Issues with FO Clusters – Hidden Risks

No doubt there is wide acceptance of failover 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 failover (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.
   
• The entire process of monitoring and failover depends on scripts. Scripts can contain bugs and 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 second node, the same problem of file loss exists.
   
• Maintenance and backup is a challenge. In the case of an active/active architecture for a database cluster, the executables and other related files that are stored 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 failover process.
   
• Configuration of the critical resources that make up resource or service groups is critical and careful thought has to be given to the design. Dependencies have to be accurately reflected. For example, in a MQ Series service group, the database instance and listener are defined as critical resources. In this situation, even if the MQ Series fails for any reason, the database instance and listener are brought down or failover unnecessarily.

So far, we have covered many details about the FO clusters, database deployment, and the failure process. Now let us move on to basic features of parallel scalable clusters.