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What is a Oracle RAC Database?

Oracle RAC Cluster Tips by Burleson Consulting


Oracle RAC database system involves a configuration of multiple hosts or servers joined together with clustering software and accessing the shared disk storage structures. On each of the hosts in the cluster, an Oracle Database instance is launched that uses the shared storage structures to provide the logical database objects. Thus, the multiple database instances provide a common database access for the users. Users can access the same database from any of the instances. Table 1.1 summarizes the main features of the single instance stand-alone database and the multi-instances parallel data which is also called the RAC database.

SINGLE INSTANCE DATABASE

MULTI-INSTANCE RAC DATABASE

Only one instance to access and process database requests

Multiple Instances accessing same database.

One set of data files, redo files, undo  and control files etc.

One set of Data Files and Control Files, but separate Redo Log files and undo for each instance

Locking and Concurrency Maintenance is confined to one instance

Locking and Concurrency Maintenance is extended to multiple instances

Dedicated Storage Structures for the instance

Multiple instances access the same shared storage structures

Weak on High Availability and Scalability

Provides HA and Scalability Solution

Table 1.1: Stand alone single Instance v/s Multi-Instance RAC Database

A RAC System, by providing multiple instances (i.e., hosts and its associated resources) access the same database, creates multiple database computing centers and extends high availability and scalability.

A RAC Cluster database uses a SAN or other network storage device as a Shared System Disk. A RAC system must use a cluster file system or a raw partition, where any server can read or write to any disk in the shared disk subsystem. This allows access to all data files, control files, and redo and rollback (undo) areas by any instance. This ability to access all disks, allows instance recovery after an instance failure has occurred. All surviving nodes automatically absorb the failed instance?s tasks 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 are automatically shared among all of the servers in the RAC cluster.

The sharing of application processing by 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.

Before RAC

Even though Oracle introduced Oracle Parallel Server (OPS) option with version 6.2, the real usage began with version 7. The Version 7.34 OPS was generally a stable product, but needed lots of application planning and partitioned usage.

The biggest performance robber in the OPS architecture was the DB block ping. A DB block ping would occur when an instance participating in an OPS database had 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 be written out to disk, the locks transferred, and then the block re-read into the requesting instance.

With OPS, scalability was always an issue. OPS implementations used to suffer from tedious application design and coding, database management issues. There were many performance issues reported. OPS lacked good tools for management, which caused frustration among DBAs and application users.

Oracle 8i OPS implementation brought in many significant changes. The significant new feature was the introduction of Cache Fusion technology. Cache Fusion is a concept where Cache (or SGA) from the multiple instances coordinate the buffers (or cache) and manage the database access.

Oracle 8i (OPS) introduced the initial phase of cache fusion. The data blocks were transferred from the SGA of one instance to the SGA of another instance without the need to write the blocks to disk. This was aimed at reducing the ping overhead of data blocks. However, the partial implementation of cache fusion in Oracle 8i could help only in certain conditions.

 
   
Oracle Grid and Real Application Clusters

See working examples of Oracle Grid and RAC in the book Oracle Grid and Real Application Clusters.

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