Anyone who has worked with IBM i knows how important disk I/O is to system performance. You can throw a 60,000 CPW, IBM Power System running IBM i, flat on its back if it’s not given the disk resources it needs to read and write at a pace greater then what the user demands.
The example I have used in the past goes something like this…
If you have 50,000 people leaving a football stadium out of 100 exits, the number of exit doors allow the crowd to exit in a single file line, at a brisk walking pace. In this scenario, the number of exits covers the demand set by the people exiting.
OR
If you have 50,000 people leaving a football stadium out of 10 exits, the limited number of exit doors creates a bottleneck in front of each exit. People are now piling up at each exit and have to start thinking more intensely about their next action. The nice gentleman now in front of the door stops to let a women and child out before him [delay], a drunk guy is asked to finish his beer before he leaves [delay], another person is trying to push his way to the front [delay], a man confronts the guy pushing his way through [delay], and the list goes on. In this scenario, additional delays were created because people needed to think about their next move instead of just flowing out of the stadium.
A server is no different. When a server is disk constrained it forces the system to think in more depth about the next move it will make. This absolutely kills performance.
See my blog post “Benefit for Mid-Sized Business’s Running on IBM Power8 (S814 & S824) Systems” to see how IBM is giving you a little more in Power8 to battle the disk I/O challenge.
Data is growing at a staggering rate for many companies. This means that many companies are having to invest in additional storage capacity for their IT infrastructure. For the companies that have lived inside the CEC (system unit) for many years, adding external disk seems like overkill. Especially when they can replace their 6x 283gb 15k RPM drives with 6x 571gb 15k RPM drives (used) for way cheaper then adding an external expansion drawer. Yes, you’re almost doubling the overall storage capacity but I/O remains exactly the same.
With most of my customers in the manufacturing, insurance, or financial markets, when storage capacity increases so does transaction volume. Most people think of storage as a place to hold data but loose site of the way data moves to and from that place.
Taking the stadium example a step farther we can picture a large parking lot with only one entrance and exit. You can only park as many cars as you can let flow through the entrance. This becomes your bottleneck. 
You could separate the large parking lot into 4 smaller lots with each one of them housing it’s own entrance and exit. You still can only park as many cars as you can let flow through each of the 4 entrances, but compared to the large parking lot above, it yields 4x the performance. 
So at the end of the day, it is extremely important to not strictly focus on overall storage capacity but take into account the effects and value of disk arms.
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