Database Reference
In-Depth Information
Leaf nodes
Hub nodes
Leaf nodes
Figure 4-1.
Hub-and-spoke topology of Oracle Flex Clusters
In this example, four Hub nodes in the Hub center form the hub of the cluster, which is similar to a standard
cluster supported in Oracle 11gR2 Clusterware. These four Hub nodes are tightly connected with the private
interconnect, which is the same as the one in the standard Oracle 11g R2 cluster. The cluster heartbeat and the data
block transfer for RAC cache fusion among the Hub nodes are based on this interconnect network. All of these Hub
nodes are also required to have direct access to the shared storage, which stores the database files as well as the Oracle
Cluster Registry (OCR) and the voting disks for the Oracle Clusterware. The database instances running on their Hub
nodes will function in the same way as in a standard cluster.
Outside of the Hub center, 12 Leaf nodes form four groups. Each group of Leaf nodes is connected to one Hub
node, which is the attached node for all the Leaf nodes in the group. None of the Leaf nodes are connected to any
other Leaf nodes in the cluster. Oracle RAC database instance can also run on Leaf nodes. Since these Leaf nodes
do not have direct access to the shared storage, the RAC database instances on the Leaf nodes will need to get the
database through the Hub nodes. The technology to implement this remote storage access is another new Oracle 12c
feature known as Oracle Flex ASM. From here, we can see that Flex Clusters require Flex ASM to enable storage access
for Oracle database instances on Leaf nodes. For this reason, when you configure a Flex Cluster, Oracle Flex ASM is
automatically enabled.
One benefit of running the loosely coupled architecture of the Flex Cluster is to provide the high availability of
the Oracle Clusterware of the applications tier. We can run application tiers on the Flex Cluster, which provides high
availability, such as failover capability, against server hardware failover and planned maintenance.
Scalability and Availability of Flex Clusters
Oracle Flex Clusters increase cluster scalability. The hub-and-spoke topology in the Oracle Flex Clusters significantly
reduces the number of network connections among the cluster nodes. In the 16-node cluster example shown in
Figure
4-1
, where we set up 4 Hub nodes and 12 Leaf nodes with 3 Leaf nodes per Hub node, the number of private
interconnects among 4 Hub nodes is 6 and the total number of connections between the Leaf nodes and their Hub
nodes is 12. This makes the total number of interconnects 18, compared to 120 in a 16-node standard cluster. The
number of storage network connections is also reduced from 16 to 4. If we extend this cluster to a 500-node cluster
on which we set up 25 Hub nodes and 475 Leaf nodes with 19 Leaf nodes per Hub node, the interconnect network
connections consists of 300 interconnects between Hub nodes plus 475 connections between Leaf nodes and their
attached Hub nodes. The total number of the connections is 300+475 = 775, while the total number of interconnects
