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Introduction to NAS & SAN

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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.

The Wires
--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

The Protocols
--SAN uses Encapsulated SCSI

More Differences
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 blocks.
A NAS allows greater sharing of information especially between disparate operating systems such as Unix and NT. File Sharing is operating system dependent and does not exist in many operating systems.
File System managed by NAS head unit File System managed by servers
Backups and mirrors (utilizing features like NetApp's Snapshots) are done on files, not blocks, for a savings in bandwidth and time. A Snapshot can be tiny compared to its source volume. Backups and mirrors require a block by block copy, even if blocks are empty. A mirror machine must be equal to or greater in capacity compared to the source volume.
What's Next?
NAS and SAN will continue to butt heads for the next few months, but as time goes on, the boundaries between NAS and SAN are expected to blur, with developments like SCSI over IP and Open Storage Networking (OSN), the latter recently announced at Networld Interop. Under the OSN initiative, many vendors such as Amdahl, Network Appliance, Cisco, Foundry, Veritas, and Legato are working to combine the best of NAS and SAN into one coherent data management solution.

For more information, see the technical library.

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