The Concept of RAC/CRS and Database !!!!

This Concept should have come somewhere in my earlier posts. But ok.. it is never 2 late!!!!

There are 3 most important things to RAC/CRS setup. RAC is Real Application Cluster which is build on CRS (Cluster Ready Service).

We take a 2 node cluster.We try understanding the setup. Both the nodes will have a separate CRS setup. The version of CRS should be greater than or equal to that of RAC. So whenever we go for an upgrade to a newer version, we go for CRS upgrade first and then the RAC.
The reverse will make things unuseable. Reverting back the changes in the version is not possible.(as far as I have known).

Now the two nodes will have a shared location. This shared location contains the ORADATA. this oradata is what helps storing information , our data.
RAC is a normal DB installation except that the other db files are stored in the separate nodes but the oradata which manages the data is stored in the shared location. The two nodes are hence gateway to the same data. This is what increases the availability of the RAC and hence defines its use.

Now, the shared location defines the size of database.

And lets take the case of upgrading the RAC db. We first upgrade the db instance files on both nodes , and then the procedure to upgrade the oradata is not the same and is done by running a specific sql script. We can note that the script is run only from one node. This is because the oradata is accessible from both nodes.