RDF System Management Manual

Introducing RDF
HP NonStop RDF System Management Manual—524388-003
1-5
Unplanned Outages Without ZLT
Unplanned Outages Without ZLT
Without ZLT functionality, it is possible for some committed transactions to be lost
during an unplanned outage. When the RDF TAKEOVER command is issued, any
transaction whose final outcome is unknown on the backup system is backed out of the
backup database. One or more transactions might have committed on the primary
system, but, before the extractor could read and send the associated audit data to the
backup system, the primary system failed. Loss of audit data in this manner typically
involves no more than a fraction of a second.
If the primary system is unexpectedly brought down because of a disaster, the
outcome of some transactions might never be known, as illustrated in Table 1-1.
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.
Table 1-1. Audit Information At the Time of a Primary System Failure
Primary database updates
(Sequence in master audit trail file)
Updates sent to the backup
(Sequence in image trail file)
TRANS100—Update 1 TRANS100—Update 1
TRANS100—Update 2 TRANS100—Update 2
..
..
..
TRANS100—Update 10 TRANS100—Update 10
TRANS101—Update 1 TRANS101—Update 1
TRANS100—Commit record
(Primary system fails)