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Oracle 12c RAC: New Features

With Oracle 12c now established in most organizations as the standard, we wanted to highlight a few of the lesser-known features of 12c Real Application Clusters:

Oracle Flex Cluster:

Oracle introduced the Flex cluster feature with the 12c release, which uses a Hub and Leaf architecture. A Hub node is like standard Grid infrastructure nodes where the storage is mounted and it has direct access to Oracle Cluster Registry and Voting Disk. Leaf nodes are part of the cluster where storage is not mounted, but they are connected via Interconnect.


A Hub node can have multiple leaf nodes in the cluster, whereas a leaf node, which is a light weight server, cannot be part of multiple Hub nodes and cannot connect to other leaf nodes. This new architecture enhancement gives the ability to manage both application servers and database servers in a single cluster environment.

Oracle RAC 12c also now supports IPv6 for Client connectivity, however the Cluster Interconnect is still on IPv4.

Oracle Flex ASM:

Prior to the 12c release, a database instance had a dependency on the ASM instance for any given node in a RAC environment. Technically, every node in RAC should run an ASM Instance and if that instance fails on the node, the database immediately becomes unavailable. Starting with 12c, Oracle introduced a new feature called Flex ASM architecture. As the term Flex implies, this new architecture for ASM is more flexible in RAC. With this new feature, ASM can now run on a subset of nodes involved in the Oracle cluster. The ASM client communicates with the Flex ASM nodes to get the metadata and get to the data from disk. The limit on the number of ASM diskgroups also increased from 63 to 511.

Flex ASM can be implemented in two ways.

  1. Mixed Mode: ASM instances will be running on all nodes of the cluster. By setting the cardinality to ALL, we can make use of the benefit of ASM instance failover.
  2. Pure 12c Mode: The Database can connect to any available ASM instance and failover to a secondary instance. An added benefit is that we can now perform maintenance on ASM without database instance downtime.

There are no Flex ASM specific instance parameters; the default parameter setting has been modified to suit Oracle Flex ASM architecture. ASM server instances use automatic memory management (AMM).

ASM Disk Scrubbing:

Oracle ASM scrubbing is a new feature which checks for logical data corruptions and fixes them automatically using redundant data. It comes in handy for both normal and high redundancy diskgroups. This process can also be combined with rebalancing of diskgroups to reduce I/O resources.

We can perform scrubbing on a disk group, a disk or a specified file in a disk group.

SQL> alter diskgroup disk_group_name scrub repair;

SQL> alter diskgroup disk_group_name scrub file ‘+DATA/TEST/DATAFILE/USER.123.4567234’ repair power high force;

SQL> akter diskgroup disk_group_name scrub disk disk_name repair power high force;

The repair keyword is optional where a corruption will be reported and not repaired automatically.


For more information on these and other 12c Database and RAC features, contact Cintra today!

Written by Chetan Eshwaraiah, Senior Oracle DBA, Cintra NY – August 2017

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