OpenVMS Cluster Systems - OpenVMS Systems - HP

Cluster Storage Devices
6.5 Managing Cluster Disks With the Mount Utility
inconvenienced by the blocked access to disks (for example, between midnight and
6 a.m. each day). Because the SET VOLUME/REBUILD command determines
whether a rebuild is needed, the procedures can execute the command for each
disk that is usually mounted.
Suggestion: The procedures run more quickly and cause less delay in disk
access if they are executed on:
• Powerful computers
• Computers that have direct access to the volume to be rebuilt
Moreover, several such procedures, each of which rebuilds a different set of disks,
can be executed simultaneously.
Caution: If either or both of the following conditions are true when mounting
disks, it is essential to run a procedure with SET VOLUME/REBUILD commands
on a regular basis to rebuild the disks:
• Disks are mounted with the MOUNT/NOREBUILD command.
• The ACP_REBLDSYSD system parameter is set to 0.
Failure to rebuild disk volumes can result in a loss of free space and in
subsequent failures of applications to create or extend files.
6.6 Shadowing Disks Across an OpenVMS Cluster
Volume shadowing (sometimes referred to as disk mirroring) achieves high data
availability by duplicating data on multiple disks. If one disk fails, the remaining
disk or disks can continue to service application and user I/O requests.
6.6.1 Purpose
Volume Shadowing for OpenVMS software provides data availability across the
full range of OpenVMS configurations—from single nodes to large OpenVMS
Cluster systems—so you can provide data availabililty where you need it most.
Volume Shadowing for OpenVMS software is an implementation of RAID 1
(redundant arrays of independent disks) technology. Volume Shadowing
for OpenVMS prevents a disk device failure from interrupting system and
application operations. By duplicating data on multiple disks, volume shadowing
transparently prevents your storage subsystems from becoming a single point of
failure because of media deterioration, communication path failure, or controller
or device failure.
6.6.2 Shadow Sets
You can mount one, two, or three compatible disk volumes to form a shadow
set, as shown in Figure 6–9. 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 the multiple members of the shadow set, represented
by the virtual unit, appear to operating systems and users as a single, highly
available disk.
Cluster Storage Devices 6–27