Fibre Channel SANs for Graphics Applications
Storage area networks can solve a lot of problems in graphics-intensive environments, but "server SANs" and "client SANs" have different requirements.
By Doug Gowell
Local area networks play a vital role in providing "instant" access to data anywhere, anytime. Yet while today`s LANs are ubiquitous and adequate for most tasks, they do not provide instant access to all data, especially when data is very large and needs to be moved numerous times.
Trying to push more data through too small a pipe will not meet the needs of these environments. The concept of a storage area network (SAN) is one solution. The SAN architecture typically uses Fibre Channel-Arbitrated Loop (FC-AL) technology to provide a direct, high-speed connection between storage systems and servers in a network environment.
With an important modification, the SAN concept can be applied to real advantage in the printing and publishing industry, where time is measured in dollars and delays due to inadequate network throughput are unacceptable. The key challenges facing this industry are those issues that drive competitive advantage: lower costs, increased productivity, and shorter turnaround time. Within electronic prepress, these issues translate into how to store, access, and manage the massive growth of digital data.
In these environments, the SAN concept must be taken one step further by combining high-capacity file servers with FC-AL to provide direct, high-speed connections between data storage and workstations used for prepress applications such as image retouching and raster image processing. This approach provides a powerful data storage and access system that delivers the kind of performance required to support large file, media-rich applications--thereby improving competitive advantage.
While LAN performance has improved significantly over the last 20 years, the increase in throughput is more on the order of 10 to 20 times, compared to the million-fold increase in computing speed. The most common LAN today is still 10baseT Ethernet, which typically delivers 300KBps. 100baseT Ethernet, which is now common in the printing and publishing industry, delivers around 1MBps in a typical installation, and users brag about achieving 2MBps using special software on well-tuned networks.
In today`s client/server networks, storage is normally not connected directly to the devices that need the data, but to a general-purpose network server running a general-purpose operating system such as NT or UNIX (see Fig. 1). These servers are connected in turn to the computers that need the data, using standard LANs. This indirect access over a limited size pipe (the LAN) is the root of the performance problem. The ability to move huge amounts of data quickly between storage systems and devices, servers, and computers is also needed in other industries.
At least in part, the solution lies in recognizing that some users on the network often need direct high-speed access to large files. For those that do, getting them "closer" to the data by minimizing overhead and maximizing the amount of available bandwidth are key.
There are two parts to an effective solution. The first part is getting the storage closer to the systems that need it. From this comes the idea of network-attached storage (NAS). The second part is the concept of a storage area network (SAN), which provides a very fast link, separate from the LAN, to connect NAS to servers and devices requiring fast throughput.
NAS includes shared RAID arrays and dedicated file server systems. Shared RAID arrays put disk drives or RAID systems directly on the network, leaving the management of the data to the various server and workstation devices that use the data. The shared RAID approach provides fast access to raw data and is very useful in read-only environments, or where several servers are mirrored for redundancy purposes. However, because data is not managed at the file level in shared RAID arrays, they have limited use in electronic prepress and production environments.
Dedicated file servers are devices that specialize in fast file serving and are typically comprised of RAID storage and a "thin server" specifically designed for file serving. Because they are not part of a general-purpose computing environment, they can be highly optimized to serve files to the network very quickly. Workstations located on the network can then access and share data that is located on these file servers, but performance is still limited to LAN speeds.
Storage Area Networks
SANs provide a separate high-speed link to connect NAS to the servers and devices that require fast throughput, while leaving normal network traffic on the LAN. This approach not only addresses the problem of moving large amounts of data quickly to where it is needed, but also relieves the LAN from this task, greatly improving the availability of the LAN for the traffic it was designed to handle.
The storage industry is rapidly developing a solution for this fast network link. In the next few years, SANs will be widely deployed in environments where direct high-speed access to data is required. In most situations, "access" means between the data storage devices and servers. In electronic prepress, this requirement extends to a number of other devices, such as graphics workstations, application servers, and raster image processors.
FC-AL is the standard being adopted in the industry to support SANs. FC-AL runs at 1Gbps and is ideally suited for prepress applications. It can operate over reasonably long distances and can run on either copper or fiber-optic cable.
The SAN concept has its roots in mainframe system architectures. For many years, mainframe computers used dedicated "back-end" networks to attach storage devices to multiple mainframes. Unlike these mainframe architectures, where the storage network was hidden and could be completely proprietary, SANs must work in the world of open systems. Late last year, The Storage Networking Industry Association (www.snia.org) was formed in an industry-wide effort to develop standards for SAN systems.
In most client/server environments, SANs can provide a high-speed link between disks and the server portion of the client/server environment because servers are typically the only machines that need very high-speed file access. The processed data is then distributed to the clients over a standard LAN. For purposes of this discussion, these "back-end" networks are labeled "server SANs".
However, in certain industries and applications, high-end workstations and application servers must process and interact directly with large amounts of data. These devices need to be directly attached to the SAN. This environment, which we refer to as a "client SAN," demands a different set of requirements than server SANs.
Requirements for Client SANs
Requirements for deploying a client SAN are application- and industry-dependent. Read-only devices have limited value in environments in which data is modified. For electronic prepress environments, a client SAN must meet three additional requirements (other than speed):
- Files must be properly managed (i.e., read/write, sharable, and secure) over the SAN.
- The SAN must work with existing applications.
- The SAN must work in a heterogeneous platform environment.
The storage must be a read/write device. All authorized users must be able to read and write files to and from the storage system without restriction. Raw disks or RAID systems attached to the network without any file management capability are of little value in this environment. Even when these systems meet all other requirements, the inability to share files and areas of the storage system for both reading and writing is unacceptable.
Another requirement is applications compatibility. Printing and publishing firms have a large investment in software and training in their existing applications. These applications, whether commercial or custom, must work with the SAN file server. Any system that does not work with existing applications is of limited value. And any system that requires end-users to take special steps or to learn new procedures is also of very limited value.
Yet another requirement is support for heterogeneous environments: high-end graphic workstations, which are often Macintosh systems, and application servers, which tend to be Windows NT systems. A complete solution must provide storage services for this heterogeneous environment in a transparent fashion. To Mac users it should act like a Mac. To NT users it should act like NT. And all files must be shareable.
Finally, regarding performance, "specsmanship" has always been a widely played game in the computer industry. Raw numbers that can never be achieved in the real world are frequently used to promote products to unsuspecting users. Fibre Channel as used in SAN architectures is no exception.
It`s true that typical SAN installations today use 1Gbps Fibre Channel, which translates into a raw speed of 100MBps burst data transfer rate (it takes 10 bits to send an 8-bit byte due to encoding techniques). But this is not the end-to-end data transfer rate. Users need to be wary of benchmarks that claim such numbers. All that really matters is how fast your applications can access the data. Make sure the quoted numbers reflect end-to-end file transfer and then test them in your environment.
In sum, LANs are a fundamental part of modern computer facilities, but they do not provide sufficient bandwidth for handling very large files and high-throughput requirements for electronic prepress, medical imaging, and GIS environments. For these media-rich environments, solutions using Fibre Channel in a "client SAN" architecture are required. By using standard FC-AL interfaces and special software for managing Macintosh and Windows NT file access, devices like scanners, high-end image retouch workstations, and application servers can obtain immediate access to the large images and data files they need.
Fig. 1: Standard LAN environments use storage that is connected to general- purpose network servers.
Fig. 2: In a client/server environment, a "server SAN" provides a high-speed link between storage devices and servers.
Fig. 3: A "client SAN" provides a high-speed link between storage and workstations. The LAN functions as a low-volume client-to-client link.
Fig. 4: Environments like electronic prepress require a "client SAN" so devices like scanners, high-end image retouch workstations, and application servers can obtain immediate access to the large image and data files they need.
Doug Gowell is director of marketing at Augment Systems Inc., in Westford, MA.