Rack Mounting 101

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There are examples on the Overclockers Forums of people making use of a rackmount solution at home but most people don’t fully understand the whats and whys of rackmounting. In this article, Adragontattoo and Thideras explain some of the basics behind rackmounting applications and limitations.

First things first, here is a basic introduction to rack mounting, including an explanation of “U-Space”:

Your server(s) will be mounted in the rack. A rack is a metal framework that houses electronic equipment and usually contains bays designed for a piece of equipment like a server. These can be either enclosed cabinets or four post, open racks. Four post, open racks are probably not best for home use, as they need to be bolted to the floor. Two post racks are also available, but not recommended. They are intended for switches and network gear primarily. The space used by your server(s) is measured in Rack Units (RU or “U-Space”). This is the unit of measurement used for defining the vertical space used. A ‘U’ is equivalent to 1.75 inches (4.45 cm).

  • 1U Server= 1.75″ of Vertical Rack Space
  • 2U = 3.5″ of Vertical Rack Space
  • 4U = 7.00″ of Vertical Rack Space

A full rack can contain 40 to 44U, or larger in custom configurations.

  • 1/4 Rack=10U (10 servers of 1U)
  • 1/2 Rack=20U (20 Servers of 1U)

So, if you have to buy your own server you have to choose a rackmount server (or rack mounted server) that has been designed to live on a rack. You can buy rack shelves, but these get expensive when combined with case costs and are not efficient for space utilization.

Reference: http://www.askwebhosting.com/article/10/What_is_1U.html

Form Factors & Configuration

1U servers, being the smallest, are typically extremely loud due to high-speed small fans. For those that do not mind the noise, 1U servers are a great way to fit much processing power in a small footprint. Due to height limitations, these servers require custom power supplies, cooling arrangements, and motherboards. Since these are such special systems, I wouldn’t recommend them for home use for their noise, custom parts, and cost.

Typically, you will want to use a 2U, 3U or 4U server. There are cases larger than 4U, but most are specialty cases and will cost substantially more. In the 2U/3U cases, a stock AMD/Intel heatsink should have enough room to function. Depending on the case, they may use a riser card, which puts the card parallel to the motherboard. These enable you to install expansion cards (such as sound, video and network cards) into the case by turning it sideways. You will also see the ability for the case to take half-height expansion cards, which is convenient if you need more than one or two expansion cards. 3U and 4U servers are tall enough to take full-height cards.

For example, in my server rack at home, I currently have two 2U cases, as well as two 4U cases.  One of the 2U cases is my current firewall/router and the other is being built in place to replace the older firewall. I have one of the 4U cases for a HULU/Netflix/Media center box. The other is currently not being used. I have my entire entertainment center (A/V receiver, DVD player, Cable Box, Cable Modem and Digital Voice Modem) as well as my Brother Printer, Firewall, HTPC, and a spare (to be built) firewall. All of the above takes up 12U, and the footprint is contained in an area that is only 30″ square.

A typical sized 42U rack with varying sized servers.

A typical sized 42U rack with varying sized servers.

Cases, Manufacturers, & Vendors

Since the main goal of a rack is to stuff as many servers into the smallest possible area, the cases are not made with looks in mind. Norco, arguably, makes the best value rackmount cases for building home servers. However, if you are looking for something different there are a numerous vendors who offer a variety of cases for different uses. Examples of case manufacturers are SuperMicro, Norco, Antec, Chenbro, iStarUSA, amongst many others. I prefer to use Norco and SuperMicro cases because of a better build quality, as well as better cooling options.

NewEgg sells Norco and other rackmount cases. Craigslist and Ebay are also good places to look for the rack, rackmount cases, and parts. But buyer beware: many sellers do not know what they are selling and sell parts in as-is condition (no guarantee it works), or just plain lie. Make sure you do some research prior to purchasing.

Power Supplies

As mentioned at the beginning of the article, the size of the case may impose limits on what type of power supply is used. A 1U case will certainly require a specific power supply, but other sizes give more options. 2U and 3U cases have much more variety. A double-stacked 1U power supply can be used for redundancy or extra power, and possibly even an ATX power supply. If an ATX power supply is used in these smaller cases, you will be restricted to ones with 80 mm fans. 4U cases are tall enough that a standard desktop ATX power supply will fit. When purchasing a case, this is an extremely important feature to review. Make sure that the power supply that fits the chassis will have the connectors you need and supplies enough wattage to run the system.

There are also power supplies which fit in rackmount cases which are redundant and hotswappable. If one power supply dies, you can pull it out with the system still running and replace it. Most home users will not have any real use for a hotswap PSU since they would not keep an inventory of parts on hand in order to replace a broken part. Hotswap and 1U power supplies are typically the most expensive as they are a niche market, so users are willing to pay higher prices.

ATX PSU (left) vs 1U PSU (right)

ATX PSU (left) vs 1U PSU (right)

Redundant PSUs

Redundant PSUs

Cooling

Cooling in a rackmount case follows the same basic principles as in any other case. Draw in cool air and exhaust warm air. The problem then becomes “How do you get the cool air into the rack and how do you exhaust the warm air?” In normal data centers, cool air is fed from the front floor and exhausted through a mesh door in the back of the cabinet. This may not work as well at home. In my computer rack, I am running dual 120 mm intake fans and my exhaust is six 120 mm fans blowing out the top of the rack. This is not a silent solution, but it is not overpowering noise-wise either. Alternate methods are to remove the doors or, if the doors are mesh, allow a couple of feet clearance for airflow on the front and back.  Even for home use, you should keep the airflow moving in a consistent direction. You don’t want the exhaust of one server to be pulled into the intake of another. There are many different ways of getting air flowing and, depending on your rack, it may have vents to draw in cool air and a raised/vented lid on top to exhaust warm air. Options are fairly custom to the specific rack selected.

You are going to be very limited in your heatsink selection because of clearance inside the case. You will not be able to cram in that Thermalright Ultra 120 Extreme with Delta screamers strapped to it and add in more fans beyond that. Top down heatsinks might fit, but will probably restrict airflow since the fan is close to the top of the case. Side-mount fans will perform best in these cases as the fans push air horizontally, front to back. Personally, I look for cases that already have fans on both the front and back as I want to mod these cases as little as possible while ensuring as much flow as possible. Usually there are one or two 120 mm fans on the front (assuming no hotswap bays are in place) and one or two 80 mm fans on the rear of the case. This is all that is necessary, but users may want to adjust the interior case fans for more CFM/airflow or less noise. Be warned that you will need to keep an eye on your temperature if you change the fans out for slow-flow/low CFM fans. Water cooling in a rackmount case is a bad idea. I have seen a few attempts over the years, but most were hacked together. In a stacked PC situation, a leak could do more than damage a single server!

Example of Fans Set Up for Horizontal Airflow

Example of Fans Set Up for Horizontal Airflow

Conclusion

Using rackmount cases for computers and equipment is an effective way of saving space and being organized. Unfortunately, there are a few downsides. A fair amount of research is needed to effectively store computers in a rack environment. In addition to this, it is substantially more expensive than purchasing small cases and stacking them on storage shelves. In return, you gain the ability to easily access all the computers, equipment, and cables while having it stored in a small footprint. When done correctly, many benefits can be had in addition to a very good looking setup.

Original article written by Adragontattoo.
Rewritten article done by Thideras.

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Discussion
  1. Great summary. I love racks of hardware. When I did commercial electrical work I spent many hours in and in alot of cases underneath data centers. One day(probably not to far off) I hope to convert alot of my digital assets to a rack system and lighten up my clients. Once we get a bit better 3d virtualization available for free Ill likely be dumping all of my desktops in favor of a 4P setup with a couple of 670s in it to drive the entire house.
    I bought a 4U rack case. I'm discovering that it is far more of a challenge to build a system with good air flow that is quiet. After tinkering with it for over a month, I'm leaning towards using a mid-tower case and setting it on the floor. Too many things have to be a compromise for my comfort.

    My hat is off to people who do it. But I just don't see the juice being worth the squeeze.

    BachOn

    :eh?:
    Thanks, in looking back at it now. It covers the basics without going into overly large amount of detail.

    Hopefully, it gives enough info for the folks who arent sure and doesnt miss any crucial bits.