Oracle RAC
What is Oracle RAC One Node?
Oracle introduced a new option called RAC One Node with the release of 11gR2 in late 2009. This option is available with Enterprise edition only. Basically, it provides a cold failover solution for Oracle databases. It’s a single instance of Oracle RAC running on one node of the cluster while the 2nd node is in a cold standby mode. If the instance fails for some reason, then RAC One Node detects it and first tries to restart the instance on the same node. The instance is relocated to the 2nd node in case there is a failure or fault in 1st node and the instance cannot be restarted on the same node. The benefit of this feature is that it automates the instance relocation without any downtime and does not need a manual intervention. It uses a technology called Omotion, which facilitates the instance migration/relocation. “RAC one” is Oracle’s answer or solution to OS clustering solution like Veritas Storage Foundation, Sun Solaris cluster, IBM HACMP, and HP Service guard etc.
Its Oracle’s attempt to tie customers to a single vendor by eliminating the need to buy 3rd party OS cluster solutions. First, it introduced Oracle Clusterware with 10g and stopped the need to rely on 3rd party cluster software and now it intends to conquer the rest who are still using HACMP, Sun Solaris cluster etc. for cold failover.

The Oracle RAC One node provides the following benefits:

•         Built-in cluster failover for high availability

•         Rolling patches for single instance database
•         Proactive migration / failover of the instance
•         Live migration of instances across servers
•         Online upgrade to RAC
The rolling upgrade is really useful. Upgrade to the OS, and Database can be done without any downtime unless upgrade requires some scripts to be run against the database. With RAC One Node, the DBA’s and Sys admins can be proactive and migrate/failover the instance to another node to perform any critical maintenance activity.
What it’s not suited for
According to me the RAC one node is not a viable or recommended solution in the following scenarios:
•         To load balance unlike regular RAC
•         A true high availability solution
•         As a DR solution; Data guard best suits the bill
•         For mission critical applications
It is definitely not FREE. Oracle has priced RAC one at par with Active Data Guard. The RAC One node is priced separately and costs $10,000 per processor as against $23,000 for regular RAC. The licensing cost is required for ONE node only (in a 2-node setup). RAC one node is eligible for 10-day rule, allowing a customer to migrate to another without the need to buy additional license up to 10-days in a calendar year. People arguing against paying a license fee for resources they are not using will still lament.
I am still not very convinced on the usefulness of RAC one node. I think customers invest in RAC for their mission critical applications and achieving high availability and load balancing at the same time. Those who don’t go for RAC rely on Data Guard and now with 11g, on Active Data Guard. So don’t see a huge requirement for RAC One except seamless failover within a data center. The licensing is a bit disappointing; they are making clients pay $10 K. Moreover RAC is free with Standard edition though one doesn’t get enterprise features and limited to 4 CPU sockets only. So, thinking RAC One will be popular among customers who are currently using standard edition and want to switch to enterprise will be wrong. However, this is still a very new feature and as more people adopt it, we will get more clarity on its’ usability. I am planning to do a POC on it and would publish the installation steps and any findings (goods things and not so good things) of my POC.