Why fibre channel rules in the most rigorous environments

If you need reliable enterprise storage with the capacity for massive scalability, you can't beat fibre channel technology. See how it stacks up against SCSI and SAS.

In this article, I will present information about the (current) Grand Poobah of magnetic storage: fibre channel. The specification for fibre channel was ANSI-approved in 1994.

When you think of "fibre channel," you probably think about SANs (Storage Area Networks), too. In truth, unlike ATA, SATA, SCSI, and SAS disks, fibre channel disks are generally relegated for use in a SAN environment. Sure, you can use other disk types in a SAN, but, in contrast, you won't generally see fibre channel disks used for local storage. Notice that I used the word "generally" in the last sentence. Just because fibre channel disks usually live in a SAN doesn't mean that it's not possible to use them as local storage.

In considering fibre channel's location on the drive reliability chart, fibre channel definitely has enterprise-worthiness written all over it, as do SCSI and SAS. Like the latter drives, fibre channel disks are manufactured to withstand a much more rigorous environment than ATA and SATA. After all, unlike most ATA and SATA disks, fibre channel, SCSI, and SAS disks need to spin 24/7. As such, any comparisons I make in this article will be with SCSI and SAS. I'm also assuming that you're talking about using fibre channel end-to-end, from controllers to switches, to the storage. I mention this because it is possible to use SCSI disks in conjunction with the fibre channel transport mechanism.

So, just how does fibre channel stack up against SCSI? When it comes to something as simple as the number of devices available for expansion, you just can't beat fibre channel's scalability. For a single loop, fibre channel supports 126 nodes compared to SCSI's 15-device limit on a single cable. When you start including switches to build a complete fibre channel fabric, this limit jumps from 126 nodes up to around 16 million. That's a lot of disks! Even SAS--SCSI's serially-enabled upgrade--can only support up to 16,000 devices.

When you start to talk about massive storage systems, distance limitations take on a whole new importance. No longer are you worried whether or not you have enough cable to reach another point in a chassis. Instead, you worry about whether you can stay easily scale your array across various rooms, buildings, or even towns. In this area, fibre channel rakes SCSI and SAS over hot coals and leaves it for dead. SAS currently allows for cables of up to about six meters, although, with the right cabling and future specifications, longer distances are possible. Fibre channel, on the other hand, supports connections that are up to 10 KM apart, when you use single-mode fiber as the interconnect.

Like all storage mechanisms, some kind of interface is needed between the computer and the disk or array in order for the server to be able to make use of the disks. In the case of fibre channel, this is accomplished through the use of a host bus adapter (HBA), which, like SCSI adapters, helps to offload the server from having to handle storage-related tasks, resulting in better overall performance.

When it comes to speed, fibre channel would appear to be at something of a disadvantage when compared with SCSI, or even SATA-IO. Fibre channel runs at speeds of 200 MB/s or 2 GB/s while SCSI disks top out at 320 MB/s. However, when you really take a look at storage I/O with an array, it's not usually the disk transfer speed that becomes a bottleneck. Further, a change in fibre channel is currently underway that bumps this speed up to 4 GB/s, with a plan to eventually hit 8 GB/s and 10 GB/s.

It should be noted that you don't need to use fibre channel disks, even if you use a fibre channel storage infrastructure. In fact, you can use SCSI disks for the storage mechanism and use the fibre channel to transmit SCSI commands from your servers (which will use a fibre channel HBA) to your storage. It all depends on your storage needs.

It's hard to write a comparison between SCSI and fibre channel since, depending on the environment and the application, these technologies can complement each other or compete with one another. For smaller organizations with less rigorous requirements, SCSI (or SAS) is a great option, even when fibre channel is used only as the data transport mechanism. For organizations that need to massively scale their storage to millions of nodes, fibre channel is now--and for the immediate future remains--the Grand Poobah of storage.

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