VSAM dataset reaches CI or CA split pressure

VSAM performance problems — slow reads, CI/CA splits — come from cluster design not matching access patterns.

What it means

• CI/CA splits happen when inserts do not fit; FREESPACE reserves room for growth.
• Wrong sizing forces constant splitting.

Common causes

• No FREESPACE on an insert-heavy KSDS.
• CI size wrong for the record length.
• Excessive alternate indexes.
• RLS vs non-RLS mismatch with the workload.

How to diagnose

• LISTCAT shows CI/CA split counts.
• Measure read/write ratios.
• Check FREESPACE and CISZ.

How to fix it

• Reorganize (REPRO out/in) with proper FREESPACE.
• Tune CISZ to the record size.
• Drop unused AIX paths.

Example: Leave room for inserts

FREESPACE prevents split storms on growing files.

Interview tip

• Q: How would you troubleshoot a VSAM dataset reaches CI or CA split pressure in production?
• A: I would check split counts in LISTCAT, review FREESPACE and CI size, and reorganize with corrected parameters.

Prevention checklist

• Size for growth, not just today.
• Monitor split rates.