1. A method of controlling a plurality of virtual machines by a virtual machine control unit in a cloud computing system, the method comprising: monitoring a plurality of virtual storages allocated to the plurality of virtual machines; detecting a service unable virtual storage having a service unable state among the monitored plurality of virtual storages; choosing a virtual machine using the detected service unable virtual storage from the plurality of virtual machines; and temporarily interrupting the chosen virtual machine, wherein the monitoring includes: detecting errors generated at one of a hypervisor level connection and a storage level connection based on a result of the monitoring; and detecting, from the monitored virtual storages, a virtual storage associated to the detected errors and determining the detected virtual storage as the service unable virtual storage, and wherein: the cloud computing system includes: a storage configured to provide the plurality of virtual storages to the plurality of virtual machines, a hypervisor configured to allocate the virtual storages of the storage to the plurality of virtual machines and to control the allocated virtual storages in response to a request of the plurality of virtual machines, a storage interface providing an interface to the hypervisor to access the virtual storages allocated to the plurality of virtual machines, and the virtual machine control unit directly coupled between the hypervisor and the storage interface, the method comprises: directly detecting, by the virtual machine control unit, errors generated in a network between the storage interface and the storage, wherein the network between the storage interface and the storage is exclusive of the storage interface and the storage, and a network between the storage interface and the hypervisor, wherein the network between the storage interface and the hypervisor is exclusive of the storage interface and the hypervisor.