NonStop Systems Introduction

The NonStop Kernel
NonStop Systems Introduction—527825-001
6-16
Fault-Tolerant Expand Operation
communications lines. In short, the Expand software supports the message system in
a network environment.
Because it is based on the inherent networking capabilities of the message system, the
Expand networking software is a compact, efficient body of code that works in close
cooperation with the system code.
The Expand software provides the following capabilities for a NonStop network:
•
Connection of multiple NonStop systems in the same network. An Expand network
can consist of up to 255 NonStop systems.
•
Passthrough routing. Systems do not need to be connected directly to each other
to exchange data.
•
Multiple communications lines between nodes. The communications link between
two adjacent nodes can consist of several physical communications lines, all of
which can be carrying data at the same time. As in the paragraphs that follow, this
increases both the overall bandwidth and the fault tolerance of the network.
•
Best-path routing. When there are multiple paths between systems (multiple
routes that a message could follow), the Expand software routes data automatically
over the best path.
•
Enhanced routing. Users can define different transmission priorities for each
message, and the Expand software orders message transmissions on the basis of
these priorities.
Figure 6-10 on page 6-17 shows best-path routing in an Expand network. To route a
message from a process in node A to a process in node E, Expand would
automatically choose the path consisting of links A-B, B-D, and D-E rather than the
path consisting of links A-C, C-D, and D-E. This is because link A-B has a faster data
transfer rate in bits/second (bps) than link A-C.
Fault-Tolerant Expand Operation
You can configure an Expand network to be continuously available. You can configure
as many as eight lines between the same two nodes using the Expand multiline path
feature. Expand can simultaneously transmit data over all of the lines, thus increasing
overall bandwidth. Moreover, if one of the lines fails, Expand will automatically reroute
data over remaining lines.
You can configure as many as 16 paths between the same two nodes using the
Expand multi-processor feature. This feature enables you to spread the
communications load over multiple processors by connecting multiple Expand line-
handler processes on separate processors at one node to Expand line-handler
processes on separate processors on another node. Expand transmits data between
neighbor nodes over all of the paths in a multi-processor path, and will automatically
reroute data over remaining paths if one or more paths fail.
Expand also uses a rebalancing algorithm to ensure that the average communications
load of all the paths in a multi-processor path is close to equal.