Storage area networks (SANs) and network attached storage (NAS) are often positioned as rival technologies, with one seen as outselling or overcoming the other. The truth is that the two are a lot more alike than they are dissimilar.
In particular, both aim to do the same thing Ã, to split data storage from the servers and move it out onto a network. The difference is in how they achieve this and in the complexity of the resulting setup.
NAS uses storage appliancesÃ, complete file servers that can be attached to a network and made available for other servers to access, using standard protocols such as network file system (NFS). These normally share the same LAN as other servers, so the network needs to be at least Fast Ethernet and preferably Gigabit Ethernet.
Like other file servers, NAS appliances are file-based, which makes them particularly suitable for tasks that are heavily file-oriented such as Web cacheing and multimedia content distribution.
Compare and contrast
By contrast, the key feature of a SAN is that it uses a separate network dedicated to storage traffic. This is typically 1Gbit/s Fibre Channel, but it need not be, as a SAN could equally well be built on Ethernet or tunnelled over a WAN for long-distance data backup or mirroring. SANs are normally block-based rather than file-based, so servers can use the storage as they please.
Many storage experts feel that the need for a second network means SANs are suited only to centralised applications, whereas NAS would probably be better for a distributed business with a number of branch offices, especially as the branches are unlikely to have the IT staff needed to support the more complex SAN option. Even with a single site, size is also an issue, with a rule of thumb suggesting that only once an organisation needs to store 500GB of data or more will a SAN become cost-effective.
The other thing that distinguishes SANs today is that the storage devices-Ã, whether tape drives, disk drives or disk arrays-Ã, are directly attached to the network. Fibre Channel allows each device to be assigned to a specific server, with zoning added if necessary to protect devices from over-acquisitive systems such as NT4, which will attempt to seize all the devices they can locate.
The difference between the two technologies can also be seen as a tactical and strategic answer to the same question. NAS fixes the problems of storage being in short supply, for instance, and also consolidates it in one place when it can be protected properly. With the increasing speed of LANs, NAS can even provide networked storage that is faster than locally attached storage, according to Network Appliance's northern European marketing manager, Chris Gale.
What NAS does not do is relieve the LAN of that extra traffic, nor does it address the storage pooling issue. And because it works at file level, its performance may not match that of a block-based alternative.
