IP SANs and iSCSI

IP SANs use the Internet Protocol (IP), discussed in Chapter 24, "Overview of the TCP/IP Protocol Suite," instead of Fibre Channel frames to transmit data on a SAN. The difference between an IP SAN and NAS is pretty much the same as the difference between a Fibre Channel SAN and NAS. An IP SAN, like a Fibre Channel SAN, is placed on a separate network from the LAN that connects clients to the network. By offloading the SAN network traffic from the LAN, an IP SAN is similar in topology to a Fibre Channel SAN.

Tip

Don't confuse an IP SAN with tunneling Fibre Channel frames through an IP network, which was discussed earlier in this chapter. An IP SAN doesn't tunnel Fibre Channel frames. Instead, the IP frame is used in place of a Fibre Channel frame.

IP SANs typically use iSCSI network storage devices and host bus adapters. While an iSCSI host bus adapter resembles a standard network adapter, it differs by transmitting and receiving data in blocks, rather than in packets as with standard network adapters. Figure 11.6 illustrates a typical iSCSI host bus adapter.

Figure 11.6. A typical iSCSI HBA. It uses onboard memory and a processor to enable block-based transfers to and from the network.

IP Storage is an increasingly important part of SAN technology.

There are several important reasons for this:

• The differences in speed among the differing technologies, from Fibre Channel's newest incarnation to Gigabit Ethernet and 10Gigabit Ethernet, make this a limiting factor that doesn't matter as much as it used to. Even with frame overhead compared to payload ratio, at these speeds (and soon to come even faster speedsit will never end!), speed will play less of a factor.

• IP is a technology that many network technologists and administrators are already trained on, because similar equipment is used for both types of networks. Standardized Fibre Channel equipment requires a new, challenging networking set of skills. IP SANs require only a few new features to learn, and much less investment in equipment.

• Fibre Channel is coming down in price at a drastic rate as economies of scale have reduced prices a lot. However, even having to train personnel on Fibre Channel technology can be costly. And you need to be sure that discounted hardware will interact with hardware from other vendors. Don't buy older technology because of its price, unless you are sure it's not older proprietary hardware that will be difficult to incorporate into your SAN.

These points apply to medium- to large-scale networks. However, for the very large-scale network, the Fibre Channel network, with its increase in speed and low error rate, is often the choice to be made. Fibre Channel SANs aren't going to go away any time soon. Instead, IP SANs will probably develop to be a solution for an inexpensive SAN, while Fibre Channel SAN technology will continue to improve in speed, management software, and reliability.

Tip

The only downside to Fibre Channel and SAN equipment is that, although the technology has been around for about a decade in proprietary formats, it has been only the past few years that standards have been developed. Thus, if you decide to buy equipment from more than one manufacturer, be sure that it will first be tested with your existing equipment before the actual purchase. And check out your SAN management software to see whether it can be used with equipment from various vendors.

IP SANs typically are based on iSCSI technology. However, it is not necessary to replace FC with IP Storage. By using IP Storage routers between an existing FC SAN and the rest of the network, and IP Storage switches to connect individual iSCSI and FC storage devices, you can use both types of network storage on the same network (see Figure 11.7).

Figure 11.7. Using IP Storage routers and switches to interconnect IP Storage and FC storage on an IP network.