At first glance NAS and SAN might seem almost identical, and in fact many times either will work in a given situation. After all, both NAS and SAN generally use RAID connected to a network, which then are backed up onto tape. However, there are differences -- important differences -- that can seriously affect the way your data is utilized. For a quick introduction to the technology, take a look at the diagrams below.
Wires and Protocols
Most people focus on the wires, but the difference in protocols is actually the most important factor. For instance, one common argument is that SCSI is faster than ethernet and is therefore better. Why? Mainly, people will say the TCP/IP overhead cuts the efficiency of data transfer. So a Gigabit Ethernet gives you throughputs of 600-800 Mbps rather than 1000Mbps.
But consider this: the next version of SCSI (due date ??) will double the speed; the next version of ethernet (available in beta now) will multiply the speed by a factor of 10. Which will be faster? Even with overhead? It's something to consider.
--NAS uses TCP/IP Networks: Ethernet, FDDI, ATM (perhaps TCP/IP over Fibre Channel someday)
--SAN uses Fibre Channel
--Both NAS and SAN can be accessed through a VPN for security
--NAS uses TCP/IP and NFS/CIFS/HTTP
--SAN uses Encapsulated SCSI
Almost any machine that can connect to the LAN (or is interconnected to the LAN through a WAN) can use NFS, CIFS or HTTP protocol to connect to a NAS and share files. Only server class devices with SCSI Fibre Channel can connect to the SAN. The Fibre Channel of the SAN has a limit of around 10km at best
A NAS identifies data by file name and byte offsets, transfers file data or file meta-data (file's owner, permissions, creation data, etc.), and handles security, user authentication, file locking A SAN addresses data by disk block number and transfers raw disk