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Benefits of RAC 

In the beginning was Oracle Parallel Server or OPS, now we have Real Application

Clusters or RAC. It is more than just a name change. RAC provides a process known asw cache-fusion which allows direct sharing of Oracle database blocks between cache areas of Oracle instances participating in RAC via a high speed interconnect. This direct sharing eliminates the biggest performance robbers in the OPS architecture, the DB block PING.     

A DB block PING would occur when an instance participating in an OPS database would have a block in its cache that another participating instance required. In OPS if another instance required the block in the cache of a second instance, the block would have to written out to disk, the locks transferred and then the block re-read into the requesting instance. As you can imagine all of this IO was expensive in terms of  time and performance.      

The new RAC with cache fusion exploits the new high speed buses and architectures available in the new servers. By eliminating the blockage caused by pinging, RA enables better, more efficient scaling. In order to add more processing or user capacity you simply add a server to the LAN, load the appropriate software and Oracle9i and you are up and operating.

What Is a DB Cluster? 

A DB cluster consists of a group of independent servers, connected via a LAN

that share a set of disk assets via the LAN and have a high speed cluster interconnect that allows connection of their processors to the other processors in the cluster. The clusters do not share memory and act as independent servers. The servers can either be single processor or SMP (Symmetric Multi-Processor) nodes. A DB cluster provides fault-tolerance and allow for modular growth. By having the built in redundancies of multiple servers the chance that a single point of failure will prevent database access is unlikely. 

DB Cluster Architectures 

Essentially there are only two basic architectures used in DB clusters, these are the shared-nothing and shared-disk. The shared-nothing cluster uses dedicated disk assets for each server in the cluster. The shared-disk cluster uses a SAN type storage array to serve the cluster.      

Examples of systems that use the shared nothing architecture are the IBM RS/6000 SP and the federated database approach used by SQL Server. This architecture is rapidly losing ground to the shared-disk architecture.      

The shared-disk architecture is being adopted almost 100% by IBM, SUN and HP and other vendors. It offers better redundancy, and , better connectivity through technologies such as fiber-channel, hub and shared SCSI.

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