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SAN vs NAS: Spelling out the differences

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The names almost give away the difference between network attached storage (NAS) and storage area networks (SANs): you would expect a NAS to consist just of storage and a SAN to be a network, and that is true – up to a point.

Designed to be easy to manage, a NAS is fundamentally a bunch of disks, usually arranged in a Raid and consisting of either SAS (serial attached SCSI) or Sata disks just like the ones in most desktops. Users and servers attach to the NAS primarily using TCP/IP over Ethernet, and the NAS has its own IP address. The primary job of a NAS is to serve files, so most NAS systems offer support for Windows networking, HTTP, plus file systems and protocols such as NFS and Mac AFP.

The key difference is whether you need the top performance and reliability of a SAN

While a SAN deals in blocks of data, a NAS operates at the file level and is accessible to anyone with access rights, so it needs also to manage user privileges, file locking and other security measures. The processing and control of this data is performed in large enterprise systems by a NAS head, physically separated from the storage system.

In contrast, SANs allow multiple servers to share a pool of storage, making it appear to the server as if it were local or directly attached storage, and it cannot be accessed by individual users. A dedicated networking standard, Fibre Channel, has been developed to allow blocks to be moved between servers and storage at high speed. It uses dedicated switches and a fibre-based cabling system which separates it from the day-to-day traffic traversing the busy enterprise network, while the well-established SCSI protocol enables communication between the servers’ host bus adaptors and the disk system.

The SAN’s storage usually consists of several large arrays of high-speed SAS disks spinning at up to 15,000 rpm, although solid-state disks are used in situations where performance or energy saving are priorities.

SANs are used for mission-critical data such as big databases, and reliability and performance are key. SANs need to be fast and transparent to the operating system even though the data may travel some distance from the server.

The development of SANs was in part a solution to the problem of slow access to network-attached storage. However, with faster 10Gb Ethernet becoming a standard in the datacentre, we are seeing more convergence between SAN and NAS. Most vendors are offering combined storage systems, available either via block access over Fibre Channel or using file-level access, a trend that is likely to accelerate as 40Gb and 100Gb Ethernet enter the mainstream. Fibre Channel over Ethernet looks set to become the de facto standard for storage over the next decade.

Convergence offers a big advantage if you are managing datacentre storage because you will no longer need to manage, back up and balance data across two separate storage systems. However, if you need to choose between SAN and NAS, the key difference to focus on is whether or not you need the top performance and reliability of a SAN and are prepared to pay the premium. If not, you need a NAS. ®

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