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Oracle 白皮书 extended RAC PDF 下载
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Oracle RAC on Extended Distance Clusters provides a greater high availability than a local
Oracle RAC implementation. However, it does not provide full Disaster Recovery. Feasible
separation is a great protection for some disasters (local power outage, airplane crash, server
room flooding) but not all.
Disasters such as earthquakes, hurricanes, and regional floods may affect a greater area.
Oracle RAC on Extended Distance Clusters does not protect from human errors or corruptions
in the shared storage, either, as an Oracle RAC system, even on Extended Distance Clusters,
is still a tightly coupled and enclosed system.
For comprehensive data protection, including protection against corruptions and regional
disasters, Oracle recommends using Oracle Data Guard together with Oracle RAC as building
blocks of Oracle’s Maximum Availability Architecture (MAA)1
. For active-active databases
deployed across geographically separated data centers, the MAA recommendation is to use
Oracle GoldenGate. Note that Data Guard and GoldenGate also provide additional benefits
such as minimizing downtime for maintenance activities such as upgrades and migrations.
This paper discusses the potential of the extended cluster architecture, covers the required
components and design options that should be considered during implementation, reviews
empirical performance data over various distances, explores supported and non-supported
configurations, and lists the advantages that Oracle Data Guard provides to this solution.
1 For more information regarding MAA, see: http://www.oracle.com/goto/maa
Oracle RAC and Oracle RAC One Node on Extended Distance (Stretched) Cluster
What is an Extended Distance Oracle RAC Cluster?
As the name implies, an Extended Distance Cluster is a cluster, in which most or all the nodes are
not local and typically set up with a certain distance between them. Clusters of this kind have been
referred to by many names, including “campus clusters”, “metro clusters”, “geo clusters”, “stretched
clusters” and “extended clusters”. Some of these names imply a vague notion of distance range.
For this paper, this type of configuration will be referred to as either Oracle RAC on “Extended
Distance Clusters” or “Stretched Clusters”, while the names will be used synonymously. If Oracle
RAC One Node is concerned in a certain context, respective references will be made.
Unlike classic Oracle RAC implementations, which are primarily designed as scalability and high
availability solution that resides in a single data center, it is possible – under certain circumstances –
to build and deploy an Oracle RAC system in which the nodes are separated by greater distances.
For example, if a customer has a corporate campus, they might want to place the individual Oracle
RAC nodes in separate buildings. This configuration provides a higher degree of disaster tolerance, in
addition to the normal Oracle RAC high availability, since a fire in one building would not, if properly
set up, stop the database from processing. Similar, many customers have two data centers in reasonable
proximity (<100km) which are already connected by a direct, ideally non-routed, high speed link(s) and
are often on different power grids, flood plains, and the like.
Practically, there is, however, another characteristic of Extended Distance Clusters or Stretched
Clusters, at least when implemented for an Oracle RAC Database. This aspect of an Extended
Distance Cluster derives from the storage configuration and setup and is therefore independent
of the physical distance between the nodes in the cluster.
Assuming that an Extended Distance Cluster maintains servers that are physically dispersed to protect
the system as a whole from local server failures, similar considerations can lead to a storage setup that
foresees (at least two) independent storage arrays (SAN / NAS systems) in different locations. As a
matter of fact, probably all Extended Distance Clusters use this kind of storage setup. However, often
one will find that the necessity to protect the storage arrays leads to an architecture that is also used in
Extended Distance Clusters, while the nodes in the cluster are actually in rather close proximity.
Over the last decade of implementing Oracle RAC on Extended Distance Clusters, the latter aspect
might have been a driver for implementing such architecture more often than the idea of protecting
the system as a whole from server failures. While for the configuration of such systems the main driver
is of minor importance it needs to be pointed out that such a configuration cannot replace a disaster
recovery solution, protecting the shared storage from more than just a failure.
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