Oracle Database Tips by Donald Burleson

Oracle10g Grid Computing with RAC
Chapter 7 - Cache Fusion and Inter Instance Coordination

Block Access Modes and Buffer States

Only the SCUR and PI buffer states are real application cluster-specific. There can be only one copy of any block buffered in the XCUR state at any time. To perform modifications on a block, a process must assign an XCUR buffer state to the buffer containing the data block.

For example, if another instance requests a read access to the most current version of the same block, then Oracle changes the access mode from exclusive to shared, sends the current read version of the block to the requesting instance, and keeps a PI buffer if the buffer contained a dirty block

At this point, the first instance has the current block and the requesting instance also has the current block in shared mode. Therefore, the role of the resource becomes global. There can be multiple shared current (SCUR) versions of this block cached throughout the cluster database at any given point of time.

Cache Fusion Scenarios

The GCS plays a key role in performing the necessary block transfers. Three scenarios are discussed to explain the concept of cache fusion:

1.            Scenario 1: Read/Read

2.            Scenario 2: Write/Write

3.            Scenario 3: Disk Write