oracle RAC

Relative to the single-instance Oracle database, Oracle RAC adds additional complexity that can become overwhelming without investment in appropriate training and management tools. While database automation makes sense for single-instance databases, it becomes even more necessary for clustered databases because of their increased complexity.

Oracle Real Application Clusters (RAC), introduced with Oracle9i, is the successor to Oracle Parallel Server. However it required an external clusterware (known as vendor clusterware like Veritas or Sun Cluster) for most of the Unix flavors (except for Linux and Windows where Oracle provided free clusterware called Cluster Ready Services or CRS). 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, in which case you need vendor clusterware). You can still use clusterware from other vendors if the clusterware is certified for Oracle RAC.

In RAC the node performing the write-transaction must take ownership of the relevant area of the database: typically this involves a request across the cluster interconnection (local IP network) to transfer the data-block ownership from another node to the one wishing to do the write. This takes a relatively long time (from few milliseconds to tens of milliseconds) compared to single database-node using in-memory operations. For many types of applications, the time spent coordinating block access across systems is low relative to the many operations on the system, and RAC will scale comparably to a single system. Also, high read-transactional databases (such as data-warehousing applications) work very well under RAC, as no need for ownership-transfer exists. (Oracle 11g has made many enhancements in this area and performs a lot better than earlier versions for read-only workloads[citation needed].)

The overhead on the resource mastering (or ownership-transfer) is very minimal when you have less than three nodes, as the request for any resource in the cluster can be obtained in a maximum of three hops (owner-master-requestor). This makes Oracle RAC horizontally scalable with many nodes. Application vendors (such as SAP) use Oracle RAC to demonstrate the scalability of their application. Most of the biggest OLTP benchmarks are on Oracle RAC. Oracle RAC 11g supports up to 100 nodes[4].

For some applications, RAC may require careful application-partitioning to enhance performance. An application which scales linearly on an SMP machine may scale linearly under RAC. However, if the application cannot scale linearly on SMP, it will not scale when ported to RAC. In short the application scalability is based on how well the application scales in a single instance.