided. In addition, logically ascending data is no longer physically adjacent so any sequential access to the file is slowed down.
If CI splits occur over the period of several days, the file needs to be reorganized. If they occur in a period of a day or less, increase the CI FREESPACE parameter and reorganize the file. CI free space is a percentage of the CI size and must be large enough to insert a whole record.
Whereas CI splits represent minimal to moderate overhead, the amount of processing incurred by a CA split is much greater. A CA split occurs when there is not enough room to contain the new CI created during a CI split. When this happens, VSAM must create a new control area, move 50 percent of the data from the old CA to the new one, update the sequence set, update part of the index set, and then continue with the original CI split. With Sequential Insert Strategy (SIS), this process is similar except the old CA is split at the CI being split instead of in the middle. With cylinder CAs (no SIS), both half a cylinder of data is moved and a minimum of one level of index is located and updated before the original CI split can occur. Like CI splits, CA splits call for reorganizations and FREESPACE.
As mentioned above, CA splits are of greater concern than CI splits. Generally it is a matter of how much free space you can afford to allocate to a file. Putting free space in a file increases the number of data CIs and CAs, thereby increasing index CI size requirements and levels as well as the file's space allocation. Splits are probably the number one cause of poor CICS response time. All the queuing that takes place while CICS is nondispatchable uses a lot of CPU time, requires additional I/O, and makes CICS less responsive. Although CICS puts a file in LSR, it does not activate Deferred Write (DFR), meaning CICS waits on physical I/Os during splits. VPE can help by turning on Sequential Insert Strategy (SIS) and Deferred Write (DFW) for files that CICS has put into LSR as well as to identify which files contain splits.
A split in an index component CI or CA indicates the index component CI is undersized. In addition to increasing the index CI size, you can give the index component its own FREESPACE.
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