This week, we compare Network Attached Storage (NAS) and Storage Area Networks (SAN), two storage technologies that are far from similar, despite the confusing similarity of their acronyms.

by Alex Goldman ISP-Planet Associate Editor [August 8, 2002]

In previous articles, we've reviewed the pros and cons of direct attached storage (that is, the various hard drives in all of the computers in your organization). We've also looked at Network Attached Storage (NAS), a massive box, streamlined for serving files to multiple computers.

However, few vendors today are trying to sell NAS except to specialist markets. For example, a small firm might buy a NAS system capable of holding all of the files everyone in the company uses. Then again, design and professional teams that regularly share files—such as those using Computer Aided Design (CAD) software on collaborative design projects, or teams writing software code—might use a sophisticated file server to share files and track permissions so that only one person at a time can make changes. Auspex, a company that makes high-end storage equipment, is finding some demand among small businesses for a device it calls the "brat," a 22 U cabinet that holds up to 4 TB of storage and is mounted on wheels.

As far as ISPs are concerned, the collaboration and file-sharing benefits of NAS are of little interest. But the hot, new storage technology known as Storage Area Networks (SANs) is just what the ISP network doctor ordered.

Whether it's a billing application or a mail server, an ISP's operations tend to depend on one or more critical databases. By their nature, databases do not require the sophisticated file management systems offered by NAS systems. Many ISPs are content to run them on off-the-shelf DAS computers, but—especially for companies experiencing rapid growth or fluctuating use of mail—the Storage Area Network offers some distinct advantages.

Infinite bit bucket
A Storage Area Network allows a server or cluster of servers to use off-the-shelf JBOD or RAID storage to achieve virtually unlimited scalability. At the center of the SAN sits a switch, or an intelligent group of switches known as a SAN fabric, that is responsible for making sure that no two machines access any one block or device simultaneously.

This gives databases, which are mission-critical applications for ISPs and most other online businesses, access to inexhaustable storage resources. The list of database contents, known as the "index," remains on the server's DAS storage, but the actual data elements are stored remotely. The connection, either fiber or Ethernet, is so fast that there is no perceivable performance penalty for storing the data remotely.

In practice, there are some downsides. The biggest problem for small and medium-size businesses is that SANs are seen as difficult to manage, costly to buy, and slow to build. According to Bob Iacono, vice president of marketing for Auspex, it can take just a few hours to add NAS to a network, while building a SAN could take days or longer.

Then again, each storage box requires a Host Bus Adapter (HBA) to connect to a fibre channel SAN, or a network interface card (NIC) to attach to an Ethernet SAN. These can be expensive, and products from different manufacturers will not necessarily work well together. To fight this problem, manufacturers are collaborating to certify specific products and groups of products as interoperable, but many analysts still recommend buying from one vendor to avoid compatibility issues. Even when they work well, HBAs and NICs add to network costs.

Another issue is that the SAN can be more flexible than the servers and software it supports. The SAN can adjust the size of the storage resources available to a database, but some databases can't handle the amount of storage a SAN can make available. Others can handle massive storage, but were not designed to deal with changing disk sizes. When a SAN gets really big, it is possible to provide more storage than an operating system can handle. In either case, the result can be a total crash.

Finally, administrators are more likely to make errors when setting up new technology. Among numerous stories we've heard (none of which were the manufacturer's fault) are instances of databases trying to write gigabytes of data to the same storage block, due to configuration errror. Such tales are not surprising, given that the industry is young and the products are new.

SAN souci
The problems with SANs are real enough, but ISPs should find the rewards compelling.

Balaji Baktha, vice president of marketing for Adaptec's storage networking group business unit, sums up the SAN's many advantages: "You have detached the servers from the hard disk drives, so you can scale storage and servers independently. Users can reach storage through multiple servers. You can add disk space without bringing down the LAN, and you can up the number of servers too. The most advantageous aspect of SAN is the network behind the server. The activity on the storage network, such as daily or up-to-the-second backups, does not impact the Ethernet LAN."

At its core, the SAN deals with data blocks common to every operating system, and uses only the simplest of commands (NAS, remember, deals with files). Dealing with blocks makes the system simpler, and theoretically more secure, but also makes it impossible to share data in a group.

Nelson Nahum, chief technology officer of StoreAge, says, "the block level is the basic unit of every operating system. The commands are simple, like read and write. You cannot lock a file at the block level or share data. If you wish to do so, you need to add servers that will ensure that no computer writes to a block that another computer is trying to read."

Once users accept those limits and build networks around those limitations, the rewards, especially for critical database applications, are tremendous.

Stephen Beer, director of product marketing for Brocade, which dominates the SAN switch market, concludes, "with a SAN, you get over scalability thresholds. You've got one repository of storage, not a variety of servers, and the result is an environment that is easier to administer and back up."

All in all, ISPs should find investigating SANs a worthwhile exercise, even if they choose not to implement one immediately.

Next week, we'll look at technologies that could make storage networks (both NAS and SAN) available to more businesses, and also extend the reach of storage networks.

—End