Abstract: MVS mainframe computer systems employ the ICF (Integrated Catalog Facility) catalog environment to manage numerous data sets. To provide 24×7 availability of those data sets, the BCS (250,270) must be re-organized while leaving the catalog open to access by applications. To perform a re-org while open, a data CI correlation table (500) is constructed (314) and used to lay the data CIs into a backup file in logical order (316), so that they can be loaded into the new BCS (324) without sorting, thereby reducing downtime. Additionally, if the BCS is damaged during normal operation, repair of the structure must also be performed to ensure that correct access is maintained to all data that is cataloged in the BCS. Throughout the re-org process, structural integrity checks are performed on the data and index component of the BCS, and if errors are encountered, they can be repaired.