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There are two big disadvantages to Oracle Parallel Server (OPS). Which
one is the "biggest" all depends on your perspective. First, OPS is
very complex. This complexity does bring high availability and it can
bring higher performance. But it is not an easy thing to set up and
administer OPS. Second, OPS has a problem known as "disk pinging." To
illustrate this, we'll assume that there are two instances in the OPS
configuration. Instance1, running on Server1, currently has some dirty
data in its buffer cache. An application connected to Instance2 on
Server2 wants to access that data. Before it can access that data,
Instance1 writes the data back to the shared disk storage. Then
Instance2 reads this data into it's buffer cache. This data going back
and forth on disk is known as disk pinging and has been known to cause
some performance problems.
Oracle addressed this last issue in Oracle9i's Real Application
Clusters (RAC), the replacement for OPS with a product called Cache
Fusion. Actually, Cache Fusion appeared late in Oracle 8.1.7's OPS.
With Cache Fusion, Instance2 could read the buffer cache of Instance1
through a high-speed interconnect between the two servers. RAC with
Cache Fusion eliminated the disk pinging that plagued OPS deployments.
This only leaves the complexity issue.
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