HSM - Hierarchical Storage Manager

Enhanced Definition

HSM (Hierarchical Storage Manager) is a z/OS component that automates the management of data across different levels of storage, from fast, expensive online DASD to slower, less expensive offline tape or virtual tape. Its primary purpose is to optimize storage utilization, improve data availability, and reduce manual storage administration tasks by automatically migrating infrequently accessed data and backing up critical data sets.

Key Characteristics

- Automated Data Migration: Automatically moves data sets between primary DASD, migration DASD, and tape storage based on user-defined policies (e.g., age, last access date).
- Space Management: Reclaims unused space on primary DASD by migrating eligible data sets, ensuring efficient use of high-performance storage.
- Availability Management: Provides automated backup and recovery services for data sets, allowing users to restore lost or corrupted data from backup versions.
- DFSMS Integration: Works seamlessly with DFSMS (Data Facility Storage Management Subsystem) to implement storage policies, often driven by SMS classes (Storage Class, Management Class, Data Class).
- Data Set Level Operations: Manages data at the individual data set level, providing granular control over migration, backup, and recovery processes.
- Command Interface: Offers a comprehensive set of TSO commands (e.g., HSM MIGRATE, HSM RECALL, HSM BACKUP, HSM RESTORE) for manual intervention and administration.

Use Cases

- Archiving Infrequently Used Data: Automatically migrates older, less frequently accessed production data sets from expensive online DASD to tape, freeing up primary storage.
- Automated Daily Backups: Configures policies to automatically back up critical application data sets daily, ensuring recoverability in case of data loss or corruption.
- Disaster Recovery Preparation: Utilizes HSM's backup and recovery capabilities to create offsite copies of vital system and application data, crucial for disaster recovery planning.
- Space Reclamation: Periodically runs space management cycles to identify and migrate inactive data, preventing DASD volumes from filling up and impacting system performance.
- User-Initiated Data Recall: Allows end-users or applications to recall migrated data sets back to primary DASD when needed, often transparently.

Related Concepts

HSM is a core component of DFSMS (Data Facility Storage Management Subsystem), working in conjunction with SMS policies (Storage Class, Management Class) to implement automated storage management. It interacts directly with DASD (Direct Access Storage Devices) and tape storage (physical or virtual tape libraries) as the tiers for data migration and backup. While HSM manages data sets, JCL can be used to invoke HSM utilities or to define data sets that will be managed by HSM policies. It is fundamental for storage administrators and system programmers to maintain optimal storage environments and ensure data availability.

Best Practices:

Define Clear Management Policies: Establish well-defined Management Class (MC) policies within DFSMS to dictate migration, backup, and retention rules based on data set characteristics and business requirements.
Monitor HSM Activity and Resources: Regularly monitor HSM logs, space utilization, and performance metrics to identify potential issues, ensure policies are effective, and plan for capacity.
Test Recovery Procedures: Periodically test the recall and restore processes for critical data sets to verify that backups are valid and recovery procedures work as expected.
Optimize Migration Thresholds: Tune migration thresholds and selection criteria to balance the cost of storage with the performance impact of recalling migrated data, avoiding excessive recalls.
Integrate with Disaster Recovery Plans: Ensure HSM backups and recovery strategies are fully integrated into the organization's overall disaster recovery plan, including offsite storage of backup tapes.