VOL SHAD SPD.PDF

Volume Shadowing for OpenVMS, Version 6.2, is a System Integrated Product for VAX and Alpha processors that implements a RAID Level 1 storage strategy, often called disk mirroring. Its primary goal is to provide high data availability and prevent data loss or system interruptions caused by media deterioration, controller, or device failures.

The software achieves this by maintaining redundant copies of data across a "shadow set" of one to three compatible disk volumes. It ensures that read/write operations continue transparently even if a member disk becomes unavailable. Reads are optimized from the best available member, and writes are duplicated in parallel to all members. It can also repair unreadable data by rewriting it to good areas on DSA and certified SCSI disks.

Key features include:

• Transparency: Users and applications interact with a single "virtual disk," unaware of the underlying multiple physical disks.
• Dynamic Membership: Shadow set members can be added or removed dynamically without interrupting system or user operations.
• Host-Based Implementation: It uses a host-based shadowing approach (Phase II), with previous controller-based (Phase I) methods no longer supported.
• Cluster Support: Fully supports OpenVMS Clusters (including mixed VAX/Alpha environments), allowing cluster-wide shadowing of Digital SCSI and DSA disk storage systems, and MSCP-served disks.
• Fault Tolerance: Operations continue with remaining members after a disk failure; inoperative disks are automatically removed.

Configuration limits allow for a maximum of 400 disks in multimember shadow sets per OpenVMS Cluster. Key restrictions dictate that all shadow set members must have identical disk geometry, cannot have hardware write protection enabled, and third-party SCSI disks must support READL/WRITEL commands for full functionality. OpenVMS Cluster quorum disks cannot be shadowed.

The product requires OpenVMS Version 6.2 and operates on any supported CPU with disk controllers and drives compatible with the OpenVMS Operating System. Licensing is available on either a capacity or a per-disk basis.