IP Storage Networking: Straight to the Core

NAS devices overcame several barriers when introduced to the storage market. First, NAS broke the DAS captive model. Second, it provided a mechanism to link storage across Ethernet and IP networks, a completely new footprint for storage administrators. Most importantly, and in part to support the architectural change, NAS moved the file system and volume management functions from the server to a NAS device, often called a filer . Keep in mind that the "filing" function remains no matter what the storage model. The location of this function has traditionally separated the distinction of NAS and SAN. See the following section for a more complete outline of the differences between NAS and SAN.

2.5.1 STORAGE GUIDANCE: SAN and NAS

A discussion about accessing storage via IP and Ethernet, such as with IP storage adapters, invariably begs the question about the difference between SAN and NAS. The SNIA shared storage model outlines file-level and block-level distinctions, which are further clarified in Figure 2-7.

SANs provide scalable performance with direct access to storage devices. Since the storage system resides on the distributed hosts , this architecture works well for databases and online transaction processing.

NAS provides crossplatform scalability, but not the performance scalability due to its use of a high level of file abstraction between applications and storage. While a NAS filer offloads some server workload required to map files into physical storage, it adds significant workload to the filer itself. NAS delivers a central storage system (file system) that is ideal for file sharing, Web serving, and similar applications.

A NAS device is essentially a highly optimized file-delivery server. Depending on the specific implementation, NAS devices may have internal storage (similar to a traditional server), or they may be directly connected to a RAID device for additional capacity or to a tape library for backup functions. Additionally, some NAS devices can connect directly to a block-based storage network.

Traditional servers access NAS devices through the Ethernet and IP network by means of a file request. Since the file system resides on the NAS device, servers with different operating systems can all easily access the same data. This crossplatform access gives NAS a considerable advantage in terms of simplicity. It also allows the NAS device to provide lock functions for particular files. Since the device knows if one server is accessing a file, the second server will not be able to write or modify that file.

NAS has made tremendous inroads to certain market segments where the architecture suits the applications. Web- related functions, such as serving HTTP requests , are ideal applications. The relatively small file size combined with the crossplatform access across Ethernet and IP networks makes NAS a winner.

Similarly, corporate file sharing is easily addressed by NAS. To add more storage capacity to such a configuration, a storage administrator would simply need to plug another NAS device into the IP network. Of course, if for the new NAS device to be completely integrated with other NAS devices in a network ”for example, where the NAS devices could pool storage capacity ”purchasing the unit from the same vendor becomes a requirement.

Figure 2-8 outlines a basic NAS architecture. As shown, application servers access NAS filers over the IP network, providing greater scalability and crossplatform access than DAS provides. However, NAS doesn't alleviate LAN congestion, and the expansion of block-oriented storage remains disruptive. Additionally, each filer owns the storage that it maintains (except in some homogeneous, pooled NAS environments), and in general, NAS has a unique OS that must be maintained across all filers for ease of management. Finally, the high level of file abstraction inherent to NAS design doesn't suit certain application requirements accustomed to block-level storage.

The gap between high-performance DAS and flexible NAS became the sweet spot for SANs. Traditionally, block-oriented SANs solved a combination of problems between DAS and NAS with Fibre Channel networks. More recently, SANs have been able to fill this gap with block-oriented IP SANs, using IP storage protocols such as iSCSI. The following sections cover the details of both Fibre Channel and IP SAN infrastructure.