VSAM file status 21 sequence error

VSAM file status 21: Sequence error — records written out of key sequence to a KSDS.

What it means

• Sequence error — records written out of key sequence to a KSDS.
• The key of the new record is lower than the previous one.

Common causes

• Input file not sorted by key.
• Key built incorrectly.
• Restart resumed at the wrong point.

How to diagnose

• Sort the input by key before the run.
• Verify key construction.
• Check sort stability for duplicate keys.

How to fix it

• Sort input by key.
• Fix key construction.
• Correct the restart point.

Example: Always check FILE STATUS after I/O

FILE STATUS after every I/O turns mystery failures into clear codes.

Interview tip

• Q: How would you troubleshoot a VSAM file status 21 sequence error in production?
• A: I would confirm the exact file status code, check the JCL DD and catalog, then apply the specific fix.

Prevention checklist

• Check FILE STATUS after every OPEN/READ/WRITE.
• Keep FD layouts in sync with cluster definitions.