RDF/IMP and IMPX System Management Manual (RDF 1.3+)

Triple Contingency
Compaq NonStop™ RDF/IMP and IMPX System Management Manual—522204-001
10-2
How Does It Work?
How Does It Work?
In general, the triple contingency feature works as follows:
•
The RETAINCOUNT configuration parameter on both backup systems prevents
the purger process from purging image trail files that might be needed for triple
contingency recovery.
•
If the primary system fails, you execute two takeovers: one on each backup system.
Upon successful completion of both takeovers (signalled by a 724 message in the
EMS event log of both backup systems), the databases on the two backup systems
will almost assuredly not be identical: one of the extractors will have been ahead of
the other in its RDF processing when the failure occurred.
•
Examine the EMS event log on both backup systems for a 735 message. That
message, which follows the 724 message in the log, specifies the last position in the
MAT that was seen by the receiver process. Compare the MAT positions in the two
735 messages and determine which of the two systems was further behind in its
RDF processing when the failure occurred (that is, which system had received the
least amount of audit data from the extractor by the time the primary system was
lost).
•
On the backup system that was further behind (had the least amount of audit data),
issue the COPYAUDIT command specifying the name of the other backup system
and its RDF control subvolume. That command copies over all missing audit
records from the designated system.
•
Upon successful completion of the COPYAUDIT operation, do a second takeover on
that system. When the second takeover has completed successfully, initialize and
configure the two backup systems as a new primary-backup pair (either system can
be the primary), turn on AUDIT on the pertinent database files on the new primary
system, and then restart application processing on the new primary system.
The remainder of this section discusses the hardware and software requirements, the
RETAINCOUNT parameter, and the COPYAUDIT command in detail.
Hardware Requirements
Both backup systems must have exactly the same hardware configuration with respect to
RDF operation (in particular, the RDF disk configuration must be identical between the
two systems). It is also strongly recommended that the Expand bandwidth between the
primary and backup systems be the same for both configurations.
WARNING. To be able to use the triple contingency feature, it is imperative that you carefully
obey the instructions and caveats presented in this section.