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

Introducing RDF
Compaq NonStop™ RDF/IMP and IMPX System Management Manual—522204-001
1-5
Planned Outages
In the example illustrated in Table 1-1, a disaster has brought down the primary system
immediately after the commit record for transaction 100 was written to the MAT, but
before the RDF extractor process was able to send the commit record to the backup
system. For transaction 101, a single update was logged in the MAT and sent to the
backup system, but the primary system was brought down before the transaction was
completed.
When the command for a takeover is issued, the updater processes treat all transactions
whose outcomes are not known as aborted transactions. In this scenario, only the
changes related to transactions known with certainty to have been committed on the
primary system are left in the backup database. Therefore, in the example illustrated in
Table 1-1
, the audit information associated with transactions 100 and 101 is backed out
of the backup database.
Typically, the extractor process sends audit information to the backup system within a
second after it has been written to the MAT on the primary system, so a minimum
number of transactions are lost when a disaster brings down the primary system.
Planned Outages
RDF can be very useful when a planned shutdown of the primary system is necessary.
For example, you might need to bring the system down to install new hardware or to
perform a system software upgrade. In such a situation, you might determine it is
unacceptable to stop your business applications for the time required.
With RDF, you need only stop the applications momentarily, do a switchover from the
primary system to the backup system, and then restart the applications on the backup
system. When the primary system is ready for use again, you can use RDF to bring the
primary database up-to-date with changes made to the backup database while the
primary system was shut down. After the primary database is consistent with the backup
database, you can perform another switchover, this time from the backup system to the
primary system, and then restart the applications on the primary system. For instructions
on how to perform a switchover, see Carrying Out a Planned Switchover
.
Features
In providing backup protection for online databases, RDF offers many advantages:
•
Continuous Availability
RDF maintains an online copy of your production database on
one or more backup systems. If the primary system should go down, the backup
database(s) will be consistent and you can resume your business processing on a
backup system with minimal interruption and data loss.
•
Fault tolerance
You can restart RDF after a system crash. Single processor failures do not bring the
subsystem down. If a double processor failure occurs, RDF goes down, but it is
restartable (issue a START RDF command after the processors have been restored).