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Issues with Failover (FO) Clusters - Hidden Risks

Oracle RAC Cluster Tips by Burleson Consulting

This is an excerpt from the bestselling book Oracle Grid & Real Application Clusters.  To get immediate access to the code depot of working RAC scripts, buy it directly from the publisher and save more than 30%.

No doubt, there is wide acceptance of failover (FO) clusters, and in most situations, FO clusters provide an excellent failover mechanism, but there are some hidden risks with the nature and implementation of FO clusters. 

The hidden risks with FO clusters are as follows:

* Database Failover is a time consuming process. It is usually a cold fail-over (Active/passive). A fresh database instance starts on the surviving node. For example, the un-mounting of a file system may hang and as a result off-line activity may take longer time.

* The entire process of monitoring and failover depends on scripts. Scripts can become problematic with bugs and sometimes they may not take care of all situations.

* Cluster technology cannot protect against software corruption and human-induced failures. If the server operating system crashes in such a manner that it corrupts the file system, recovery by the other member of a cluster may not be possible.

* If the owner of a file accidentally deletes it from the file system, the cluster will be unable to recover the file. Even after failing over to the second node, the same problem of file loss exists.

* Maintenance and backup is a challenge. In the case of active/active architecture for a database cluster, the executables and other related files that you store on local storage have to be kept synchronized. Any mismatch of executables or patches might have an effect on the start up of the standby database during the fail over process.

* Configuration of the critical resources that make up the resource or service group is very significant and careful thought has to be given to the design configuration. Dependencies have to be accurately reflected, for example, in a service group, MQ Series, the database instance and Listener are defined as critical resources. In this situation, even if MQ-series fails for any reason, the database instance and listener are brought down or failover unnecessarily.

So far, many details about the FO Clusters and Database deployment and failure process have been covered. Now it is time to move on to basic features of parallel scalable clusters.

Parallel Database Clusters

The Parallel Clustered Database (PDB) is a complex application, which provides access to the same database, or group of data tables, indexes and other objects, from any server in the cluster concurrently without compromising data integrity. Well known examples include Oracle Real Application Cluster (Oracle RAC), the subject matter of this book, IBM UDB DB2 Enterprise Extended Edition (EEE), and IBM S/390 Parallel Sysplex Clusters.

Parallel Databases typically contain multiple nodes or servers accessing the same physical storage or data concurrently. PDB Architecture allows multi-server data sharing technology, allowing direct concurrent read/write access to shared data from all the processing nodes in the parallel configuration. However, this necessitates complex lock management to maintain the data integrity and resource coordination.

In terms of storage access type, a Parallel Clustered System is implemented in two ways, the Shared Nothing Model and the Shared Disk Model.

This is an excerpt from the bestselling book Oracle Grid & Real Application Clusters, Rampant TechPress, by Mike Ault and Madhu Tumma.

You can buy it direct from the publisher for 30%-off and get instant access to the code depot of Oracle tuning scripts.


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