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Attributes of an Oracle RAC Cluster  

The Oracle9i RAC allows for a single or multi system Image, what this means is that the root and other system files can either be localized to internal or non-shared disks for each server in the RAC or each can load the operating system from a central set of disks.

If the RAC cluster uses a SAN or other network storage device then this is known as a Shared System Disk (root) system.

A RAC system must use a cluster file system where any server can read or write to any disk in the shared disk subsystem. This allows access to all datafiles, control files and redo and rollback areas by any instance. This ability to access all disks allows for instance recovery after an instance failure has occurred. The work of the failed instance is automatically absorbed by all surviving nodes until the failed instance is brought back online, at which time it is fully synchronized and restored to service automatically.      

A RAC cluster provides for automatic shared execution of oracle applications. This means that for any Oracle instance application all queries and other processing is automatically shared among all of the servers in the RAC cluster.      

The sharing of application processing to all servers in the RAC cluster leads to automatic load balancing across all cluster members.     

The ability of a RAC cluster to provide shared application execution and automatic load balancing leads to the true scalability of applications without code or data changes.

One limit on shared-none or federated databases was the amount of time required for failover. Due to the requirements for rebuild or rehash of data fail over time in a non-shared disk environment can be prohibitive. In a RAC cluster fail over time is virtually null since oracle handles the redistribution of work automatically and no rehash of data is required since the same data files are shared by all participating nodes in the RAC cluster.    

The number of nodes in a RAC cluster is basically only limited by the number of servers that your LAN architecture allows to connect into the disk farm. In the SQL Server 2000, in the cluster database you have a hard limit of 2 or 4 servers depending on what level of server license for Windows 2000 you have loaded on your server. The convoy effect limits the number of servers that an IBM type structure allows. Due to the cache fusion and the high speed cluster interconnect the Oracle9i RAC allows for a virtually unlimited number of nodes in a RAC cluster.

Oracle9i RAC allows for TCP/IP/ UDP/ VIA for the protocol used by the cluster interconnect. The speed of the interconnect should be at a minimum of 100 mbit/sec. The maximum number of instances that can be interconnected is OS platform dependent. He distance allowed between nodes on a cluster interconnect is system and LAN specific.      

Oracle RAC displays great disaster tolerance. If a single or multiple nodes fail, the load is redistributed between all of the remaining nodes. If the database is globally distributed then a disaster to a single site will not effect other sites.

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