Oracle RAC Extended Distance Clusters Oracle Tips by Burleson Consulting

Oracle RAC has always had a latency issue when the RAC nodes are geographically distributed.  In these cases, many shops use Oracle Streams from back-and-forth replication.  Oracle MetaLink has just published new information of Oracle RAC "Extended Distance Clusters" for RAC nodes within 25 kilometers using Dark Fiber (DWDM or CWM) technology.

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Here are MetaLink notes on RAC extended distance clusters:

Real Applications Clusters (RAC) on Extended Distance Clusters is an architecture that provides fast sub-minute recovery from a site failure and allows for all nodes, at all sites, to be part of single database cluster. These advantages have raised interest in the architecture, have led some customers to implement, but it is critical to understand where this architecture fits best especially in regards to distance/latency and degree of protection, as well as some of the additional implementation complexity.

The high impact of latency, and thus distance, do create some practical limitations as to where this architecture can be deployed. This architecture fits best where the 2 datacenters are relatively close (<~25km) and where the extremely expensive costs of setting up direct cables with dedicated channels between the sites has already been taken. All of the known customers implemented with distances less than 25km, and because of the performance impact as the distance is increased it is unlikely they will be implementations at distances greater than 25km.

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