Levels of High Data Availability Using Volume
Shadowing
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 disk
whose data can be accessed by applications at the same time the
data is being copied to a newly mounted target disk.
Once the data is copied, both disks contain identical information
and the target disk becomes a source member of the shadow set.
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 should 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.
Levels of Availability provides a qualitative,
high-level classification of how you can achieve increasing levels
of physical data availability in different types of configurations.
Figure 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.