Specifications

Chapter 1. The problem

How can servers be protected against power failures? Usually redundant power

supplies and redundant power cords connected to different power lines are used

for basic protection.

The failure of one of these components does not affect the operation of the

server. To protect the system against a complete power loss, an uninterruptible

power supply (UPS) is required. If software such as PowerChute PLUS is

installed on the system and a communication link is set between the UPS and the

server, PowerChute PLUS can stop the applications and shut down the operating

system.

Figure 1. UPS with one server

For larger solutions such as a Microsoft Cluster Server (MSCS) configuration, a

single UPS may not provide sufficient protection. The run-time capacity of one

UPS is inadequate in most cases for two servers with shared storage.

Additionally, one UPS is a single point of failure. Thus, you need two or more

UPS units. However, in an MSCS environment with one or two UPS units you

have certain problems:

• Application handling: In an MSCS environment you cannot simply stop an

application; the cluster application must be set to offline with the cluster

administration tool. If the application is stopped by normal procedures (such

as PowerChute PLUS’s Application Shutdown or application-specific stop

procedure), then the application would be considered as failed and restarted

by the cluster resource monitor.

• Server status: If a power loss occurs on one server, a communication about

the status of the other server is required. If only the local server is affected by

the power loss, then it makes sense to move all cluster resources from this

node to the surviving one. Otherwise, if no other node is available or if the

other node will also shut down, then the resources must be set to offline.

• Server power cabling: How are power cables connected? Some servers

have a N+1 redundancy in power supplies. If you connect a server with three

power supplies and three power cords to two power circuits, you have at least

two power supplies on the same circuit. It is possible that this circuit will fail.

With the one remaining power supply, the server will not work. In the case of a

server with two power cords, you could try to connect them to different UPS

units. But then you have the communication problem as described below.

Server

UPS

Communication

Smart signaling cable

Power