The first thing you will want to understand is what you are going to use the shared storage for. The most common early use cases for shared storage is either basic file serving, clustering or most recently, virtualization. If it is just file sharing then NAS becomes the obvious choice. Clustering and virtualization are not so cut and dry.
Clustering for example may not even require shared storage. There are several solutions on the market today that will synchronously mirror data from one physical server to another, typically across a dedicated 1GBE link. In fact several of the companies offering this type of solution also provide similar functionality for the virtual server environment. Mirroring one physical host to a standby host. For environments with small server count but high uptime requirements these solutions may be ideal.
When you get beyond the simple file sharing use case or the need to provide clustering to more than a few servers, shared storage quickly becomes the best option. The good news is that regardless of the shared storage type that you select the entry price has become significantly more affordable over the last few years.
The truth is you are most likely to select a protocol based on your familiarity with it. That is why most companies will look at iSCSI or NAS for their entry level storage systems. You almost always have a network before you have shared storage and of course that network is IP. Fibre is often left out of the consideration because of it's reputation of being expensive and complicated. If your needs are relatively basic then that is a fair assessment. Understand though that as your shared storage network scales all three of the protocols becomes more complicated. My advice is to not rule out any protocol based on it's reputation but to judge the protocol on it's short term learning curve as well as it's long term flexibility.
There are some inherent advantages to each protocol. iSCSI does not have the need to have additional hardware installed and can use standard Ethernet switches. A NAS' built in sharing capabilities make setting up a virtualization environment with live migration a little easier. Fibre remains the performance leader and has become more affordable over the last few years.
Unless you have an overwhelming advantage to sticking with one protocol or another, you are going to be selecting the solution based on the capabilities of the software it comes with, not just on the capabilities of the protocol. As we discussed in our prior entry, that is where the functionality of the system comes into play.
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George Crump is lead analyst of Storage Switzerland, an IT analyst firm focused on the storage and virtualization segments. Find Storage Switzerland's disclosure statement here.