DxConsole Prioritized Failover


DxConsole Prioritized Failover

High availability is easy to implement with a built-in prioritized and automated failover solution.

DxConsole allows you to organize your SQL Server databases into virtual hosts, which can be moved around your cluster as needed.  If the system that is currently hosting a virtual host fails, the virtual host will automatically be restarted on another system.  To choose the best system to host the virtual host, DxConsole will load the list of systems that are eligible to host the virtual host, and eliminate systems that are unavailable, or lack access to storage required to run all of the SQL Server instances.  The order of this list is important, as systems closer to the front receive absolute priority.

Once the list has been generated, DxConsole will check resource allocations that have been set.  If no resource allocations have been set, the virtual host will be moved to the first eligible system.

On the other hand, if resource allocations have been set, DxConsole will sum up the resources allocated to all the instances that are part of the virtual host, and sum up the resources allocated to all instances running on the first-choice system, and check whether the grand total exceeds the resources that have been allocated on that system.  If starting the virtual host on the first-choice system does not cause the system to become over-allocated, that system will be chosen, and the virtual host will be started there.  Otherwise, if resource allocations favor starting the virtual host elsewhere, the other systems on the list will be tried in order.

If it is not possible to start the virtual host on any eligible systems without causing the system to become over-allocated, DxConsole will test different combinations of moving lower-priority virtual hosts to other systems.  This action is taken as a last resort, as restarting SQL Server instances will cause client connections to be temporarily dropped.  If a solution is found that requires fewer than three unrelated virtual hosts to be moved around, it will be carried out.

Sometimes, if more than one system fails, the total resources allocated to all of the virtual hosts will exceed the resources allocated on the remaining systems, and it is not possible to uphold resource allocations.  In this case, DxConsole will start the virtual host on the highest priority eligible system, and raise an administrative alert that the system is over-allocated.

Watch a short webcast on DxConsole’s Prioritized Failover. 

Sam Revitch