Oracle RAC Parallel Database Clusters

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Donald K. Burleson

Oracle9i RAC Tips

Parallel Database Clusters

The parallel clustered database is a complex application, which provides access to the same database (group of data tables, indexes, and other objects) from any server in the cluster concurrently, without compromising data integrity. Well known examples include Oracle Real Application Cluster, IBM UDB DB2 Enterprise Extended Edition (EEE), and IBM S/390 Parallel Sysplex Clusters.

Parallel databases typically contain multi-node servers accessing the same physical storage or data concurrently. PDB architecture allows multi-server data sharing technology, allowing direct, concurrent read/write access to shared data from all the processing nodes in the parallel configuration. This however, necessitates complex lock management to maintain the data integrity and resource coordination.

A parallel clustered system may use either of two types of storage access, the shared nothing model and the shared disk model.

Shared-Nothing Model

The shared-nothing model system, also termed a data-partitioning model, owns a portion of the database and each partition can only be read or modified by the owning system. [Fig 3.11] Data partitioning enables each system to locally cache its portion of the database in processor memory, without requiring cross-system communication to provide data access concurrency and coherency controls.

Each server in the cluster has its own independent subset of the data (a partition) it can work on independently, without encountering resource contention from other servers. The clustered nodes communicate by passing messages through a network that interconnects the servers. Client requests are automatically routed to the system that owns the particular resource (for example, memory or disk). Only one of the clustered systems can "own" and access a particular resource at a time. In the event of a failure, resource ownership can be dynamically transferred to another system in the cluster.

Fig 3.11 Shared Nothing Mode – Three Node database cluster

This architecture has several advantages:

Shared-nothing systems provide for incremental growth. System growth is practically unlimited.
Good for read-only databases and decision support applications.
Failure is local. If one node fails, the others stay up. However, the disk system of the failed node moves over to the surviving node.

However, it suffers from some drawbacks:

More coordination is required.
More overhead is required for processing or function shipping for a SQL operation working on a data/disk belonging to another node.
Data skew is a potential problem. As data is added to the database and access patterns change, data re-partition is needed to balance IO.

Shared-Disk Model

In the shared-disk model, all the disks containing data are accessible by all nodes of the cluster. Disk sharing architecture requires suitable lock management techniques to control the update concurrency control. Each of the nodes in the cluster has direct access to all disks on which shared data is placed. Fig 3.12 shows a typical three-node parallel database cluster. Each node has a local database buffer cache. IBM Parallel Sysplex and Oracle RAC systems follow this approach of shared-disk.

Advantages of shared-disk systems are as follows:

Shared-disk systems permit high availability. All data is accessible even if one node fails.
These systems have the concept of one database. There is no issue such as data skew since the data is located and accessed at a common location.
It provides for incremental growth of nodes and thus adds to processing power.

Disadvantages of shared-disk systems are these:

Inter-node synchronization is required, involving complex lock management and greater dependency on high-speed interconnect.
If the workload is not partitioned well among the processing nodes, there may be high synchronization overhead.
There is the operating system overhead of running the shared disk software.

Fig 3.12 Shared Disk Parallel Database Cluster

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