A cluster system and a method for providing service availability. The cluster system includes a first physical server having registered therein an active virtual machine; and a plurality of physical servers, wherein the plurality of physical servers comprises a second physical server having registered therein a standby virtual machine corresponding to the active virtual machine, the active virtual machine failing over to the standby virtual machine when a failure occurs in the first physical server, wherein each of the plurality of physical servers stores post-failure registration information when the failure occurs, wherein the post-failure registration information associates the active virtual machine with a physical server among the plurality of physical servers, and wherein the physical server is different from the second physical server.