Prior to Oracle 9,
network-clustered Oracle databases used a storage device as the data-transfer
medium (meaning that one node would write a data block to disk and another node would read
that data from the same
disk), which had the inherent disadvantage of lackluster performance.
Oracle 9i addressed this
issue: RAC uses a dedicated network connection for communications internal to
the cluster.
Since all computers/instances
in a RAC access the same database, the overall system must guarantee the
coordination of data changes on different computers such that whenever a
computer queries data, it receives the current version — even if another
computer recently modified that data. Oracle RAC refers to this functionality
as Cache Fusion.
Cache Fusion involves the
ability of Oracle RAC to "fuse" the in-memory data cached physically separately on
each computer into a single, global cache.
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