The next logical step beyond decentralized, workstation-based primary storage is combining all of that shared data onto a single, dedicated server. By doing this, companies can standardize their data protection and security models across all of their mission-critical data. Centralizing the data also makes it cheaper to invest in redundancy -- whether redundant disk arrays or power supplies.
Most file servers are exactly that: an industry-standard server with a general-purpose server operating system and lots of direct-attached disk dedicated to sharing files. However, many low-end NAS devices fall into this category as well. As this kind of NAS device becomes increasingly prevalent in businesses of all sizes, it's important to note that they are essentially the same as a file server.
At a certain point, though, a business will outgrow a single file server or NAS device. Usually, the most common approach is to add more file servers. As this practice continues, the same problems plaguing peer-to-peer storage emerge again. Instead of maintaining a single pool of storage, you're now tasked with managing many of them. Similarly, the exposure to data loss through hardware failure is multiplied as the number of devices increases.
File servers and NAS devices are also poorly suited for storing block-level structured data such as databases and email. These applications are usually built on their own servers with their own direct-attached storage, which further compounds the storage management challenge.
Primary data storage, rung 3: Low-end SAN (a file server by any other name)
Users: 10 to hundreds
Cost: $2,000 to $20,000
Examples: Microsoft Windows Storage Server derivatives, Overland SnapServer
In an effort to address the challenge of managing both structured and unstructured corporate data simultaneously, many storage vendors have come out with low-end SAN devices that allow both block- and file-level data to be stored on the same device. The benefit to using this kind of device is that all of a company's data -- file shares, databases, email, virtualization infrastructures, and so on -- can be combined into the same storage pool and managed and protected together.