Volume Shadowing for OpenVMS is a System Integrated Product (SIP) that runs on the Integrity server. Volume Shadowing for OpenVMS implements a RAID Level 1 storage strategy that provides high data availability for disk devices by preventing data loss resulting from media deterioration or from controller or device failure. This strategy also prevents storage subsystem component failures from interrupting system or application operations.
Volume shadowing, sometimes referred to as disk mirroring, maintains redundant copies of data on a collection of disk volumes (one copy per disk volume) called a shadow set. The shadow set can support up to six members. This duplication of data provides enhanced data availability. If data is recorded on multiple disk devices, it remains accessible if one device becomes unavailable. Disk read and write operations continue transparently with the remaining members of the shadow set. Note that the terms disk and device are used in this SPD to refer to a disk volume.
Because a shadow set is made up of multiple disks containing the same data, the Volume Shadowing for OpenVMS software can read data from any full member of the shadow set. For each read operation, the Volume Shadowing for OpenVMS software determines which disk to read from using an algorithm that maximizes performance. Volume Shadowing for OpenVMS ensures that disk write operations are duplicated on all shadow set members. For maximum performance, Volume Shadowing for OpenVMS ensures that shadow set write operations are issued in parallel to full shadow set members.
If some data on a full shadow set member becomes unreadable, the shadowing software can read the data from another available full member. Additionally, with DIGITAL Storage Architecture (DSA) disks, the member with unreadable data can be repaired by the controller by rewriting the data to viable, replacement areas provided on each disk. Small Computer Systems Interface (SCSI) devices certified by HP for use with volume shadowing support data repair. For other SCSI devices, replacement and repair algorithms are device specific.
OpenVMS Integrity Files-11 On-Disk Structure 2 (ODS-2), and Files-11 On-Disk Structure 5 (ODS-5) data disks that are certified by HPE can be volume shadowed.