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Network Attached System (NAS)
When server computers need to use the same data, a Network File System (also called NAS - Network Attached Storage) can be used. The Network File System is implemented using a File Server and a network. The File Server is a regular computer or specialized OS that has a regular File System and regular disk devices controlled with this File System. Since only the File Server computer has direct access to the physical disk, all servers computers connecting to these disks use the same File System (The File System of the NAS). That File System guarantees the data consistency
Storage Area Network
Storage Area Network is a special type of network that connects computers and disk devices; in the same way as SCSI cables connect disk devices to one computer. SAN provides Shared Disks, but SAN itself does not provide a Shared File System. The space of the SAN disk is called a LUN (Logical unit number). If you have several computers that have access to the LUN (via SAN or dual-channel SCSI), and try to use that space the disk logical structure will be damaged very quickly. So by default a single server can see a single LUN at any one time. So the area (LUN) on the SAN could not be accessed by the 2 servers at the same time.
Internet SCSI (ISCSI)
The iSCSI (for "Internet SCSI") protocol allows clients (called initiators) to send SCSI commands to SCSI storage devices (targets) on remote servers. It is a popular Storage Area Network (SAN) protocol. Unlike Fibre Channel, which requires special-purpose cabling, iSCSI can be run over long distances using existing network infrastructure. However we will have the same problem than with the SAN
