Network Attached Storage

As an alternative to SANs, especially for smaller networks, consider Network Attached Storage (NAS). NAS technology is now available for all levels of networks, including SOHO networks.

NAS has some advantages over SAN technology. Consequently, in some situations it may be a better solution for your network storage needs:

• Your network traffic on the LAN is minimal and the impact of one or more NAS servers will not affect performance greatly. An example of this is using NAS to store data that is not used by the network clients frequently, such as reference material, software installation kits, or archived files.

• Your budget doesn't allow for the purchase of an expensive SAN. Some NAS devices sell for a few thousand dollars. Some SANs devices sell for a few hundred thousand dollars (and don't forget the maintenance costs), and some are over the million mark. You can always upgrade your NAS devices to newer technology in the future when your business can afford it. SANs are definitely not for the home or SOHO market.

• Your need for storage isn't as great as that which a SAN can provide. Smaller additions to storage can be done with NAS devices that can be shared among more than one server. If you need only a small amount of space, NAS devices might be cheaper to use. For larger storage needs (in the terabyte range) you'll probably do better with a SAN.

As you can see, NAS can be ideal for the small office environment, for a small departmental deployment that needs a large amount of storage, or even for home users with large storage demands.

Network Appliances

Many NAS solutions come as a packaged product. The "network appliance" will have its own operating system and network connection, and in many instances is pretty much a plug-and-play type of operation. Just connect it to the network and make some minor configuration tweaks, and you've got storage on the network that you can restrict to specific servers or other computers.

Note

Network NAS devices can be complex devices. They do not just offer simple file shares for the disk storage they provide. Instead, using management software for these devices, you can configure RAID sets, such as mirrored disks or stripe sets, that can be used to increase up-time by enabling a disk subsystem that can tolerate the loss of one or more disk drives. These functions are also performed by SANs. The point is that you don't have to pay more to obtain this level of data integrity. For more information, RAID is discussed later in this chapter.

You'll find many vendors of NAS, from HP to IBM to many other smaller firms. Some SOHO network hardware vendors, such as Linksys, also offer NAS devices especially designed for SOHO networks. Check the specs first to be sure that the device you want to buy is one that will work on your network. For example, consider whether the device is expandable, or whether you'll have to buy additional NAS devices in the future to keep up with storage demands. You may want to make sure that the device has upgradable firmware to address future features and devices (such as newer tape drives). And you should be sure that any NAS device you purchase can be upgraded to incorporate additional storage so that you do not have to go through the process of adding yet more separate NAS devices to accommodate future needs. Except for the smallest network, a NAS device should provide an upgrade path to attach additional storage capabilities, be it disk or tape backup hardware.

Tip

Some NAS devices come with backup software that allows you to back up the NAS data directly to tape drives attached to the NAS device. This approach alleviates copying data across the network to a backup device and thus using a significant amount of the network bandwidth.

NAS Protocols

Most NAS devices support the major players in the file protocols just as they do the major disk filesystem formats. Most NAS devices will support NetWare's now-legacy IPX/SPX protocols, Microsoft's NetBEUI, and Sun's Network File System (NFS). The SMB (now CIFSCommon Internet File System) protocol/name service is becoming increasingly popular, despite its age. The Storage Networking Industry Association has just published a document titled Common Internet File System (CIFS) Technical Reference, Revision: 1.0. You can download this from www.snia.org. Just use the search text "CIFS" and you should find this 150-pluspage PDF document!

The fact that SNIA (which is a trade association that supports both NAS and SAN) has published one of the few volumes that consolidate a lot of diverse CIFS information is indicative of the fact that CIFSthe overhauled Microsoft SMBwill still be a major player in networking protocols in the near future. One advantage of CIFS devices on a Windows network is that a CIFS device is listed in My Network Places (Network Neighborhood), making it easy to use.

NAS Capacity LimitationsBandwidth and Storage

The major trade-offs between SANs and NAS are storage capacity, response time, and the "backup window." Although it is quite possible to attach terabytes and terabytes of NAS storage to your LAN, there will come a time long before that when NAS begins to impede normal network response time for end users. Your network utilization will go through the roof and your NAS investment will give you no definite advantage. This assumes that the NAS devices are used by a large number of clients. If you have distributed data to the departmental level, and there is little interaction between those data stores and clients in other departments, then this may not impact your LAN performance.

For larger installations that tend to use higher-end storage solutions, the bandwidth factor divides NAS and SAN technologies. But this is just one of a few considerations you should think about before employing either in a production network.

If you have a need for a SAN to enable fast transfers of large amounts of data, it may be because you have a large number of client workstations to service with a high volume of file requests from a large data bank. Other situations in which you might use a SAN, when fast access to data is required, is workstations used in video and other mastering environments where speed is of the utmost importance. What would take hours or days to compute just five years ago would take only a few minutes today with modern processors. With the capability to process more data faster, workstations must have fast access to storage to prevent storage from becoming a bottleneck.

Or perhaps your environment is an industrial one that requires constant access to inventory, accounting, and other traditional computer resources, as well as manufacturing resource planning (MRP) software to help keep costs in control. Even instruments or computers used on the factory floor for data entry can generate voluminous amounts of information. Is a NAS sufficient? Or should you consider a SANs solution?

The other major reason that SANs are used in large network environments revolves around the backup window mentioned earlier. Only the more expensive NAS devices will be able to perform timely and efficient backups of data stored on their disks. Compare this to the capability of a large SAN device. Using both proprietary techniques and standards such as RAID technology, SAN devices can be enabled to take snapshots of their file systems and commit them to backup devices without involving the CPU of the computer systems that make use of SANs. The file server can continue offering resources to clients while a backup is being performed.

One of the techniques of this type of backup is to simply create mirror sets with three or more members and then break the mirror by removing a disk. Similar to a snapshot backup, a disk from a mirror set (all three disks contain the same data) is taken out of the mirror set and used for the backup. The remaining members of the mirror set provide redundancy (in case one of those disks fails!), so users can continue working. When the backup is completed, the third disk that was taken out of the mirror set is brought back in, and everything it "missed" while users had access to the others is updated so that within a short time all three disks are again complete copies of each other.

Another method of creating a snapshot of data is by using checkpoints in file systems so that the backup can proceed with copying data from a disk, and the operating system can keep track of changes that need to be applied to the disk (from the checkpoint onward) after the backup has completed.

In the next section you'll learn about the basics of a SAN, along with more differences between SAN and NAS.