Disks and other storage devices have become over time more and more intelligent. A Network Attached Storage (NAS) device has its own network interface. A filer for example would be a storage device that interface at the level of a distributed file system. For example, a NAS might have an attached disk array that uses a variant of SCSI, but access to the device is through the Network File System (NFS). NAS offer certainly flexibility, but as they are on a network, they are exposed to network intrusion and to network congestion.
Figure 1: NAS (SUN)
Storage Area networks (SAN) are a low cost, high transfer alternative, that connects storage devices to servers through a dedicated storage network. Typically, the NAS interface is at the lower end of the file system, whereas SAN devices interface at the block system. NAS and SAN technology can be mixed, see the SNIA Shared Storage model that describes the interfaces. In Figure 2 for example, a NAS processor (the NAS head) might use a SAN to access the internal storage (e.g. a disk array) over a SAN. SANs are normally chosen for high performance applications that require gigabit speeds, and throughput at the block device level. NAS excell for heterogenoous file access and where simplified management is needed.
Figure 2: SNIA Shared Storage Model - NAS/SAN Example
The storage area network is completely separated from the network through which the servers receive requests from clients. The storage devices can be of various kind, such as JBODs, RAIDs, disk arrays, tapes, etc. Typically, the servers interact with the storage devices using blocks, though something more advanced like storage objects is equally possible. Since the storage area network is completely separated from the LAN, an intruder would have to subvert a server first (which in itself gives complete access to all storage units).
Figure 3: Storage Area Network
The switch technology in a SAN can be of many kinds. SSA (IBM) (serial storage architecture), a gigabit serial storage transport has already been superseeded by the standardized Fibre Channel. Fiber Channel itself is in danger of being superseded by IP based technology such as iSCSI.
Additional ReferencesSUN: NAS white paper
|© 2003 Thomas Schwarz, S.J., COEN, SCU SCU COEN T. Schwarz COEN 180|