Oracle RAC clusters disk system

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Donald K. Burleson

Oracle9i RAC Tips

Oracle9i RAC Disk System

The disk or storage system is the most crucial component and is the only place to store data in a persistent form. Oracle9i RAC needs shared storage to store a variety of files. RAC follows the shared disk model, where all the cluster nodes share the same disk or storage volumes.

The RAC architecture includes the following main file structures. They must be available simultaneously and updateable by all the nodes, and hence, to all instances in the cluster.

Control Files - The control file contains entries that specify the physical structure of the database.
Redo Log Files - A redo log is made up of redo entries (also called redo records). The primary function of the redo log is to record all changes made to data.
Tablespaces – Comprised of one or more data files, these are the storage containers where various relational database objects are created, such as tables and indexes.
UNDO Tablespaces -- These are special tablespaces and cannot contain user application tables or indexes. They contain “before images” of blocks involved in uncommitted transactions. As such, they are the primary support structure allowing a transaction to “rollback” if the decision is made to not commit the transaction. These tablespaces also support the Oracle functionality known as Multiblock Read Consistency. If configured to use “System Managed Undo,” there must be one UNDO tablespace available for each instance in the RAC setup.
Server Parameter File and server configuration file
Cluster quorum disk -- Depending on what clusterware is in use, this may or may not be a file owned by Oracle.

Each of the above mentioned structures are either on raw partitions or on a cluster file system. Raw partitions are a set of unformatted devices on a shared disk sub-system. The RAC shared storage files can be stored on the cluster file system wherever it is available.

There are other file systems such as the Oracle Home, the Oracle executables, the file system where the archive logs are written, and the file system where Oracle log files and trace files are written. These file systems are usually mounted on the storage volumes from either the local storage or cluster file system.

Fig 5.2 Six Node Cluster with shared storage

As we can see in Fig 5.2, the storage system is the key component for both the survival of the database system and its high availability.

For example, we have a 6-node cluster with a very reliable architecture comprised of powerful processors, a large memory in each server, and all 6-nodes sharing the storage system. The 6 nodes jointly provide both the parallel computing environment and high availability. Imagine we have lost the storage unit for whatever reason and all 6-nodes are still functioning, but cannot provide the database service. The storage unit is the ‘Achilles Heel’.

There are many methods that solve the problem by providing adequate redundancy at the hardware level. We will examine some of these methods in the later sections.

Storage occupies an important place in the overall architecture of the RAC system. It is crucial to plan and design carefully to get the right storage array in a compatible environment. Next, we will look at the storage or disk technology in more detail.

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