HP 3000 Manuals

Silhouette Reference Manual

Secondary System Failure 

When the secondary system fails, the Communication Process (CP) on the
primary system does not receive confirmation of its last transmission to
the Secondary Communication Process (SP). To the CP, this appears to be a
simple communication line failure.  The CP informs the Silhouette
Operator of the problem and begins to keep track of the records added to
the primary system logfile.  Since the secondary system may not return to
operation for an undefined period of time, the CP requests that a tape be
mounted and begins to write log records to tape when the number of new
records in the logfile exceeds the high water mark.  This number is set
by the Silhouette Operator in the configuration file for the Silhouette
process.  When the high water mark is exceeded, all logfile records not
previously sent to the secondary system are stored to tape.  Figure 3-8
shows that when this is done duplication occurs even when the secondary
system is not operating.


          Figure 3-8.  When the Secondary is Down 

The tape is handled by the Silhouette Tape Process (TP) which is created
by the CP at startup time.  When the high water mark is exceeded, the CP
informs the TP which requests that a tape be mounted.  When the
Silhouette Operator answers the tape request, the TP reads its records
directly from the logfile and transfers them to tape.  Should the
secondary system be returned to service, the normal catch up process
operates automatically to bring the secondary system into step with the
primary.  In this event the tape is not used in bringing the secondary
up-to-date.  In fact the tape produced by the TP is useful only if the
primary system fails some time after the secondary system has failed.  If
such a double failure occurs, the tape can be used to bring the secondary
system up-to-date without the presence of the primary so that processing
can be switched to that system.

The high water mark is set by the Silhouette Operator in the
configuration file for the Silhouette process.  The value set for the
high water mark represents a compromise between convenience and risk.  If
the mark is set too low, the Silhouette Operator may have to mount a tape
and dismount it every few minutes when the TP process alternately falls
behind and catches up as the processing load on the primary system
fluctuates.  Setting the mark higher represents a convenience for the
Silhouette Operator.  However, the high water mark is also a measure of
the number of records which might be lost if the primary fails after the
secondary fails but before the high water mark is reached.  In this
situation, there is no backup anywhere of the latest records in the
primary logfile.  Setting the mark lower represents less risk of losing
records should both systems fail.

MPE/iX 5.0 Documentation