𝗣𝗼𝘁𝗲𝗻𝘁𝗶𝗮𝗹 𝘇/𝗢𝗦 𝟯.𝟭 𝗗𝗲𝗳𝗲𝗰𝘁: 𝗕𝗮𝘀𝗶𝗰 𝗙𝗼𝗿𝗺𝗮𝘁 𝗦𝗲𝗾𝘂𝗲𝗻𝘁𝗶𝗮𝗹 𝗗𝗮𝘁𝗮𝘀𝗲𝘁𝘀 𝗘𝘅𝗰𝗲𝗲𝗱𝗶𝗻𝗴 𝘁𝗵𝗲 𝟲𝟱𝗞 𝗧𝗿𝗮𝗰𝗸 𝗟𝗶𝗺𝗶𝘁

A Mainframe Storage Group reached 99% utilization, resulting in unexpected abends for production jobs utilizing that storage group. As an immediate workaround, additional volumes were added to the storage group to restore capacity and prevent further job failures.

Following the temporary fix, we investigated which datasets were consuming the most space within the storage group. We identified the top storage-consuming datasets and analyzed the jobs responsible for creating them. The investigation revealed that a DB2 Image Copy job was generating these datasets.

The Image Copy job created backup datasets as GDG (Generation Data Group) generations on a daily basis. Since a new generation was created every day, the storage group's utilization continued to grow. To manage disk space, the job included logic to migrate older image copy generations to tape while retaining only the latest generation on disk.

Our analysis showed that most image copy datasets were being successfully migrated to tape. However, a small number of datasets remained on disk and were not being migrated. We further analyzed these datasets and compared the storage group growth trend. The investigation showed that the abnormal growth started after the z/OS 3.1 upgrade.

We then focused on understanding why these specific image copy datasets were not migrating to tape after the z/OS upgrade. The problematic datasets were found to be multi-volume datasets, and occupying more than 65,535 tracks in one or more volumes. These datasets were Basic Format Sequential Datasets.

According to IBM documentation, Basic Format Sequential Datasets are limited to 65,535 tracks per volume. However, these datasets exceeded that limit. As a result, SMS was unable to migrate them to tape, causing the datasets to remain on disk and contributing significantly to the continued growth of the storage group.

Based on the findings, it appears that the z/OS 3.1 was allowing Basic Format Sequential Datasets to exceed the documented 65K track limit per volume. This behavior is likely a defect.