The conventional wisdom is that network-attached storage is easy to set up but slow, and a storage area network is difficult but fast. New storage technologies merge these models into a lower-cost Internet Protocol-based architecture; real products, however, are a year or two away.
Storage Area Network
A SAN is a private, high-speed network that sits outside the local area network. Since it does not affect normal corporate traffic, the approach is well-suited for moving large amounts of data. SANs are too expensive to make connecting large numbers of systems to storage devices appealing, however, and because vendors vary on the specifics of implementation, management can be difficult.
Network Attached Storage
NAS servers connect directly to the corporate backbone—easy and inexpensive to set up, but bad for network traffic. NAS servers run a very thin, proprietary operating system optimized for network file access. Though growing rapidly in popularity for business and Internet applications, NAS lacks the performance and quality-of-service guarantees required for high-end data-center applications.
Companies managing multiple storage architectures can achieve faux convergence through "virtualization," which puts an open-protocol access point in front of storage networks and systems. Some virtualization devices use iSCSI (below), while others use NAS protocols; all simplify management by reducing the number of interfaces.
The technologies that connect systems and devices have improved greatly in the past five years. Emerging IP versions increase interoperability and reach.
The American National Standards Institute (ANSI) combined several standards it had built in the mid-90s into the Fibre Channel (FC) spec. FC—the de facto foundation of SANs—can transfer data at 100 MBps to 4 Gbps, has wide vendor support and can reach up to six miles over optical fiber. FC lets you set quality-of-service levels. But its expensive—$900 per port, on average—and compatibility among vendors is a problem.
In 2001, the Internet Engineering Task Force (IETF) upgraded the aging SCSI standard to iSCSI, which puts commands within IP packets. Used with Gigabit Ethernet (below), it competes with FC on speed and distance. Its also relatively inexpensive to buy and, being based on well-known standards, easy to manage. But vendor support isnt wide, and implementations vary.
The Institute of Electrical and Electronics Engineers (IEEE) finished work in 1998 on a speedier version of the popular Ethernet protocol. Gigabit Ethernet is better-standardized and less expensive than FC but doesnt support quality of service. And its reputation for being superfast is relative: To reach speeds of 80 MBps to 90 MBps, it requires a "jumbo frame" mode that not all hardware supports.
IBM, Intel, Microsoft and others merged competing proposals for next-generation I/O to create InfiniBand, which uses virtual direct connections between devices and processors (instead of a PCI bus, say) to move data at 500 MBps to 6 Gbps. The spec supports 128-bit IP addressing, quality-of-service controls and distances of just over half a mile. InfiniBand 1.0 was released in 2001—whether it performs as promised wont be seen until hardware ships in earnest.
Fibre channel and IP
The IETFs development work includes tunneling FC packets over IP networks (FC/IP); connecting FC gateways over IP (iFCP), and running IP packets over FC.
10 Gigabit Ethernet
By Q2, the IEEE hopes to complete this standard, which would make Ethernet as fast as—or faster than—FC.
The growing popularity of the NAS architecture is driving the search for faster, IP-based protocols.
- Network File System Developed by Sun Microsystems, NFS is the standard for Unix and NAS file-sharing. Its cross platform, works with nearly every application and is deeply rooted in the industry. But the aging technology has a lot of overhead and is not optimized for high-volume I/O. In addition, NFS offers little in the way of security—developers must turn to Kerberos and other authentication systems. Sun developed a version for the Internet, WebNFS, in which few vendors have shown interest.
- Server Message Block Protocol SMB has gone through many iterations since Microsoft first developed it as part of Microsoft LAN Manager and Windows NT. Its currently the standard for NAS vendors for file-sharing with Windows workstations; an open-source version, called Samba, is available for Unix and is included in most Linux distributions. SMB is faster than NFS and, as a standard part of Microsoft operating systems since Windows 95, is almost as entrenched. But its not integrated with directory structures, and using it in multiplatform environments requires that passwords are plain text. Microsoft has proposed an open-standard version, the Common Internet File System (CIFS), as a replacement for FTP.
- Direct Access File System The Storage Networking Industry Association wants to replace NFS and CIFS with DAFS, a faster file-sharing protocol for application servers. DAFS is based on the virtual interface (VI) architecture, a standard for linking together server clusters promoted by Microsoft, Intel and Compaq. VI connects systems so that they can directly access each others memory and storage without the intervention of an operating system. DAFS is designed to work over InfiniBand, Gigabit Ethernet and Fibre Channel with little or no overhead.
Background Reading The Storage Networking Industry Association is a leading proponent of SAN-NAS convergence.