HP Volume Shadowing for OpenVMS ensures that data is available
for applications and end users by duplicating data on multiple disks.
Because the same data is recorded on multiple disk volumes, if one
disk fails, the remaining disk or disks can continue to service
I/O requests. This ability to shadow disk volumes is sometimes referred
to as disk mirroring.
Volume shadowing supports the clusterwide shadowing of DIGITAL
SCSI and DSA storage systems. Volume shadowing also supports shadowing
of all mass storage control protocol (MSCP) served DSA disks and DIGITAL
SCSI disks. For more information about Volume Shadowing supported
devices, refer to the Volume Shadowing for OpenVMS Software
Product Description.
You can mount one, two or three compatible disk volumes, including
the system disk, to form a shadow set. Each
disk in the shadow set is known as a shadow set member.
Volume Shadowing for OpenVMS logically binds the shadow set devices
together and represents them as a single virtual device called a virtual
unit. This means that multiple members of the shadow
set, represented by the virtual unit, appear to applications and
users as a single, highly available disk.
Volume Shadowing features include:
Controller independence. Shadow set
members can be located on any node in an OpenVMS Cluster that has
Volume Shadowing enabled.
Ability to survive controller, disk, and media failures
transparently.
Shadowing functions that do not affect application
I/O.
Applications and users read and write data to and from a shadow
set using the same commands and program language syntax and semantics
that are used for nonshadowed I/O operations. Volume shadowed sets
are managed and monitored using the same commands and utilities
that are used for nonshadowed disks. The only difference is that
access is through the virtual unit, not to individual devices.
SHDRIVER, the driver that controls the virtual unit functions,
is described in
Driver Functions.
For more detailed information on HP Volume Shadowing for OpenVMS,
refer to the Volume Shadowing for OpenVMS manual.