OpenVMS Cluster Systems - OpenVMS Systems - HP

OpenVMS Cluster Interconnect Configurations
3.2 OpenVMS Cluster Systems Interconnected by CI
3.2.3 Star Couplers
What appears to be a single point of failure in the CI configuration in Figure 3–1
is the star coupler that connects all the CI lines. In reality, the star coupler is
not a single point of failure because there are actually two star couplers in every
cabinet.
Star couplers are also immune to power failures because they contain no powered
components but are constructed as sets of high-frequency pulse transformers.
Because they do no processing or buffering, star couplers also are not I/O
throughput bottlenecks. They operate at the full-rated speed of the CI cables.
However, in very heavy I/O situations, exceeding CI bandwidth may require
multiple star couplers.
3.3 OpenVMS Cluster Systems Interconnected by DSSI
The DIGITAL Storage Systems Interconnect (DSSI) is a medium-bandwidth
interconnect that Alpha and VAX nodes can use to access disk and tape
peripherals. Each peripheral is an integrated storage element (ISE) that
contains its own controller and its own MSCP server that works in parallel with
the other ISEs on the DSSI.
3.3.1 Design
Although the DSSI is designed primarily to access disk and tape storage, it has
proven an excellent way to connect small numbers of nodes using the OpenVMS
Cluster protocols. Each DSSI port connects to a single DSSI bus. As in the case
of the CI, several DSSI ports can be connected to a node to provide redundant
paths between nodes. However, unlike CI, DSSI does not provide redundant
paths.
3.3.2 Availability
OpenVMS Cluster configurations using ISE devices and the DSSI bus offer high
availability, flexibility, growth potential, and ease of system management.
DSSI nodes in an OpenVMS Cluster configuration can access a common system
disk and all data disks directly on a DSSI bus and serve them to satellites.
Satellites (and users connected through terminal servers) can access any disk
through any node designated as a boot server. If one of the boot servers fails,
applications on satellites continue to run because disk access fails over to the
other server. Although applications running on nonintelligent devices, such as
terminal servers, are interrupted, users of terminals can log in again and restart
their jobs.
3.3.3 Guidelines
Generic configuration guidelines for DSSI OpenVMS Cluster systems are as
follows:
• Currently, a total of four Alpha and/or VAX nodes can be connected to a
common DSSI bus.
• Multiple DSSI buses can operate in an OpenVMS Cluster configuration, thus
dramatically increasing the amount of storage that can be configured into the
system.
OpenVMS Cluster Interconnect Configurations 3–3