Database Reference
In-Depth Information
Some RAC nodes can run Oracle Database instances without an Oracle ASM instance. For the RAC nodes where
there is no local ASM instance or the local ASM instance fails, the database instance on the RAC nodes can remotely
connect to an Oracle ASM instance on another RAC node. Therefore in Oracle 12c, depending on whether or not
the Oracle Flex ASM option is enabled, Oracle ASM can run in two distinct modes: the Standard ASM and the Flex
ASM. The standard ASM works similarly to Oracle 11gR2 ASM, with every RAC node running an Oracle ASM instance
and all the Oracle Database instances on the cluster connecting to the local ASM instances. Figure
5-7
shows the
dependency in Oracle 11gR2 and Oracle 12cR1 Standard ASM without the Flex ASM being enabled.
Database
1 Instance
Database
1 Instance
Database
2 Instance
Database
2 Instance
Grid Infrastructure
Grid Infrastructure
ASM
Instance
ASM
Instance
Clusterware
Clusterware
Shared Storage
Figure 5-7.
Dependency 11gR2 ASM and 12cR1 standard ASM
In this architecture, a single ASM instance per RAC node provides a clustered pool of storage to all the database
instances running on that node; the ASM instance has access to the shared storage volumes and presents the ASM
diskgroup as the file system for all the database instance(s) running on the RAC node.
In Oracle 12cR1, by enabling the Flex ASM, a small set of cluster nodes run Oracle ASM instances to provide
storage access to a large number of cluster nodes. Figure
5-8
shows an example of the Flex ASM configuration. By
default, three RAC nodes (nodes 1, 2, and 3) run Oracle ASM instances, and all the database instances, including ones
on node 4 and 5, remotely run these ASM instances. Oracle Flex ASM often works with Oracle Flex Cluster. The nodes
that have a direct connection to the shared storage are called the Hub nodes (such as nodes 1, 2, 3, and 4 in Figure
5-8
),
and the nodes (such as node 5 in Figure
5-8
) that don't have a direct connection to the shared storage are Leaf nodes.
Usually one or more Leaf nodes are connected to a Hub node to access shared storage. In Figure
5-8
, Leaf node
5 accesses shared storage through Hub node 3. Chapter 4 discusses the details of Oracle Flex ASM and Oracle
Flex Clusters.
