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Observer
database database
Figure 1 Fast-Start Failover Configuration
4
to install the Oracle Client Administrator (choose the Administrator option from
Oracle Universal Installer). Installing the Oracle Client Administrator results in a
small footprint because an Oracle instance is not included on the observer system.
The underlying rule is that out of these three participants, (primary, standby &
observer) whichever two can communicate with each other will determine the
outcome of fast-start failover. For example, if the primary database becomes
unavailable, the Observer confirms with the target standby database that the
primary database is unavailable and that the target standby database is
synchronized with the primary database, and if so, initiates a fast-start failover to
the target standby database. Lacking such agreement, an automatic failover cannot
occur. This insures two important characteristics of Fast-Start Failover:
• There can be no event where more than one database in a Fast-Start
Failover configuration can assume the primary role at the same time. This
avoids what is commonly referred to as a “split brain” scenario by
guaranteeing that only one database in a Fast-Start Failover configuration
is able to accept transactions.
• Automatic failover can only occur if there is a guarantee that zero data will
be lost.
Following a fast-start failover, the Observer periodically attempts to contact the
old primary database. If a reconnection to the old primary database is made, the
Observer automatically reinstates the old primary database so that it can become a
standby database to the new primary database. This quickly restores high
availability to the Data Guard configuration.
EVENTS THAT TRIGGER FAST-START FAILOVER
The following database conditions will trigger a fast-start failover:
• Database instance failure (or last instance failure in a RAC configuration)
• Shutdown abort (or shutdown abort of the last instance in a RAC
configuration)
• Datafiles taken offline due to I/O errors
The following network condition will trigger a fast-start failover:
• When both the Observer and the standby database lose their network
connection to the primary database, and when the standby database
confirms that it is in a “synchronized” state.
5
ORACLE TEST RESULTS
Oracle tested a Fast-Start Failover configuration comprised of a primary database,
standby database, and observer, all running Redhat Linux 3.0. The results of the
test are provided in Figure 2 below.
The test databases were each 100GB in size. Each host was connected to the next
over a Gigabit Network. The workload on the primary database generated 3
MB/second of redo. Both single instance databases and multi-instance RAC
configurations were tested. Tested configurations included failover to a physical
standby database (Redo Apply), and a logical standby database (SQL Apply). In all
cases, the failover threshold (or time to detect the failure) was not included in the
failover timing calculation, the test measured only the time required to complete
the actual database failover. Total time to complete failover ranged between 10
and 25 seconds, depending upon the configuration.
APPLICATIONS WELL SUITED TO FAST-START
FAILOVER
Data Guard is typically used to maintain a synchronized standby system in a data
center remotely located from the primary production site for the purposes of data
protection and disaster recovery. Data Guard has enabled zero data loss
protection and fast database failover since Oracle9i, assuming an administrator is
immediately available to execute the failover when needed. Enhancements in
Oracle Data Guard 10g have further reduced the time it takes to execute a manual
failover (for more information please reference the MAA paper, Oracle Data
Guard 10g Release 2 Switchover and Failover Best Practices [6]).
0
5
10
15
20
25
Average
Failover
Time
(seconds)
Physical Standby Logical Standby
Single Instance RAC
Figure 2 – Fast Start Failover Test Results
“Airbus testing of Data Guard
10g Fast-Start Failover
produced impressive results.
Failover executed in less than a
minute. This was much faster
than a cold failover using third
party cluster technology. With
Data Guard, Airbus can
achieve continuous data
protection and high levels of
availability using a standard
feature of the Oracle
Database.”
—Werner Kawollek
Application Management
Operations
Airbus Deutschland GmbH
中国航空网 www.aero.cn
航空翻译 www.aviation.cn
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