Databases Reference
In-Depth Information
Figure 13.16
Oracle RAC.
pages that are accessed by multiple servers. RAC has become the dominant availability
solution for collocated servers today, largely because of its ease of use. However, as we
will see, RAC is not without issues, and there are pros and cons to the major offerings in
the market today. Oracle RAC does not support geographic failover.
RAC's shared-disk active-active solution has many benefits. It is easy to use. All
servers in the configuration can be used for active work. However, there are some little
discussed issues that are important to know about RAC related to the possibility of
incorrect answer sets and log storage requirements.
Oracle RAC data consistency: Every Oracle data page has a system change number
(SCN) that indicates the “time” the page was changed. The SCN is an ever-increasing
number so that Oracle knows the order of the updates. Oracle uses this virtual “times-
tamp” to determine what data to return to a query. By default the current SCN is not
broadcast to all other nodes for 7 seconds. This means a query can return the wrong
results by default. Oracle can be configured to force the right results to be returned,
though this incurs a significant performance penalty.
Oracle RAC has been used in industry-standard benchmarks, such as TPCC, to
demonstrate its efficiency for use in OLTP workloads. The results of these benchmarks
have shown that RAC appears to require an order of magnitude for more log storage
than the non-RAC version of Oracle.
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Oracle 10g = 2,460GB of log per million tpmC
Oracle 10g RAC = 20,676GB of log per million tpmC
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