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Don Burleson Blog 







Oracle Maximum Availability Architecture (MAA)

Oracle Database Tips by Donald Burleson


RAC protects against instance and server failure by providing multiple servers with which one can be connected.  However, remember that all data will be in centralized storage.  There is still a possibility of data failure or data center loss.


Data failure is the worst of the three that have been seen thus far (instance and system failure), resulting in the loss or corruption of data.  Some disk failures are non-disastrous; for instance, if a disk is mirrored with hardware or software RAID.  Even then, if excessive disks are lost, it is possible that production data could be lost as well, thereby requiring some form of recovery.  User error can also cause data loss if an operating system user removes database files with a command such as rm.  In this case, the file will be removed, and the disk mirror will provide no protection.  Lastly, corruption can occur if hardware or software bugs result in inappropriate data being written to the datafiles.


Data center loss occurs when a system is completely lost, usually as the result of some sort of natural disaster.  A hurricane, flood, or tornado may destroy or seriously disable an entire data center resulting in a combined loss of servers and disk.  This is by far the worst unplanned-downtime scenario and can only be protected against with extensive, and usually expensive, disaster recovery methods.


Oracle provides many options for preventing downtime and data loss, all of which make up the Maximum Availability Architecture.  The MAA provides redundancy on all components and employs different Oracle tools.  RAC only makes up one piece of the MAA; it does not take into account all possible problems.


These tools, as recently mentioned, must provide protection for planned and unplanned downtime. They must also protect against varying levels of unplanned downtime ranging from single server outages, which RAC covers, to entire data center loss, which RAC does not cover.


Some businesses choose not to follow all the guidelines for maximum availability.  When considering a high availability strategy, the DBA must consider:

  • Recovery Time Objective (RTO)

  • Recovery Point Objective (RPO)

  • Downtime Cost-per-Minute

  • Available Resources

The RTO defines the allowable downtime for the database.  An advertising company may allow hours of downtime; however, a bank will usually allow no downtime whatsoever.  RPO defines the allowable data loss if a failure occurs.  If batch processes load the data, it may be that hours or even days of data could be reloaded.  However, for a system that allows direct access by the end user, such as an online store or ATM machine, zero data loss is allowed.


Downtime can be expensive.  Depending on the system, costs can range from dollars per minute to tens of thousands of dollars lost for every minute the database is unavailable. However, uptime is expensive as well.  It has been shown how costly RAC can be for a business. Now it can be seen that even more may be required for a fully bulletproof system.


Figure 7.3: Example of an HA Configuration using MAA Best Practices

Many other HA solutions require the backup server to sit uselessly idle.  A solid HA solution like Oracle 11g RAC is good for the users, management, System Administrators and DBAs.


With multiple instances, the RAC system gives a near zero failure environment. Even when one or more nodes fail in the cluster, for whatever reason, as long as there is one instance running, the database resources are provided.


With the help of the transparent application failover (TAF) configuration, operations are transferred automatically to the surviving instance.


Users will appreciate the ability to always connect to their apps even when a server node experiences a total hardware or instance failure.  Management and the other System Administrators are happy when the users are happy.  Then the DBA can sleep more soundly and work a more balanced 9 - 5 schedule.  A DBA can now take a node offline knowing the other nodes will prevent the users from noticing.


The major benefits of the RAC database system are scalability and high availability.  No business operations can run without the use of database resources.  That is why the geeks, and more specifically, the DBAs shall inherit the earth.

Oracle Grid and Real Application Clusters

See working examples of Oracle Grid and RAC in the book Oracle Grid and Real Application Clusters.

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