Oracle Parallel Processing

Chapter 8. Locking Mechanisms in OPS

Synchronization among multiple instances is the single most crucial factor in achieving the benefits of Oracle Parallel Server. This chapter explains the various issues involved with synchronization and also explains the locking mechanisms and other techniques used by OPS to attain it.

In a standalone database system, locks play a crucial role in maintaining concurrency, thereby allowing multiple transactions to access the same data at the same time. In a parallel server environment, locks also play an important role in allowing multiple instances to access the same data at the same time. This chapter describes the many types of locks you’ll need to be aware of as you administer an OPS system: transaction and instance locks, Parallel Cache Management (PCM) and non-PCM locks, hashed and fine-grained PCM locks, and fixed and releasable locks. We’ll also discuss lock modes, conversion, and lock allocation.

Cache Coherency

In an OPS environment, multiple instances execute transactions at the same time. Each instance maintains a buffer cache in its System Global Area (SGA) to hold copies of database blocks read from the datafiles. It is possible that more than one instance may attempt to modify the same database block at the same time. An instance always modifies the copy of the block contained within its buffer cache. If multiple instances were to modify the same block concurrently, each instance would be modifying a different copy of that block. ...