In this last
part of my series on the different types of storage hard drives and controllers
(Parallel ATA
and SATA 1, SATA
II, SCSI,
SAS), 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.

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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.