Storage Area Networks

As companies rely more and more on e-commerce, online transaction processing, and databases, the amount of information that needs to be managed and stored can intimidate even the most seasoned of network managers.

While servers do a good job of storing data, their capacity is limited, and they can become a bottleneck if too many users try to access the same information. Instead, most companies rely on peripheral storage devices such as tape libraries, RAID disks, and even optical storage systems. These storage devices are effective for backing up data online and storing large amounts of information.

But as server farms increase in size , and as companies rely more heavily on data- intensive applications such as multimedia, the traditional storage model isn't quite as useful. This is because access to these peripheral devices can be slow, and it might not always be possible for every user to easily and transparently access each storage device.

Recently, a number of vendors from all walks of the industry have been pushing a concept called the Storage Area Network (SAN). SANs provide more options for network storage, including much faster access than Network Attached Storage (NAS) and the flexibility to create separate networks to handle large volumes of data.

Storage Space

Before going into a detailed discussion of SANs, I'll examine how other methods of adding storage to the network work, and why the need arose to develop something beyond them.

The most basic way of getting storage devices on the network is to hang disk arrays or other storage devices off of servers, using an interface such as the Small Computer Systems Interface (SCSI) to make the connection. SCSI is a relatively high-speed interface that was developed more than 15 years ago.

This popular I/O interface connects not just storage- related devices such as tape and optical drives but also printers, scanners , and external drives.

SCSI has gone through a number of changes over the years, especially the speed the interface supports. Initially designed to handle speeds of 5Mbytes/sec, it supports a throughput rate of 160Mbytes/sec in its current iteration, Ultra3 SCSI. Within the Ultra3 specification is a subset implementation, Ultra160 SCSI, that's gaining popularity among SCSI vendors as well as server and workstation manufacturers.

While SCSI has been a workhorse over the years for connecting peripherals at a relatively fast speed, distance limitations have kept this particular interface from evolving very rapidly . The SCSI standards put a bus length limit of about 6 meters on devices. While this distance limitation doesn't really affect connecting storage devices directly to a server, it does severely restrict placing RAID and tape libraries at other points on the network.

This is where the concept of NAS comes in. NAS is straightforward in that disk arrays and other storage devices connect to the network through a traditional LAN interface such as Ethernet (or whatever the topology of choice might be). Storage devices would attach to network hubs much the same as servers and other network devices (see Figure 1).

Figure 1: In a Network Attached Storage (NAS) scenario, storage devices such as RAID and tape drives are part of the LAN, making them accessible to any other network resource. Drawbacks of this model include bandwidth limitations, because NAS implementations are dependent on the underlying network topology.

NAS makes storage resources more readily available and helps alleviate the bottlenecks commonly associated with access to storage devices. However, NAS does have a few drawbacks.

First, network bandwidth places throughput limitations on the storage devices. Most NAS servers are placed on 10Mbit/sec or 100Mbit/sec Ethernet LANs, but even if a network is running at gigabit speeds, most NAS vendors today only offer interfaces up to Fast Ethernet.

Another downside to NAS is the lack of cohesion among storage devices. While disk arrays and tape drives are on the LAN, managing the devices can prove challenging since they are separate entities and are not logically tied together.

NAS has its place as a viable storage architecture, but larger enterprises need something a few steps beyond.

SANs Arrive On The Scene

SCSI-based storage and NAS-based configurations are both important ways of bringing storage to the network, but they are best utilized in situations where there is a relatively low volume of data traversing the links.

Large enterprises that want the ability to store and manage large amounts of information in a high-performance environment now have another option: the SAN.

In a SAN environment, storage devices such as DLTs and RAID arrays are connected to many kinds of servers via a high-speed interconnection, such as Fibre Channel. This setup allows for any-to-any communication among all devices on the SAN. It also provides alternative paths from server to storage device. In other words, if a particular server is slow or completely unavailable, another server on the SAN can provide access to the storage device.

A SAN also makes it possible to mirror data, making multiple copies available.

The high-speed interconnection that links servers and storage devices essentially creates a separate, external network that's connected to the LAN but acts as an independent network.

There are a number of advantages to SANs and the separate environments they create within a network. SANs allow for the addition of bandwidth without burdening the main LAN. SANs also make it easier to conduct online backups without users feeling the bandwidth pinch .

And, when more storage is needed, additional drives do not need to be connected to a specific server; rather, they can simply be added to the storage network and can be accessed from any point.

Another reason SANs are making big waves is that all the devices can be centrally managed. Instead of managing the network on a per-device basis, storage can be managed as a single entity, making it easier to deal with storage networks that could potentially consist of dozens or even hundreds of servers and devices.

Fibre Loops

The interconnection of choice in today's SAN is Fibre Channel, which has been used as an alternative to SCSI in creating high-speed links among network devices.

Fibre Channel was developed by ANSI in the early 1990s, specifically as a means to transfer large amounts of data very fast. Fibre Channel is compatible with SCSI, IP, IEEE 802.2, ATM Adaptation Layer for computer data, and Link Encapsulation, and it can be used over copper cabling or fiber- optic cable.

Currently, Fibre Channel supports data rates of 133Mbytes/sec, 266Mbytes/sec, 532Mbytes/sec, and 1.0625Gbits/sec. A proposal to bump speeds to 4Gbits/sec is on the drawing board. The technology supports distances of up to 10 kilometers, which makes it a good choice for disaster recovery, as storage devices can be placed offsite.

SANs based on Fibre Channel may start out as a group of server systems and storage devices connected by Fibre Channel adapters to a network. As the storage network grows, hubs can be added, and as SANs grow further in size, Fibre Channel switches can be incorporated.

Fibre Channel supports several configurations, including point-to-point and switched topologies. In a SAN environment, the Fibre Channel Arbitrated Loop (FCAL) is used most often to create this external, high-speed storage network, due to its inherent ability to deliver any-to-any connectivity among storage devices and servers (see Figure 2).

Figure 2: With a Storage Area Network (SAN), a high-speed interface such as Fibre Channel connects storage devices to servers and the rest of the network. A Fibre Channel switch or hub lets servers access any storage device on the loop.

An FCAL configuration consists of several components , including servers, storage devices, and a Fibre Channel switch or hub. Another component that might be found in an arbitrated loop is a Fibre Channel-to-SCSI bridge, which allows SCSI-based devices to connect into the Fibre Channel-based storage network. This not only preserves the usefulness of SCSI devices but also does it in such a way that several SCSI devices can connect to a server through a single I/O port on the server. This is accomplished through the use of a Fibre Channel Host Bus Adapter (HBA). The HBA is actually a Fibre Channel port. The Fibre Channel-to-SCSI bridge multiplexes several SCSI devices through one HBA.

The FCAL provides not only a high-speed interconnection among storage devices but also strong reliability. In fact, you can remove several devices from the loop without any interruption to the data flow. Also, packets sent over an FCAL are error-checked, and, if need be, packets can be re-sent if any are lost or corrupted.

Fibre Channel is really what has made SANs a reality, and future developments on the interface will likely bring more features, such as higher bandwidths.

Archiving The Future

While a SAN architecture does appear to be the next step in the evolution of network storage, there are a few points that need to be more adequately addressed before SANs become more widely used.

One crucial piece to running a large SAN is software that administers and controls all devices on the network. While a SAN configuration inherently makes management easier than in the case of NAS, most companies will require a customized application to manage their SAN.

In a relatively small SAN implementation, customized software can be written to ensure communication among all devices. But as SANs grow, and as more vendors enter this space, simply writing management software will not be enough. Rather, there needs to be a standard way for components from different vendors to interact within the context of a SAN.

Vendors in the storage, and specifically the SAN, market have realized this shortcoming. Through vendor-neutral organizations and traditional standards bodies, these issues are being raised and dealt with.

SANs may require a bit more thought and planning than simply adding one storage device to one server, but as companies wrestle with reams and reams of information on their networks, this high-speed alternative that's always available should make wading through the information age a bit easier.

Resources

For tutorials, white papers, and FAQs on Fibre Channel and how it relates to Storage Area Networks (SANs), visit the Fibre Channel Industry Association (FCIA) at www.fibrechannel.com. The FCIA was created in August 1999, when the Fibre Channel Association and Fibre Channel Community merged.

Strategic Research is a market research firm specializing in the storage market. An informative introduction to SANs can be found at www.sresearch.com/wp_9801.htm.

The Storage Networking Industry Alliance (SNIA) is an industry group of more than 75 companies working on storage networking standards for improved management. SNIA's Web site can be reached at www.snia.org.

A group of vendors has created an alliance focusing on the new 160Mbyte/sec version of SCSI. For more information, see www.ultra160-scsi.com.

This tutorial, number 136, written by Anita Karv, was originally published in the November 1999 issue of Network Magazine