OpenVMS Shadowing is a high-availability and fault-tolerant feature in the OpenVMS operating system. It provides data redundancy and continuous operation in the event of hardware or software failures. The concept of shadowing involves maintaining duplicate copies, or shadows, of data on multiple physical storage devices.
In a shadowing configuration, data is organized into volumes, and each volume has one or more shadow sets. A shadow set consists of a primary volume and one or more secondary volumes. The primary volume holds the original data, while the secondary volumes maintain synchronized copies of that data.
When a write operation occurs, the data is simultaneously written to both the primary and secondary volumes, ensuring that the copies are consistent. The shadowing software uses various techniques to keep the copies synchronized, such as disk write queuing and data interleave.
In the event of a failure, such as a disk failure or a system crash, OpenVMS can automatically switch to using the secondary volumes as the primary source of data. This failover process is transparent to the applications running on the system. Once the primary volume is repaired or the system recovers, the shadowing software resynchronizes the data between the primary and secondary volumes to restore redundancy.
Shadowing provides several benefits, including:
High availability: By maintaining synchronized copies of data, shadowing ensures that if a primary volume becomes unavailable, the system can continue to operate using the secondary volumes.
Data protection: The redundancy offered by shadowing protects against data loss caused by disk failures. If a disk fails, the data remains accessible from the secondary volumes.
Improved performance: OpenVMS can leverage the multiple copies of data to parallelize read operations, potentially improving performance.
Simplified maintenance: Shadowing allows for hot-swapping of disks, which means failed disks can be replaced without shutting down the system.
It's important to note that shadowing requires dedicated hardware support, such as specialized disk controllers capable of handling shadow sets. Additionally, configuring and managing shadowing involves specific administrative tasks to set up and monitor the shadow sets.
Overall, OpenVMS Shadowing is a powerful feature that provides fault tolerance and continuous operation for critical systems running on the OpenVMS platform.