NonStop Systems Introduction

NonStop Server Architecture
NonStop Systems Introduction—527825-001
7-3
Fault Tolerance of NonStop Servers
Significantly, the hardware is designed to provide two forms of continuous availability
when an individual component fails:
•
Continuous execution of processes
•
Continued access to databases
Figure 7-3 on page 7-3 illustrates both of these design goals.
The system achieves fault tolerance for user processes and system processes by
running programs as process pairs in different processors. The primary process
executes actively in one processor while the backup process remains in a wait state in
another processor. At critical points, the primary process sends checkpoint messages
over the dual ServerNet fabrics to the backup process.
If the processor that is executing the primary process fails, the backup process in the
healthy processor takes over where the primary process left off. It resumes execution
of the work from the last valid checkpoint and accesses the same database on disk
that the primary process had been using.
The system achieves fault tolerance for the user’s database by ensuring that data
transfers between processes and storage devices can take place over two separate
and independent paths. In this way, if a component in one of the paths fails, the
system can use the other path to access the database. As a result, the failure of a
single component does not prevent access to the database, just as the failure of a
single processor does not halt the execution of processes.
The specific mechanisms used by the system to provide two separate data paths
include dual-ported ServerNet adapters (either on the ServerNet side or on the
peripheral side) and mirrored disks. As Figure 7-3 shows, mirrored disks are disks
that contain identical copies of the database. Although both disks are always in use for
optimum efficiency, if one of these disks fails, the process can continue accessing only
Figure 7-3. Continuous Availability of a NonStop Server
Processor 0
Processor 1
Checkpoint
message
s
Primary
process
Backup
process
Mirrored
disk volumes
Volume
s
VST092.vsd