Availability Guide for Application Design

Overview of Server and Network Fault Tolerance
Availability Guide for Application Design—525637-004
2-25
The HP Integrity NonStop NS-Series Server
Provides Another Level of Availability
Processes called synchronization and rendezvous at the LSUs perform two main
functions:
•
To keep the individual processor elements (PEs) in a logical processor in loose
lock-step through a technique called rendezvous. Rendezvous occurs to:
°
Periodically synchronize the PEs so they execute the same instruction at the
same time. Synchronization accommodates the slightly different clock speed
within each PE.
°
Allow each PE to individually and deterministically respond to asynchronous
incoming interrupts and then to respond together as a logical processor.
°
Exchange software state information when performing operations that are
distributed across PEs; for example, memory reintegration, error handling, and
memory scrubbing.
•
To compare output from each PE. If identical, the output is transmitted over the
ServerNet fabrics. If the PE outputs are not the same, appropriate actions occur to
recover from the failure. Under some failure conditions, it can be necessary to stop
normal operations of a PE.
A process known as reintegration is used to start processing in a PE where processing
has been stopped due to either a failure or a service action. Reintegration requires that
all of the memory and processor state be copied from a functioning slice to the target
slice. Once the memory and processor state information are copied, rendezvous is
used to complete the reintegration. This entire reintegration operation is invisible to the
running applications.
As shown in Figure 2-9, input/output components also connect to the ServerNet fabrics
through ServerNet adapters. The I/O adapter module (IOAM) houses Fibre Channel
ServerNet adapters (FCSAs) and gigabit 4-port Ethernet ServerNet adapters (G4SAs)
that provide the system I/O to storage and communications networks. Connection to
the ServerNet fabric via the NonStop S-series I/O enclosure and the IOMF 2 provides
additional ServerNet interfacing for the most commonly used peripherals. The
ServerNet adapters handle all I/O devices such as disk drives, tape drives, and
workstations.
As Figure 2-9 shows, each NS-series server has two processor switches, or
P-switches, one for the ServerNet X fabric and one for the ServerNet Y fabric. Each
P-switch acts as a hub for its ServerNet fabric, providing connectivity for processors,
I/O, ServerNet cluster switches, and the local area network (LAN).
The entire system is easily networked with other NonStop systems because all
systems use the same message system and the same network software—even though
the internal hardware architecture might differ from one series of servers to another.