Building and Using a SAN: Part I

W. Curtis Preston

In the past few months, I've discussed many things about storage area networks. I first talked about what life has become like for those who do not have a SAN, and how a SAN could make things a whole lot better. Then I talked about Fibre Channel, the underlying technology behind SANs (see Figure 1). I followed that with a summary of the different building blocks that are used to build a SAN, which include the elements in the following list:

• Servers -- The servers are what give a SAN its purpose. Each server will connect to the SAN by one or more Fibre Channel connections.

  Host Bus Adaptors (HBAs) -- An HBA is the proper term for what is commonly called a "Fibre Channel card." The Fibre Channel cable connects to the server via the HBA, which resides in the server.

• Hubs -- By connecting a server to a hub, you are connecting it to an arbitrated loop. An arbitrated loop can contain only 126 devices, but a public loop can be connected to a switched fabric, just as a shared Ethernet network can be connected to a switched Ethernet backbone by connecting the hub into a port on a switch.

• Switches -- In a pure switched fabric topology, each server will connect to the SAN via its own port on a Fibre Channel switch. Multiple switches can be cascaded together to create larger SANs, up to 16 million nodes.

• Routers -- A router (sometimes referred to as a bridge) bridges the gap between serial SCSI (i.e., Fibre Channel) and parallel SCSI, allowing you to connect legacy SCSI devices, such as tape drives and older disk drives, to your SAN.

• Disk Systems -- Disk systems come in all shapes and sizes, including single Fibre Channel disks, JBOD arrays, RAID in a box, and enterprise-level storage arrays.