But these devices, though technically SANs (most of them support iSCSI to allow remote, block-level storage access), are really nothing more than a standard server with different software in place to allow the device to serve iSCSI requests in addition to file serving. In general, they offer no more redundancy than a normal server, nor do they scale beyond a normal server in terms of performance.
In short, these devices may allow you to efficiently manage all of your storage needs, but they lack the performance, scalability, and reliability of enterprise-class SANs.
Instead of using industry-standard server hardware and software, enterprise-class SANs employ highly redundant, dual-controller architectures, boasting such features as mirrored caches and redundant interconnect interfaces. Similarly, enterprise-class SANs are also highly scalable -- supporting a much higher level of capacity and far greater performance than their low-end brethren.
This field of devices includes not just the typical block-level SAN, but also higher-end, multicontroller NAS devices that are capable of serving both block- and file-level data with the same redundancy and performance. In addition, these devices allow storage admins to mix different capacities and speeds of physical storage media (both disks and SSD), making it possible to present the right type of storage to each storage consumer while still maintaining a unified management architecture.
Only a few years ago, the entry level for this type of device was well above $50,000. That price tag has fallen precipitously. As a result, the number of enterprises that can afford to own a SAN has sharply increased.
Primary data storage, rung 5: Network-based storage virtualization
Users: Thousands to tens of thousands (and beyond)
Cost: Sky's the limit
Examples: EMC Invista, HP SVSP, NetApp V-series
As scalable and redundant as enterprise-class SANs are, the largest enterprises will eventually outgrow a single SAN platform and need to field multiple SANs to achieve the levels of performance and reliability they require. As this happens, the same inefficiencies -- in terms of both capacity and management -- rear their heads once more. To combat this problem, large enterprises often employ network-based storage virtualization to unify heterogeneous SAN storage platforms together into a single logical infrastructure.