SAN and NAS as Storing and Filing Applications

The two major product classes in storage networking are SAN and NAS. Analyzing these technologies within the system of connecting, storing, and filing, they can be easily differentiated by stating the following:

• SAN is the application of storing over a network.

• NAS is the application of filing over a network.

SANs are used to convey storing commands and data over a network, while NAS conveys filing commands.

NOTE

When storage networking was first getting started, there was enormous confusion over the merits of SAN and NAS: Was one inherently superior to the other?, Would one win out in the market in the long run?, and all sorts of other meandering speculations. The whole thing wasted an obnoxious amount of time.

It really bugged me that so many people were spinning their wheels on discussions that would not be resolved due to the lack of a fundamental functional analysis. That's what led me to develop the three-legged stool analysis of connecting, storing, and filing. For the most part, when people understand this analysis, their confusion over SAN and NAS positioning melts away.

In the remainder of the chapter, we'll look at how connecting, storing, and filing work together in storage networking processes.

If there were a "stack" for storage processes, it would look like Figure 2-5, where connecting is the bottom-most layer, storing is in the middle, and filing is at the top-most layer.

Figure 2-5. The Storage Process "Stack"

For people familiar with networking stacks, Figure 2-6 shows the traditional TCP/IP network stack alongside the storage process stack. Notice that most of the interesting parts of the TCP/IP stack are part of the connecting component of the storage process stack.

Figure 2-6. A Comparison of the Storage Process Stack and the TCP/IP Protocol Stack

Now we'll start connecting various products and show their relative function within the storage process stack. Figure 2-7 shows a host bus adapter on the left connected to a disk subsystem on the right. The connecting function is provided by the SCSI bus and the communication function in the HBA and subsystem controller. The storing function is at a higher level in both controllers. In Figure 2-7 both the HBA and the disk subsystem controller have dual roles.

Figure 2-7. Storage and Connecting Processes Between an Interacting HBA and a Disk Subsystem

Figure 2-8 is the exact same figure but with a SAN switch between the HBA and the disk subsystem. Notice the changes in this figure from the last: the SAN switch and SAN cables replace the SCSI connection between the HBA and the subsystem. In Figure 2-8, the switch has only a connecting role.

Figure 2-8. A Storage Switch and Cables Replace the SCSI Bus in Figure 2-7, Making a SAN

Figure 2-9 is the same figure again, but this time it shows the HBA residing in a host system along with application and file system software. Notice that there is now a filing function on the left, but not on the right.

Figure 2-9. An Expansion of Figure 2-8, Adding the Host System and a File System

Figure 2-10 converts the system we have been building with this sequence of figures by turning the system from Figure 2-9 into a NAS appliance system with an integrated storage subsystem. This system also has a network interface card (NIC) for communicating with client systems over a LAN.

Figure 2-10. Enlarging the Figure to Include NAS

The connecting functions in Figure 2-10 are suddenly much more interesting than in the previous figures. Historically, the connecting technology on the LAN (NAS) side of the drawing has been a different technology from the connecting technology on the SAN side of the drawing.

However, as both filing (NAS) and storing (SAN) functions are at the application layer of the underlying connecting network, there is no reason why the two connecting functions (LAN and SAN) could not use the same technology. In fact, they could even be the same physical network, as shown in Figure 2-11.

Figure 2-11. A LAN with NAS Connected to a SAN, All Using the Same Connecting Technology

NOTE

This example is not intended to suggest that putting SANs and LANs on the same physical network is the preferred way of doing things. But it does help drive home the point that connecting, storing, and filing are, in fact, three independent and orthogonal functions.