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 Oracle Synchronization of Concurrent Tasks
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Oracle 11g Grid & Real Application Clusters by Rampant TechPress is written by four of the top Oracle database experts (Steve Karam, Bryan Jones, Mike Ault and Madhu Tumma).  The following is an excerpt from the book.

Resource Coordination

Since the RAC system allows many users to connect and process database SQL operations concurrently, many resources, such as data blocks and enqueues, are used simultaneously. This situation demands an effective synchronization of the concurrent tasks.


Within the shared cache (global cache or fused cache of multiple instances), the coordination of concurrent tasks is called synchronization. Resources such as data blocks and enqueues are synchronized as nodes within a cluster as they acquire and release ownership. The synchronization provided by real application clusters maintains a cluster wide concurrency of resources, and in turn ensures the integrity of the shared data. Even though there is seamless and transparent synchronization of concurrent tasks within the shared cache, it does not come without a cost in overhead. Processing within the local buffer cache is always faster than processing blocks across instances.


The key to successful cluster database processing is to divide the tasks that require resources among the nodes so that very little synchronization is necessary. The less synchronization necessary, the better the system’s performance and scalability will be. The overhead of synchronization can be very expensive if there is excessive inter-node communication.  According to Oracle studies, accessing a block within the local cache is many times faster than accessing a block in a remote cache (Figure 2.10).  Accessing a block from disk is even more expensive. A combination of local cache block access with occasional access to remote cache blocks gives good performance levels.


Figure 2.10:  Block Access Times

GCS Resource Modes and Roles

Because of data block transfers among multiple instances, the same data block can exist in multiple caches. The resource mode is defined as a concurrency control that establishes global access rights for instances in a cluster. The data block, or GCS resource, can be held in different resource modes depending on whether a resource holder intends to modify the data or read the data. The modes are as follows:

  • Null (N) mode - Holding a resource at this level conveys that there are no access rights. Null mode is usually held as a placeholder, even if the resource is not actively used.

  • Shared (S) mode - When a resource is held at this level, it will ensure that the data block is not modified by another session but will allow concurrent shared access.

  • Exclusive (X) mode - This level grants the holding process exclusive access. Other processes cannot write to the resource. It may have consistent read blocks.

The resource mode is an important mechanism to maintain data integrity and avoid data corruption issues. GCS resources are specified to have global roles or local roles. These roles are mutually exclusive.

  • When a block is first read into the cache of an instance and other instances have not read the same block, then the block is said to be locally managed and is therefore assigned a local role.

  • After the block has been modified by the local instance and transmitted to another instance, it is considered to be globally managed, and is therefore assigned a global role.

Basically, the concept of role supplements the access mode characteristic. A typical data block has both of these characteristics.

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