Branch office virtual server deployments require a minimum of two servers for high availability. They also require shared storage, which can come in the form of a SAN, a NAS appliance, a single shared RAID array, or a software-only Server SAN.
When considering software-only Server SAN solutions, it's advisable to have a quorum server, also called a neutral storage host (NSH). The quorum server connects to both servers and decides which of the servers in a mirrored pair can update data, if the servers are in a split-brain condition. A split-brain happens when the connection between two servers fails. With a quorum server, in order to keep writing data, a production server has to be able to speak with either its mirrored pair or the quorum server. Otherwise it’s taken off-line to prevent data corruption.
Some companies put a quorum server in each location. But when they do that, they increase the number of devices by 50%, which is costly and more difficult to deploy. Others consolidate all of the quorum servers in a centralized location, typically a network operations center or data center that is monitoring the branch offices. By doing this, they reduce the hardware in the branch locations, and reduce operating expense by 30% or more.
Wikibon's Server SAN Market Definition identified a number of software-only Server SANs, including:
Some software-only Server SAN solutions are designed for large data-center deployments, some for small and medium-sized businesses, and others for large-enterprise remote-site deployments.
Action Item: Organizations should always use a quorum server to prevent data corruption in software-only Server SANs. In large-enterprise remote-site deployments, they should consider centralizing quorum servers, to reduce hardware expense and deployment complexity. With centralized quorum servers, however, organizations should ensure the storage virtual appliance they select is designed to tolerate longer latencies with the quorum. WAN connections are often the weakest link in a remote IT deployment.