The host-based
implementation of volume shadowing allows disks that are connected
to multiple physical controllers to be shadowed in an OpenVMS Cluster
system. There is no requirement that all members of a shadow set
be connected to the same controller. Controller independence allows
you to manage shadow sets regardless of their controller connection
or their location in the OpenVMS Cluster system and helps provide improved
data availability and flexible configurations.
For clusterwide
shadowing, members can be located anywhere in an OpenVMS Cluster
system and served by MSCP servers across any supported OpenVMS Cluster
interconnect, including the CI (computer interconnect), Ethernet
(10/100 and Gigabit), ATM, Digital Storage Systems Interconnect
(DSSI), and Fiber Distributed Data Interface (FDDI). For example,
OpenVMS Cluster systems using FDDI and wide area network services
can be hundreds of miles apart, which further increases the availability
and disaster tolerance of a system.
Shadow Sets Accessed Through the MSCP Server shows how shadow-set
members are on line to local adapters located on different nodes.
In the figure, a disk volume is local to each of the nodes ATABOY
and ATAGRL. The MSCP server provides access to the shadow set members
over the Ethernet. Even though the disk volumes are local to different
nodes, the disks are members of the same shadow set. A member that
is local to one node can be accessed by the remote node via the
MSCP server.
Figure 3 Shadow Sets Accessed Through the MSCP Server
The shadowing software maintains shadow sets in a distributed
fashion on each node that mounts the shadow set in the OpenVMS Cluster
system. In an OpenVMS Cluster environment, each node creates and maintains
shadow sets independently. The shadowing software on each node maps
each shadow set, represented by its virtual unit name, to its respective
physical units. Shadow sets are not served to other nodes. When
a shadow set must be accessed by multiple nodes, each node creates
an identical shadow set. The shadowing software maintains clusterwide
membership coherence for shadow sets mounted on multiple nodes.
For shadow sets that are mounted on an OpenVMS Cluster system, mounting
or dismounting a shadow set on one node in the cluster does not
affect applications or user functions executing on other nodes in
the system. For example, you can dismount the shadow set from one
node in an OpenVMS Cluster system and leave the shadow set operational
on the remaining nodes on which it is mounted.
HP Volume Shadowing for OpenVMS
Introduction to Volume Shadowing for OpenVMS
