Oracle RAC 11g Overview

Article by author Jeff Hunter

The concept of clustering computers actually started several decades ago. The first successful cluster product was developed by DataPoint in 1977 named ARCnet. The ARCnet product enjoyed much success by academia types in research labs, but didn't really take off in the commercial market. It wasn't until the 1980's when Digital Equipment Corporation (DEC) released its VAX cluster product for the VAX/VMS operating system.

With the release of Oracle 6 for the Digital VAX cluster product, Oracle Corporation was the first commercial database to support clustering at the database level. It wasn't long, however, before Oracle realized the need for a more efficient and scalable distributed lock manager (DLM) as the one included with the VAX/VMS cluster product was not well suited for database applications. Oracle decided to design and write their own DLM for the VAX/VMS cluster product which provided the fine-grain block level locking required by the database. Oracle's own DLM was included in Oracle 6.2 which gave birth to Oracle Parallel Server (OPS) - the first database to run the parallel server.

By Oracle 7, OPS was extended to included support for not only the VAX/VMS cluster product but also with most flavors of UNIX. This framework required vendor-supplied clusterware which worked well, but made for a complex environment to setup and manage given the multiple layers involved. By Oracle 8, Oracle introduced a generic lock manager which was integrated into the Oracle kernel. In later releases of Oracle, this became known as the Integrated Distributed Lock Manager (IDLM) and relied on an additional layer known as the Operating System Dependant (OSD) layer. This new model paved the way for Oracle to not only have their own DLM, but to also create their own clusterware product in future releases.

Oracle Real Application Clusters (RAC), introduced with Oracle9i, is the successor to Oracle Parallel Server. Using the same IDLM, Oracle 9i could still rely on external clusterware but was the first release to include their own clusterware product named Cluster Ready Services (CRS). With Oracle 9i, CRS was only available for Windows and Linux. By Oracle 10g, Oracle's clusterware product was available for all operating systems. With the release of Oracle Database 10g Release 2 (10.2), Cluster Ready Services was renamed to Oracle Clusterware. When using Oracle 10g or higher, Oracle Clusterware is the only clusterware that you need for most platforms on which Oracle RAC operates (except for Tru cluster, where you need vendor clusterware). You can still use clusterware from other vendors if the clusterware is certified for Oracle RAC. This guide uses Oracle Clusterware 11g.

Like OPS, Oracle RAC allows multiple instances to access the same database (storage) simultaneously. RAC provides fault tolerance, load balancing, and performance benefits by allowing the system to scale out, and at the same time since all instances access the same database, the failure of one node will not cause the loss of access to the database.

At the heart of Oracle RAC is a shared disk subsystem. Each instance in the cluster must be able to access all of the data, redo log files, control files and parameter file for all other instances in the cluster. The data disks must be globally available in order to allow all instances to access the database. Each instance has its own redo log files and UNDO tablespace that are locally read-writeable. The other instances in the cluster must be able to access them (read-only) in order to recover that instance in the event of a system failure. The redo log files for an instance are only writeable by that instance and will only be read from another instance during system failure. The UNDO, on the other hand, is read all the time during normal database operation (e.g. for CR fabrication).

The biggest difference between Oracle RAC and OPS is the addition of Cache Fusion. With OPS a request for data from one instance to another required the data to be written to disk first, then the requesting instance can read that data (after acquiring the required locks). With cache fusion, data is passed along a high-speed interconnect using a sophisticated locking algorithm.

Not all database clustering solutions use shared storage. Some vendors use an approach known as a Federated Cluster, in which data is spread across several machines rather than shared by all. With Oracle RAC, however, multiple instances use the same set of disks for storing data. Oracle's approach to clustering leverages the collective processing power of all the nodes in the cluster and at the same time provides failover security.

Pre-configured Oracle RAC solutions are available from vendors such as Dell, IBM and HP for production environments. It is possible to put together your own Oracle RAC 11g environment for development and testing by using Linux servers and a low cost shared disk solution; iSCSI.