1. A virtualized network storage system, comprising at least two network storage devices connected to a same Storage Area Network, SAN, wherein each of the network storage devices comprises a device management unit and a global volume management unit, the device management units are adapted to exchange status information of the network storage devices with each other; one of the device management units functions as a primary device management unit, and is adapted to synchronize a mapping relationship between global volume and local volumes which is stored on network storage devices and redirect an access to a global volume to a network storage device at which a global volume management unit of the global volume is located; the other of the device management units function as backup device management units, when the primary device management unit has a fault, one of the backup device management units may be upgraded to be a new primary device management unit; the global volume management units are adapted to execute an access to a global volume according to the mapping relationship between global volume and local volumes; wherein the network storage devices employ Internet Small Computer System Interface protocol, iSCSI protocol, a network storage device at which the primary device management unit is located has a virtual network address for receiving the access to the global volume, the redirection of the access to the global volume is implemented by iSCSI redirect protocol; when receiving an iSCSI access request, the primary device management unit determining a global volume management unit located on a storage device for the accessed global volume according to the status information of the network storage devices, notifying the determined global volume management unit to the accessing client by using an iSCSI redirect protocol notification; the determined global volume management unit receiving another access request to the global volume initiated by the client and mapping the global volume to corresponding local volumes according to a table of mapping relationship between the global volume and local volumes.