Storage Area Networks For Dummies®

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Par t I: SAN 101
For example, if you’re just running a word processor over the SAN, then you
don’t need 4 Gbps Fibre Channel parts.(In fact, you wouldn’t even need a
SAN!) On the other hand, if you’re running on-demand video applications for
a chain of hotels, then you definitely require the extra speed of 4 Gbps-to-10
Gbps SAN parts.
Making sure that everything runs at the same speed helps avoid wasting your
money on fast parts that can’t be fully utilized, or inviting bandwidth problems
by using slower parts in a fast network. Meanwhile there’s an upgrade
“gotcha” to consider: In the near future, as faster speeds start coming out,
most 1 Gbps and 2 Gbps SAN components will start to disappear from the
market. Make sure that any storage array you purchase comes with at least
4 Gbps ports and can run in a 4 Gbps SAN, or is at least upgradeable to new,
faster ports as they become available.
Too much distance between components
Distance issues can be the cause of all kinds of flaky intermittent gremlin-type
problems. Hop back to Chapter 2 to review Table 2-1, which covers the type
of cables to use as well as what kind of Gigabit Interface Converter (GBIC)
goes with which type of cable. (GBICs are where the little lasers hang out.
You need either a short-wave or long-wave GBIC, depending on distance.) Use
that chart as your bible when deciding on how far your servers can be from
your storage.
Here’s a classic example: A SAN that uses 50μ multimode cables connecting 1
Gbps devices can have no more than 500 meters between each device native
(without using expensive SAN extension gear). This means you can have 500
meters between your server and SAN switch and another 500 meters between
the switch and the storage. That’s pretty far. But if you use a lot of patch
panels for your fiber-optic cables, you might be close to the edge of how far
you can go, even though you’re nowhere near the distance limits of the cables.
Patch panels are devices that consolidate cable connections into one place.
Patch panels are very useful for keeping all your cables nice and neat, off the
floor and away from clumsy folks’ feet. (Read more about patch panels in the
upcoming section “Macro- and micro-bends, and the patch-panel pain.”)
Using a patch panel, however, limits your distance because of signal loss
from the connection. Think about it: The core of a fiber-optic cable (where
the light travels through) is made of a tiny thread of glass. Whenever you
splice an optical cable together with a patch panel (or any other way), you’re
not going to get the glass in the core to line up perfectly with the core of the
other cable. This means that light is lost at the splice (see Figure 4-1).