Dell PowerEdge R720xd Dell High Availability and Disaster Recovery Solutions U - Page 10

Dell High Availability and Disaster Recovery Solutions Using Microsoft SQL Server 2012 AlwaysOn Availability Groups Limitations:  Limits on storage scalability.  No enhanced SAN capabilities. AVG deployments on shared storage In addition to the failover capability provided by AlwaysOn Availability Group, we may also set up a secondary layer of availability at the server instance level. This is done by implementing SQL Server FCI together with the underlying WSFC cluster. FCI is configured with shared storage to host the SQL server instances. FCIs are basically designed to provide High Availability at instance level whereas AlwaysOn Availability Group (AVG) is designed to provide HA and DR capabilities at database level. - Consider the following when you plan to combine the FCI with AVG.  AVG alone does not require shared storage, but when we use FCI to host one or more availability replicas, each of the nodes which are part of FCIs will require shared storage as per standard SQL Server failover instance installation.  An FCI may be used together with an availability group to enhance the availability of an availability replica. However, to prevent potential race conditions in the WSFC cluster, automatic failover of the availability group is not supported to or from an availability replica that is hosted on a FCI.  Since SQL Server FCIs do not support automatic failover by availability groups, any availability replica that is hosted by an FCI can only be configured for manual failover.  All the prerequisites and restrictions that need to be addressed before proceeding with AVG deployment may be found at http://msdn.microsoft.com/enus/library/ff878487.aspx#FciArLimitations. Care should be taken to make sure all the specified hotfixes are installed on the appropriate nodes. Figure 3 shows the typical AVG deployment along with SQL Server FCI. 10