Posey's Tips & Tricks

How To Choose a Server Rack

Size, airflow and security should be at the forefront of your decision.

• By Brien Posey
• 01/27/2023

I recently completed a server hardware refresh in my environment and made the decision to switch to rack mount hardware. Even though I am no stranger to rack mount hardware, I have never run rack mount systems in my own home until now. As weird as it may sound, the toughest part of the entire migration was finding a rack that would be suitable for my needs.

Given the amount of work that went into selecting and setting up a server rack, I wanted to share my experiences with you. Putting my own personal situation aside though, I keep hearing that on-premises environments seem to be making a bit of a comeback. I have heard several stories in recent weeks of organizations bringing certain workloads back in house because of the rising costs associated with keeping those workloads running in the cloud. Of course, edge computing has also gained rapid popularity over the last few years and this trend has led to many organizations deploying server racks in branch offices, on a manufacturing floor, or in their warehouses. That being said, rack selection just seemed like a timely topic.

Physical Size
When selecting a server rack, many of the biggest considerations come down to physical size. The most obvious such consideration is vertical height, which is measured in rack units or Us.  A 42U rack, for instance, can theoretically hold 42 1U components. Of course in the real world, hardware is often larger than 1U in size, so you probably aren't going to be installing 42 components into a single rack. Even if you were only installing 1U hardware, it's a good idea to leave a bit of space between hardware components to allow air to flow.

The component size is not the only factor that comes into play when selecting the rack size. You also have to consider the height of the room. In my case, I used a 42U rack. This particular rack is just over 7 feet tall (counting wheels, top mounted fans, etc.). The issue that I ran into was that the rack had to be assembled upside down and then turned upright. At just over 8 feet, my ceiling was barely high enough for me to reorient the rack.

Another important sizing consideration is the rack's width. Most racks are designed to accommodate 19-inch hardware. Although 19 inches has long been an industry standard, it is not the only standard. Rack mount components can be 24 inches, 32 inches or even a few other sizes. The point is that it is important to make sure that your rack is designed to handle the hardware that you are planning on installing.

Just as you have to consider the rack's width, you must also consider its depth. Simply put, not all rack mount hardware is of the same depth, and so the rack has to be deep enough to hold your largest component. In my case, I have a switch that is only 6 inches deep, but my servers are 24 inches deep. As such, the rack has to be deep enough to accommodate a 24-inch deep server, with a bit of room left over for cabling.

It's also worth noting that there is such a thing as a rack that is too deep. Rack mount servers are heavy and must therefore be mounted on rails. These rails attach to bot the front and the back of the rack. Many of the available rail kits can fit a variety of rack sizes, but there is always a limit as to how large and how small of a rack a rail kit will work with. In my case for example, my rack is 33 inches deep. The rails that I am using for my servers were designed to fit racks ranging from 27 inches to 36 inches.

Physical Security
Physical security is another key consideration that must go into server rack selection. If you plan on placing servers in a colocation facility or in another location where physical access could be a problem (or if you need to adhere to physical security regulatory requirements) then you should select a fully enclosed rack with locking panels.

Cooling
Air flow and cooling are among the most important considerations when selecting a server rack. Air must be able to flow through the rack to keep your components cool. Admittedly, this essential requirement was the one thing that I messed up on when selecting my server rack.

The rack that I selected was a fully enclosed model. Because the rack is in my home, I thought that an enclosed cabinet might make for a nice way to hide cables and to make my servers less of an eyesore.

Of course, you can't have a rack that is truly enclosed because there would be no way for air to get in and cool the components. The rack that I selected has a perforated front door. There is also a large cut out on the bottom of the rack and several large fans on the top of the rack. In other words, the rack is designed to pull cool air in through the front and bottom and then discharge warm air out the top. Unfortunately, the rack seems to be designed for use in a datacenter with a raised floor (where the rack can be placed directly on top of a chilled air vent). In a residential setting, the components mounted in the rack ran too hot.

My solution was to simply keep the rack's front and rear doors open. While leaving the doors open at least partially defeated the purpose of purchasing an enclosed rack, it did allow air to flow well enough to keep all of my components cool. In case you are wondering, the ambient temperature inside the rack (as measured at the exhaust fans) was 84 degrees (Fahrenheit) with the doors closed and 78 degrees with the doors open. As such, my advice would be to use an open rack unless you are using raised floor cooling.