Answer Posted / waseem
RAC is Cluster software available from Oracle. This is
available 10g onwards.
In this Cluster environment we can have multiple instances
running with a single database. Users are connected to
different instances. Availability and load balancing is the
main purpose behind Oracle RAC.
VD (Voting Disk) and OCR (Oracle Cluster Registry) are the
components of RAC. VD is similar to control file and OCR has
all the registered services to start the cluster.
The connection of user could be established on any node but
later depending on the number of users connections
established and the number of instances(nodes) available the
load of all the established connections is balanced across
the Cluster.
The first node created in the Cluster is the Master Node and
all other nodes in the Cluster nbehaves like Slaves.
If any slave node fails to communicate with Master node,
internally the failed node is rebooted by the Master node by
writing a kill block into VD. If the Master node fails, then
any of the existing node responding first will become the
Master node and reboots the failed node.
Daemons checks for the availability of VD,nodes and OCR and
cluster is started.
We can add nodes to the existing cluster from any exiting
node which is already a part of the cluster. We can also
delete a node from the cluster by stopping all the services
of that node from the CRS inventory and home.
We can have multiplexed copies of VD and OCR. VD (Min 1 and
Max 310....OCR(Min 1, Max 2).
We can take backup of VD and OCR with the help of dd (disk
dump). For OCR we can also perform logical backup using
export utility.
Note: Use ASM (logical location for physical diskgroups) for
the best performance).
Create ASM instance for each node.
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