Real Application Clusters (RAC) along with Oracle 12c databases have some special hardware requirements that a single instance or a non-RAC database does not have. Hardware areas to focus on include network interfaces, central storage, and nodes.
Nodes and Real Application Clusters in Oracle 12c
A node is a server running an Oracle instance. A real RAC configuration contains at least two nodes.
The number of nodes in a RAC configuration depends on hardware and software limitations. According to Oracle’s documentation and support sites, the Oracle software itself can support over 100 nodes, but other forces may limit you to fewer.
If you’re getting into many nodes (more than eight), check all of your hardware and software vendors to see what your limit is.
Add knots as you measure your group. You can add and remove them with or without minimal disruption to your app. This ensures high availability. Usually each node has its own installation of Oracle.
You can have one central software directory shared for each node to use. However, a configuration like this limits the potential for high availability.
For example, one of the advantages of installing Oracle software on each node is the ability to patch the nodes individually by removing them one by one. This rolling patch avoids complete app interruption. You cannot apply all corrections this way. Check with your patch documentation to be sure. On the other hand, a centralized installation requires you to close the entire block to apply the patch.
Each node must have its own oracle code tree if you want high availability.
Centralized Storage and Real Application Clusters in Oracle 12c
Here are some storage central requirements for a RAC configuration:
- All database files, control files, redo logs, archive logs, and spfile must all be on a shared storage space. In this way, each nodes can access all the files required to access, restore and configure the data.
- Connect central storage to each node as some high speed media. Lots of high-speed connections (fiber channel or iSCSI, for example) are available from different storage vendors.
Make sure that storage and attachments are approved for Oracle RAC before making your decisions. (For example, NFS mount drives per server is usually not a supported configuration.) You can use almost any shared storage configuration with decent marking and testing results.
- When choosing a storage vendor, consider the performance needs of your applications. Your disk subsystem should be able to scale as easily as RAC nodes. While adding nodes, you may need to add physical disks to support the increasing demand for the storage subsystem. You should be able to do this in no time or no downtime.
- The disk on the shared storage subsystem must be configured for shared access. You may have up to four choices for this:
>>Raw file system (unformatted disks)
>>Oracle Cluster File System (OCFS) (available on Windows and Linux only)
>>Oracle Automatic Storage Management (ASM) (an Oracle-supplied volume manager of sorts for database-related files)
>Third-party solution (such as Veritas)
You may have to combine options. For example, you might use Oracle ASM for your database files, but you might want something other than ASM for your RMAN backup files.
Cluster threading and real application clusters in Oracle 12c
A cluster thread is a dedicated piece of hardware that manages all communications between instances. A lot of cross-instance communication happens in a RAC configuration: maintaining consistency, sharing lock information, and transferring data blocks.
Oracle Cache Fusion is used to manage the transfer of data between nodes. Cache Fusion requires a highly reliable, private, and high-speed network that connects all nodes.
Cache Fusion is an important component of making RAC work well. Interconnection should be at gigabit speeds or better.
When you have cluster connection performance issues, the ability of the interconnection to provide the required bandwidth is called into question. It is a necessary expense to create a proper RAC environment. Would you spend thousands of dollars on a race car and then put street tires on it?
Network Interfaces and Real Application Clusters in Oracle 12c
Make sure you have the correct network interfaces on the server for the correct connection. This includes several network interface cards:
- One for the public or user connections to the device
- One for the special interconnection of the cluster to exchange information across nodes
At a minimum, the RAC configuration must contain two network interface cards:
- One for private network cluster interconnection
- One for public network
The public network is the connection for all group communication, from applications and end users (including you and your system administrator).