Multivolume Sets

The OpenVMS file system provides support for several types of multivolume disk sets, including volume sets, shadow sets, hardware-and software-based stripe sets, and RAID arrays.

Volume sets are multiple volumes treated as a single unit, wherein a portion of a new file may be stored at any available location on any volume. No data redundancy is provided; each cluster of file data resides exclusively on a single volume. The term volume set may apply to ISO 9660 volume sets, also supported by OpenVMS, as well.

Shadow sets are two-or three-disk sets in which all volumes contain exactly the same information. Each write operation is applied to all volumes, and read operations can be performed by a single volume. Read performance can be increased by assigning separate read operations to separate volumes.

Stripe sets are multivolume sets that have their logical block numbers interleaved between volumes. They can be roughly envisioned as several disks "stacked" atop one another so that a single cylinder encompasses the same track on all volumes.

RAID arrays are typically hardware-based multivolume sets that split each data byte across all volumes, such that if one disk were to fail, the data could be reconstructed from the remaining members.

A given multivolume set can be constructed so as to use two or more of the techniques discussed above. For example, two RAID sets could be combined into a volume set.