A key component of overall system availability
is availability or accessibility of data. Volume Shadowing for OpenVMS
provides high levels of data availability by allowing shadow sets
to be configured on a single-node system or on an OpenVMS Cluster
system, so that continued access to data is possible despite failures
in the disk media, disk drive, or disk controller. For shadow sets
whose members are local to different OpenVMS Cluster nodes, if one
node serving a shadow set member shuts down, the data is still accessible
through an alternate node.
You can create a virtual unit, the system representation
of a shadow set, that consists of only one disk volume. However, you
must mount two or more disk volumes in order to “shadow,”
that is, to maintain multiple copies of the same data. This configuration
protects against either failure of a single disk drive or deterioration
of a single volume. For example, if one member fails out of a shadow
set, the remaining member can be used as a source device whose data can be accessed by applications at the same time
the data is being copied to a newly mounted target device. Once the data is copied, both devices contain identical
information and the target device becomes a source member of the shadow
Using two controllers provides a further guarantee
of data availability in the event of a single-controller failure.
When setting up a system with volume shadowing, you must connect each
disk drive to a different controller I/O channel whenever possible.
Separate connections help protect against either failure of a single
controller or of the communication path used to access it.
Using an OpenVMS Cluster system (as opposed to
a single-node environment) and multiple controllers provides the greatest
data availability. Disks that are connected to different local controllers
and disks that are MSCP-served by other OpenVMS systems can be combined
into a single shadow set, provided the disks are compatible and no
more than three are combined. (Starting with OpenVMS Alpha Version
7.3-2 and OpenVMS Integrity servers Version 8.2, disks of different
sizes can be combined into a shadow set, as described in “Supported Devices ”.)
Figure 2-1 provides a qualitative, high-level classification of how you can
achieve increasing levels of physical data availability in different
types of configurations.
Figure 2-1 Levels of Availability
“Repair and Recovery from Failures” describes how you can configure your shadowed
system to achieve high data availability despite physical failures.