The explosion of the Internet, the accompanying e-business revolution and the expectation that everything data and applications should be available and accessible all the time are driving an unprecedented demand for storage. One only needs to ask data center managers how much new storage capacity they will bring online this year to understand the role of storage as part of the infrastructure of the Internet and e-commerce.
Of course, as demand rises, technological innovation surely follows. As technological innovation accelerates, user confusion over the number of products and technologies available increases as well. The storage market is no exception to this rule.
One of the primary areas of innovation in storage today is in the application of networking technology to storage connectivity. This has given rise to two new, and commonly confused, storage topologies: network attached storage (NAS) and storage area networks (SANs). Many people don't understand the difference between NAS and SAN. Some think of them as competing technologies; journalists occasionally depict a battle between NAS and SAN for storage technology hegemony, while investment analysts muse over whether NAS vendors or SAN providers are a better investment choice. Users often ask which they should choose as their storage architecture.
The truth is a little less dramatic, but for IT managers even more exciting. Network attached storage and storage area networks are both the products of the coming together of storage and networking technologies. But far from being competitive, they are, in fact, complementary technologies that productively co-exist in many data centers and are even starting, in some ways, to converge. Taken together, they represent the future of storage: storage networking.
A storage area network is, quite simply, a network dedicated to storage. More precisely, the technical dictionary published by the Storage Networking Industry Association (SNIA)1 defines a storage area network as:
A network whose primary purpose is the transfer of data between computer systems and storage elements and among storage elements. Abbreviated SAN. A SAN consists of a communication infrastructure, which provides physical connections, and a management layer, which organizes the connections, storage elements and computer systems so that data transfer is secure and robust.
Unlike the traditional direct attach storage model, a SAN attaches storage devices to servers in a networked fashion, using hubs, switches, routers and bridges to build the topology (see Figure 1). Both the systems and the storage devices can, in theory, be heterogeneous in nature, though today interoperability concerns limit some customers to building homogeneous SANs. Although the network could conceivably be built with any networking technology, Fibre Channel has emerged as the technology of choice for SANs.
Figure 1: Topology of a SAN
SANs provide a number of advantages over direct attached storage. They provide any-to-any connectivity between servers and storage devices, making possible the sharing of storage resources between multiple servers and thus enabling IT managers to consolidate storage on a few large storage platforms. They also provide any-to-any connectivity between the storage devices themselves, opening the way for direct movement of data between storage devices, vastly improving efficiency of data movement and processes such as data backup or replication. The use of Fibre Channel, or most any other networking technology proposed for SANs, enables longer connectivity distances and higher performance than currently possible with SCSI technology. Over time, SAN technology will ease the task of centralized storage management and drive the adoption of remote management and data protection strategies, storage consolidation, system clustering and cross-platform data sharing.
The SAN market is made up of the vendors of Fibre Channel interconnect technology, as well as the vendors of the systems and storage devices that attach to the network. The Fibre Channel vendors are primarily new, relatively small companies such as Brocade, Vixel, Gadzoox and Crossroads. The storage companies are the same ones that have been providing direct-attach storage for years, such as EMC, Hitachi, Sun, HP and Compaq; and it is no exaggeration to say that every storage company is involved in the SAN market.
Network attached storage, on the other hand, describes file storage attached to a network. The SNIA Technical Dictionary defines network attached storage as:
- A term used to refer to storage elements that connect to a network and provide file access services to computer systems. Abbreviated NAS. A NAS storage element consists of an engine, which implements the file services, and one or more devices on which data is stored. NAS elements may be attached to any type of network. When attached to SANs, NAS elements may be considered to be members of the SAS (SAN attached storage) class of storage elements.
- A class of systems that provide file services to host computers. A host system that uses network attached storage uses file system device driver to access data using file access protocols such as NFS or CIFS. NAS systems interpret these commands and perform the internal file and device I/O operations necessary to execute them.
Note that the SNIA definition says that a NAS system may be connected to any type of network. This is an important future consideration, which will be discussed shortly. Today, however, NAS systems are generally connected to a local area network (LAN).
In common usage, a NAS system is a special- purpose device that is designed to serve files to clients over a LAN (see Figure 2). The clients request access to files using standard network file system (NFS) or common Internet file system (CIFS) commands. NAS devices typically contain embedded processors hosting a specialized operating system, or microkernel, and a highly optimized file system both designed to enable the NAS device to serve up files to clients with very high performance. Because they can serve multiple heterogeneous clients, NAS devices provide a form of heterogeneous data sharing.
Figure 2: NAS system serves files over a LAN.
Although the attributes of specific NAS products vary, NAS vendors generally attempt to adhere to the "appliance" model of computing. That is, NAS devices are designed to do one thing file serving and to do it very well. Moreover, they are typically designed to be very simple to install and configure. The storage they provide is often housed within the device's enclosure, though some NAS devices allow for the attachment of external storage.
The NAS market was pioneered by companies such as Network Appliance and Auspex, which provide NAS systems for workgroup and enterprise customers. As the NAS market has grown, new vendors such as Connex and CDS, are attempting to stake out niches in the mid-range and low end, while system and storage vendors such as HP, Sun and EMC have also entered the market.
Can SAN and NAS Co-Exist?
SAN describes a networked storage topology and NAS describes a highly optimized network file server. The questions asked by the IT managers, then, typically come down to some variation of the following:
- Can SAN and NAS be used together or must I choose to base my infrastructure on one or the other?
- When do I choose which technology?
The first question arises because, just as NAS provides high-performance shared access to (file system) data, one of the promises of SAN is also to provide high-performance storage and data sharing. The good news is that the choice between SAN and NAS is not an either/or decision. SAN topologies and NAS devices do, in fact, peacefully co- exist in many data centers. For example, a SAN in the data center may network database and application servers with a number of large storage devices on which their data resides, while one or more NAS devices are attached to the LAN providing file access to clients (see Figure 3).
Figure 3: SAN and NAS can co-exist.
The choice of which technology to use is driven mainly by the requirement being addressed and partly by timing. If the requirement is to provide shared file access to a number of clients, NAS is generally the answer. NAS devices meet this need today with great efficiency. Because NAS systems are built on existing LAN and file system protocols, NAS technology is relatively mature in comparison with SANs. While a few SAN file sharing solutions exist, they are generally aimed at specialized markets such as video editing. Generalized SAN file sharing solutions will probably require a distributed SAN file system, which could be years away from appearing and maturing.
On the other hand, many IT managers are grappling with the need to consolidate data used by large databases or applications such as Microsoft Exchange onto a small number of shared storage platforms to improve centralized management. Or, they want to take advantage of device-to-device data movement for applications such as backup or data replication. In this case, SAN topologies can provide unique capabilities to address these requirements.
Will SAN and NAS Converge?
While SAN and NAS today are similar, but distinct, technologies, over time the lines between them are likely to blur. In fact, this process is already beginning.
This technology convergence will likely take two forms. The first, which is already underway, is the use by NAS systems of SAN infrastructures for their back-end storage. It was previously noted that while the storage capacity of many NAS systems is contained within the NAS device's enclosure, some NAS devices allow for the attachment of external storage. In fact, many NAS systems now have Fibre Channel ports, which allow them to connect into a SAN and enable the NAS file system to reside on a SAN device.
In many respects, this gives the IT administrator the best of both worlds. Clients requiring file access still get the performance benefit of a highly optimized file server. The IT manager can take advantage of the efficiencies of storage consolidation by placing the NAS file system on a shared SAN storage device. And the IT staff benefits from the plug-and-play features of NAS setup and administration.
The second avenue of potential technology convergence is somewhat more speculative. As noted previously, the SNIA definition of NAS specifically allows for a NAS device to be connected to any type of network including a SAN. For this to be meaningful, the SAN would have to be capable of carrying file traffic in addition to the block protocols (i.e., SCSI) that it typically carries today.
This is, in fact, possible. Fibre Channel, for instance, is capable of carrying both SCSI and IP traffic simultaneously. This capability is occasionally exploited today, but mainly for the transmission of management commands to a device via IP. It is relatively rare for clients to use Fibre Channel as the interconnect for accessing file servers. While theoretically possible, few people advocate the use of Fibre Channel as a generalized messaging network technology.
We have seen that SAN and NAS technologies can offer the IT manager distinct and complementary capabilities. SAN topologies offer the ability to consolidate storage and improve data protection and storage management processes with a dedicated, high-performance storage network. NAS systems offer high-performance, low-administration file serving and file sharing for heterogeneous systems. Used together, they provide a potent one-two punch for addressing data center requirements. The fact that the two technologies can co-exist and work together means that investments in either or both will pay off well into the future. SAN and NAS are the reason that the future of storage is networked.
- The definitions of SAN and NAS used in this article are drawn from the SNIA Technical Dictionary, ©2000 Storage Networking Industry Association. This excellent reference work contains definitions of nearly 1,000 storage and storage networking terms and can be found on the SNIA Web site at www.snia.org.
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