NonStop Systems Introduction for H-Series RVUs

The NonStop Operating System
NonStop Systems Introduction for H-Series RVUs—540083-001
6-16
Process Pairs
checkpoint messages to its backup in Processor 1. If Processor 0 fails, the primary
disk process fails. Processor 1 then informs the backup disk process that Processor 0
has failed, and the file system automatically reroutes requests to this disk process,
which has become the new primary process.
As a result, the users of any programs that access the disk drive attached to the failed
processor (Processor 0) can still perform data retrieval and update operations on disk
files without any interruption. This fault tolerance would not be possible without the
reliable delivery of checkpoint messages by the message system.
Most operating system processes, like the disk process in Figure 6-10, are
implemented as process pairs because all the application programs running in the
system require the continuous availability of these system processes for successful
execution.
It is also possible to implement user processes as process pairs, and you can do so
without writing special code for the checkpointing.
Figure 6-10. Software Fault Tolerance With Process Pairs
Dual ServerNet fabrics
Processor 0 Processor 1 Processor 2
Primary
disk process
Backup
disk process
Checkpoint
messages
to backup
process
ServerNet
adapter
ServerNet
adapter
VST080.vsd