Availability

Enhanced Definition

Availability, in the mainframe context, is a critical measure of a system's uptime and its ability to provide continuous, uninterrupted service for business-critical applications and data. It quantifies the proportion of time a system is operational and accessible to users and other systems, often expressed as a percentage (e.g., "five nines" or 99.999%).

Key Characteristics

- High Uptime Requirements: Mainframes are engineered for extremely high availability, often targeting 99.999% (five nines) or greater, meaning less than 5 minutes of unplanned downtime per year.
- Redundancy: Extensive hardware and software redundancy is built-in, including redundant power supplies, processors, I/O channels, network paths, and disk subsystems (e.g., RAID, mirroring).
- Fault Tolerance: The ability of the system to continue operating without interruption despite the failure of individual components, often achieved through automatic failover and recovery mechanisms.
- Disaster Recovery Capabilities: Robust solutions like GDPS (Geographically Dispersed Parallel Sysplex) and z/OS Global Mirror enable rapid recovery and failover to a remote site in the event of a catastrophic disaster.
- Dynamic Reconfiguration: Many components can be added, removed, or reconfigured dynamically (e.g., LPARs, CPUs, I/O devices) without requiring a system outage.
- Planned vs. Unplanned Outages: Differentiates between scheduled maintenance (which can often be performed with minimal or no service interruption) and unexpected system failures.

Use Cases

- Online Transaction Processing (OLTP): Ensuring continuous operation of CICS and IMS TM systems for critical applications like banking transactions, airline reservations, and retail point-of-sale.
- Database Services: Maintaining constant access to DB2 and IMS DB databases for applications that require real-time data retrieval and updates.
- Business Continuity: As a core metric and objective for all business continuity and disaster recovery planning, ensuring services remain available even after major disruptions.
- System Software Operations: Guaranteeing that z/OS and its core components (e.g., JES2, VTAM) are continuously available to support all workloads.
- Critical Batch Processing: Ensuring that time-sensitive batch jobs can complete within their designated windows without interruption, impacting downstream processes.

Related Concepts

Availability is intrinsically linked to Reliability, Availability, Serviceability (RAS), a foundational principle of mainframe design. It is heavily enabled by Parallel Sysplex technology, which allows multiple z/OS systems to share workloads and data, providing continuous availability even if one system fails. Workload Manager (WLM) contributes by ensuring critical applications receive necessary resources, preventing performance bottlenecks that could lead to perceived unavailability. Furthermore, robust Disaster Recovery solutions are designed specifically to restore or maintain availability after catastrophic events.

Best Practices:

Implement Parallel Sysplex: Leverage Parallel Sysplex for high availability and workload balancing across multiple LPARs, enabling continuous operations during planned and unplanned outages.
Regular Disaster Recovery Drills: Periodically test disaster recovery procedures and failover mechanisms to ensure they function as expected and meet recovery time objectives (RTO).
Automated Monitoring and Alerting: Deploy comprehensive monitoring tools (e.g., OMEGAMON) to proactively detect potential issues, resource constraints, or component failures that could impact availability.
Strict Change Management: Implement rigorous change control processes for all hardware and software modifications to minimize the risk of introducing instability or outages.
Capacity Planning: Continuously monitor and plan for sufficient system resources (CPU, memory, I/O) to handle peak workloads and growth, preventing performance-related availability issues.