Oracle RAC and Why it is better than individual VM

RAC stands for Real Application Cluster

In oracle RAC we have multiple instances on multiple nodes accessing the same database (disk). So we will have 2 machine whose RAM will be used as part of SGA and PGA and they will be accessing the database that is hard disk

Oracle RAC has technology called cache fusion which makes sure that cache maintained by the two separate machine is store in global area. This makes sure that we always see the current data irrespective of changes

Why RAC are better than Virtual Machines

Oracle running on VM has advantages when it comes to deployment,

A scalable environment enables you to improve performance and add capacity by adding nodes. On some platforms you can add nodes dynamically while your cluster is running. So with more nodes you have more processing power

Classic Statement -- If a application cannot scale up on single node system. It will not scale up on RAC. That is if your SQL is limited by processors and adding processor in single node system increases it then the same will hold true in RAC. Consider you are limited by I/o adding more node will not help

When you execute a statement in parallel from one node does the other node comes into play because it will also have processors

when we execute query in parallel the session which fires query become the coordinator and it generates lot of slave processes. Slave processes can be in any of the nodes. They use the node interconnect to transfter the data. Sometime the amount of data trasferred by slaves can be so large it can be itself a bottleneck .