Clusters require such a locking model because each machine in the cluster
must be aware of the data locks held by other, physically separate machines in the
cluster.
Today, that Oracle solution has evolved into Real Application Clusters (RAC)
(replacing the Oracle Parallel Server (OPS) that was available prior to Oracle9i). RAC
is most frequently used for Windows, Linux, or Unix-based clusters. Oracle provides
an integrated lock manager that mediates between different servers, or nodes,
that seek to update data in the same block.
RAC introduced full support of Cache Fusion, where locks are maintained in memory
without frequent writing to disk. Cache Fusion is different from the standard
locking mechanisms that are described in Chapter 8, in that it applies to blocks of
data, rather than rows. The mediation is necessary since two different nodes might
try to access different rows in the same physical block, which is the smallest amount
of data that can be used by Oracle.
Cache Fusion initially greatly increased performance for read/write operations compared
to the previous OPS and later added improved performance for write/write
operations in Oracle9i RAC.
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