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

Introducing RDF
Compaq NonStop™ RDF/IMP and IMPX System Management Manual—522204-001
1-24
Triple Contingency
The control subvolume on the backup system contains these files:
•
$SYSTEM.<control-subvolume >.CONTEXT
•
$SYSTEM.<control-subvolume >.CONFIG
•
$SYSTEM.<control-subvolume >.RDFTKOVR
•
$SYSTEM.<control-subvolume>.RDFLOCK
•
Exception files for each updater
•
Image files for image trail subvolumes (for example, $IMAGE.<control-
subvolume >.AA000001).
•
$IMAGE.<control-subvolume >.ZTXUNDO (master image trail only)
Triple Contingency
If you are replicating your database to multiple backup systems, you can perform an
RDF takeover to any of the backup systems upon loss of the primary system and
continue application processing on the new system within minutes. To proceed with full
RDF protection, however, you must:
1. Initiate a takeover on two of the backup systems.
2. Synchronize the two databases.
3. Configure the two systems as a primary-backup pair.
4. Initialize and start RDF on the system that you want to be the new primary system.
Depending upon the size of your database, the second step listed, database
synchronization, could take days to accomplish without the RDF triple contingency
feature. Triple contingency, however, streamlines this step, enabling you to achieve rapid
database synchronization after a takeover operation. Triple contingency allows your
applications to resume, with full RDF protection, within minutes after the loss of your
primary system, provided that the two systems are not too far behind.
The triple contingency feature builds upon the ability to replicate to multiple backup
systems. To use this feature, you establish two essentially identical RDF configurations,
as follows:
RDF Configuration #1
\A ---------> \B
RDF Configuration #2
\A ---------> \C
With both RDF configurations in operation, your data is replicated to the two backup
systems (\B and \C). Because the two configurations operate independently of one
another, if the primary system fails, it is unlikely that the databases on the two backup
systems will be logically identical to each other after RDF takeover operations. The
extractor in one RDF configuration may have just read and sent new audit to its backup
system, but the primary system might have failed before the other extractor could send