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A few years ago, it was common to see articles in the trade press hotly debating the respective merits of network-attached storage (NAS) and storage area networks (SANs). SANs were denounced as being too difficult to deploy and manage, according to NAS zealots, while NAS was condemned as being too slow and unscalable, according to the SAN fundamentalists. Over time, however, the NAS/SAN jihad abated, as both NAS and SAN vendors realized that each technology has its place and that, besides, both NAS and SAN vendors were having considerable success in the market.
The advent of iSCSI is now changing the terms of the NAS versus SAN debate. Like its Fibre Channel predecessor, iSCSI offers a new means to access storage over a network. Unlike Fibre Channel, iSCSI and NAS operate over the same type of network and so would appear to compete for the same storage market. The response of the NAS and SAN vendors has been to simply provide both NAS and iSCSI as customer options. Network Appliance, for example, is supporting iSCSI on its NAS platform, while vendors such as EMC and HDS are providing NAS processors to front-end their large SAN storage arrays. These products give customers greater flexibility but do not address the basic conundrum of when to use NAS and when to use SAN.
The File (NAS) vs. Block (SAN) Storage Dilemma
Technically speaking, a network-attached storage device is not really "storage attached to a network." The SCSI block I/O characteristic of storage devices occurs between the NAS processor and its attached storage arrays, not between the storage disks and the user network. The network-attached portion of a NAS device is therefore in reality network-attached file serving. When a NAS processor receives a request for a file, the processor must query the file system's metadata for that file, identify the data blocks on disk that compose the file, reassemble the appropriate data blocks in the proper sequence, and wrap the file content in TCP/IP for transmission onto the network. The heavy lifting of writing and reading blocks of data to disk, therefore, occurs behind the scenes, not on the network.
What differentiates a NAS device from a common file server is that the file server operating system has been stripped of superfluous functions and optimized for sending and receiving files via IP protocols such as NFS (network file system) or CIFS (common internet file system). The operating system may be a streamlined version of Unix or, as with Microsoft's new NAS initiative, a streamlined version of Windows. In either case, the NAS processor serves files onto the network to clients, as with traditional file servers, while performing its block SCSI I/O on the back end.
In contrast, SAN technology -- whether Fibre Channel or iSCSI -- serves block I/O directly onto a network infrastructure. Block data transfer over a SAN has an inherent performance advantage, as affirmed by the fact that vendors such as Network Appliance use SAN-attached disks for mass storage. Gigabit transport of data blocks over a SAN enhances throughput and enables the NAS processor to more quickly retrieve the raw material for file assembly/disassembly from blocks. The performance of NAS is thus enhanced by SAN technology.
Like Fibre Channel, iSCSI is a block storage protocol. Whereas Fibre Channel encapsulates SCSI commands, status, and data in Fibre Channel framing, iSCSI performs the same encapsulation in TCP/IP. In a pristine iSCSI environment, both hosts and storage targets have Ethernet or Gigabit Ethernet interfaces, and the IP network serves as the SAN infrastructure. Today, iSCSI device drivers and iSCSI network adapters provide host connectivity, but the mainstream SAN storage targets are Fibre Channel-attached. This requires a high performance protocol gateway such as the Nishan IP storage switch to bring iSCSI hosts to Fibre Channel targets.