Geographically Dispersed Cluster and SQL Server... Advantages and Limitations

We know about the benefits of using cluster technology as high availability solutions of SQL Server. One of the major fascinating advantages clustering technology offers is the nodes of the cluster can be residing in geographically dispersed location.

Unlike a local cluster, a geographically dispersed Cluster requires multiple storage arrays. At least one storage arrays must be present at each site to ensure that in the event of failure at one site, the remaining site will have local copies of the data. Also the nodes of a geographic cluster are connected to storage in such a way that when there is a failure at one site or a failure in communication between sites, the functioning nodes can still connect to storage at their own site.

From SQL Server perspective, setting up and managing a Geographically Dispersed Cluster is not different than setting up and managing a normal cluster as long as the cluster is set up in the similar way a locally connected cluster would have been set up. However please note that there are still some limitations of SQL Cluster (As of SQL 2008 EE) with latest stretch clustering technology. SQL Cluster setup will fail if different nodes of the cluster are on different subnets (which is true for less expensive starch cluster than a traditional Geographically Dispersed Cluster). However Microsoft acknowledged this limitation as a bug and a future release (post SQL 2008 R2 release) will fix it.

So technically there is no limitation what can be maximum distance between different nodes of the cluster if you are using Geographically Dispersed Cluster technologies.