Understanding Discrepancies in Tape Consumption Reporting Between Disk and CA 1 TMS

Products

• Disk Backup and Restore - MVS
• CA 1 Tape Management - Add-On Options
• CA 1 Flexible Storage
• CA 1 Tape Management - Copycat Utility
• Vtape Virtual Tape System

Issue/Introduction

When evaluating tape storage consumption for VTS capacity planning, discrepancies are often observed between Disk reporting (LISTV/LISTD) and CA 1 TMS reporting. For example, Disk may report a significantly lower or higher volume of data than CA 1 for the same set of resources.

Cause

The discrepancy in totals between these products is due to differences in reporting metrics:

1. Application vs. Physical Data: Disk (LISTD/LISTV) typically reports "raw" application-level data. It calculates what the application believes it wrote based on record counts and logical lengths.
2. Compression and Optimization: Modern tape environments use Subsystem Compression and Dynamic Block Size Allocation (e.g., via DFDSS or FDR). These optimizations reduce the physical footprint on the media.
3. Reporting Variables: CA 1 TMS tracks both the raw data sent to the drive and the actual physical bytes stored after hardware compression. If comparing a "Logical" report from Disk to a "Physical" report from CA 1, the numbers will not match.

Resolution

To accurately calculate tape consumption and account for compression/optimization for capacity planning, use the CA 1 EARL report TMETPUSE. This report provides two critical variables:

• T_DESCMBYTES (Bytes Used): This represents the raw volume of data actually written by the application.
• T_DESCMBYTEST (Bytes Stored): This represents the physical footprint after device-level compression. For VTS capacity planning, Bytes Stored is the metric that reflects true media/cache consumption.

Steps to Run the Report:

1. Locate the sample JCL member TMSEARL in your *.CTAPJCL library.
2. Modify the SYSIN to point to member TMETPUSE in your *.CTAPEARL library.
3. Ensure the TAPEDB DD points to your current Tape Management Catalog (TMC).
4. (Optional) To filter for specific High-Level Qualifiers (HLQs), modify the EARL code: IF DSN != 'YOUR.HLQ.-' THEN GOTO START
5. Submit the JCL and review the GRAND TOTAL in the SYSEARL output.

Note: If using small block sizes (e.g., 2K or 4K), ensure you have applied maintenance for TMETPUSE or utilize related EARL reports TMEENC01 and TMEENC02 for more granular accuracy.