From:Eric McPhail 
     Scottish Widows
     eric.mcphail@scottishwidows.co.uk

Subject:KSDS CISIZEs
Files: CALC3380.XLW, CALC3390.XLW

For KSDSs we normally choose a 4K Data CI and a 2K index CI, unless we have large keys.
Then we choose a larger Data CI and sometimes if the key is really big a larger Index 
CI as well.


What's big ?

Generally we could store 150 (the number of 4K CIs in the CA which is a Cylinder) keys
in a 2K index record. When we had 3380s the exceptions were keys greater than 30 bytes. 
These usually merited a larger Data CI.

We had to reconsider the 30 byte limit when we moved to 3390s. For a 4K Data CI we now
had to store 180 keys in the index record. We experienced large growth in DASD usage
and an extraordinary number of CI/CA splits. The cut off figure is now 24 bytes


What's really big ?

Keys > 60 bytes.

These two spread sheets for Microsoft Excel calculate the optimum CI sizes.

NB. The above should be used as a rule of thumb. VSAM compresses the keys and the amount
of compression depends on the variation of keys.

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