Multi-Version Consistency Model Architecture

Oracle RAC Cluster Tips by Burleson Consulting

Oracle?s multi-version consistency model architecture distinguishes between a current data block and one or more consistent read (CR) versions of the same block. It is important to understand the difference between the current block and the CR block. The current block contains changes for all the committed and yet-to-be-committed transactions. A consistent read (CR) block represents a consistent snapshot of the data from a previous point in time.

Applying undo/rollback segment information produces consistent read versions. Thus, a single data block can reside in many buffer caches under shared resources with different versions.Multi-version data blocks help to achieve read consistency. The read consistency model guarantees that the data block seen by a statement is consistent with respect to a single point in time and does not change during the statement execution.

Readers of data do not wait for other writer's data or for other readers of the same data. At the same time, writers do not wait for other readers for the same data. Only writers wait for other writers if they attempt to write. As mentioned earlier, the undo (rollback) segment provides the required information to construct the read-consistent data blocks. In case of a multi-instance system, like the RAC database, the requirement for the same data block may arise from another instance.

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