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What is a Rack Server? [Definition, Examples & Use Cases]

by Brett Daniel, on May 7, 2020 11:05:50 AM

There are several types of rugged computers out there supporting a variety of different military, industrial and commercial applications. The most common, however, is the unmistakable rack server.

These slidable, hot-swappable, high-powered rugged computers are ideal for harsh environments and applications that emphasize space conservation, scalability, upgradability and expandability.

In this blog post, we'll discuss how a rack server works, the different sizes of rack servers and server racks, and how you can choose the best of both to fit your program or application.

What is the definition of rack server?

A rack server, also known as a rack mount server, rack-mounted server or racked server, is a computer designed to be situated in a rectangular structure called a server rack.


Photo: Rugged rack servers are often found supporting a variety of military applications, such as on-site surveillance operations.

What is a rack server used for?

Like all servers, rack servers provide data and specific services to clients. They’re commonly found in data centers full of dozens, or even hundreds, of server racks and server rack cabinets.

Rugged rack servers and computers are often found in the field supporting military and industrial applications. Certified to military and industrial standards like the MIL-STD-810H, MIL-STD-461, CE, FCC or DO-160, these reliable, high-powered machines are stress-tested to withstand harsh environmental conditions, such as extremely high or low temperatures, vibration during transport or operation and humidity in parts of the world where atmospheric moisture is pervasive.

In military applications, rack servers can often be found situated inside compact and sturdy MIL-STD-810-certified rack server cases.


Photo: A rack server being removed from a server rack cabinet

How does a rack server work?

Mechanically, rack servers are able to slide in and out of a server rack with ease. This feature comes in handy, allowing systems administrators, technicians and operators to diagnose technical problems and hot-swap parts without shutting down and disassembling the entire system. Such a feature is vital to mission-critical applications, in which lengthy periods of downtime could result in financial loss, injury or even death.

Computationally, a rack server’s performance, resources and services are specific to the needs of a program or application. A remote military installation in the desert may need a high-powered rack database, email, web, file or application server to support on-site computation and access to critical resources and intelligence, while a commercial warehouse or industrial oil rig may operate a series of rack servers that store and process data from security cameras, assembly line computers or fracking control systems.


Photo: The number of servers a server rack can hold depends on the depth of the rack.

How many servers does a rack have?

Server racking is the process of determining how many rack mount servers a rack can hold.

The number of servers a rack contains depends greatly on server rack dimensions. Server rack sizes vary from application to application. Industry server racks typically come in three widths: 19 inches, 23 inches and 24 inches. The height of both server racks and rack servers is typically measured in rack units. One rack unit (U) is equal to 1.75 inches.

When shopping for rack mount servers, you’ll often see the rack unit measurement expressed as "#U." To determine the height of your server or rack in inches, simply multiply the rack unit number by 1.75 inches. For example, a rack server may have a 1U rack height, which equates to 1.75 inches of vertical space; therefore, a rack needs to have at least 1U rack dimensions, or at least 1.75 inches of vertical space, for the rack server to slide into it properly.

Consider a rack server with 4U rack dimensions, which equates to 7 inches of vertical space. A typical small server rack with a 4U rack height can hold one 4U rack server, which would be a perfect 7-inch fit, but it can also hold:

  • Four 1U rack servers, requiring 1.75 inches of vertical space per server
  • Two 2U rack servers, requiring 3.5 inches of vertical space per server
  • One 3U rack server and one 1U rack server, requiring 5.25 and 1.75 inches of vertical space, respectively
  • One 2U rack server and two 1U rack servers, requiring 3.5 inches of vertical space for the 2U server and 1.75 inches of vertical space for each 1U server

Naturally, there are much taller server racks than a 4U rack. Other common rack heights include the 42U and 44U racks, which equate to 73.5 and 77 inches of usable space, respectively. You could fit a myriad of different-sized rack servers into racks of this height, or you could simply go with twenty-one 2U rack servers for the 42U rack and twenty-two 2U servers for the 44U rack.

But there are even taller racks than the 44U. Rack Solutions claims its $969 70U Open Frame Server Rack is “possibly the tallest server rack in the world” that meets all Electronics Industries Alliance (EIA) standards. Talk about possibilities.


Photo Collage: A tower server (left), a rack or rack mount server (center) and a pair of blade servers

Tower server vs. rack server vs. blade server

Rack mount servers are the most commonly used servers in military, industrial and commercial applications due to their high scalability, expandability, upgradability and ability to support compute-intensive applications, but tower and blade servers also make an appearance from time to time.

A tower server is a solo, upright computer that resembles the common desktop PC tower. It’s commonly found in small business environments and other commercial applications. Tower servers are standalone, meaning they cannot be inserted into server racks, and they are taller and bulkier than their rack mount and blade counterparts. Tower servers aren’t usually very costly, but they take up a lot more space, making them difficult to scale. Furthermore, many tower servers require disassembly of the entire system to repair, replace or upgrade parts.

A blade server is a thin, lightweight, modular computer that favors a nightstand drawer in both size and functionality. Blade servers can also be considered rack servers, since they're often housed inside racks in what are called blade enclosures or blade systems. Like rack servers, they’re slidable and hot-swappable, but they're much smaller than both their tower and rack counterparts. Applications that emphasize space conservation and high processing power will find the blade server quite useful.

Tower server Rack server Blade server
Low scalability, good expandability and upgradability, difficult to repair and replace parts High scalability,  expandability and upgradability, easy to repair and replace parts Extremely high scalability and upgradability, easy to repair and replace parts low power consumption
Consumes a lot of space Consumes a lot of power Low expandability


Table: Comparison of tower, rack and blade servers

Blade servers also consume less power than tower and rack mount servers since the chassis is usually the system's sole power source. In addition, they use a lot less cabling than their counterparts; however, they’re greatly limited in expandability due to their small size. Each server type can support high processing power, considerable storage and PCIe expansion. The primary dividing factors are size, weight and power (SWaP), scalability, expandability and upgradability.

Rack unit graphic, 1U = 1.75 inches

Graphic: Knowing the depth of your rack servers will help you tremendously when choosing the right rack size

How do I choose a server rack?

The server rack you choose will depend on the needs of your program or application.

You’ll need to determine how many servers your program or application will require, as well as the height (RUs) and width of your server rack. Compare these measurements with the height and width of your servers to choose the appropriate rack size.

5U rugged rack server

Photo: A Trenton Systems 5U rack server

Which rack server is best?

The rack server that's best for your program or application will depend on the host of variables we discussed earlier: size, weight and power (SWaP), as well as scalability, expandability and upgradability.

Generally, the bigger the server, the more expansion and storage options you have, but keep in mind that, as you add more components, the server gets heavier and may need more power.

Let’s look at a couple of examples.

1U vs. 2U rack server

A 1U rack server will generally be able to support the same CPUs and RAM as a 2U server; however, it will generally have less space for PCIe slots and storage due to its small size.

With most any rack server, however, slots and storage can be expanded using a PCIe expansion kit or JBOD enclosure.

Trenton Systems 1U Rack Server
Rugged Chassis Power Supply Size (W x H x D)
TRC1001 1200W Removable 19" x 1.75" x 19"
(48.3 cm x 4.45 cm x 48.3 cm)
TRC1002 1200W Removable 19" x 1.75" x 19"
(48.3 cm x 4.45 cm x 48.3 cm)


Trenton Systems 2U Rack Server
Rugged Chassis Power Supply Size (W x H x D)
TRC2010 650W Fixed or 600W Redundant 19" x 3.5" x 18"
48.3 cm x 8.9 cm x 45.7 cm
THS2085 500W 1U Removable or 650W 2U Fixed or 600W Redundant 19" x 3.5" x 18.2"
48.3 cm x 8.9 cm x 46.2 cm
TRC2019 650W Redundant or 600W Fixed 19" x 3.5" x 28"
48.3 cm x 8.9 cm x 71.1 cm


Tables: Power supply and dimensions for Trenton Systems' 1U and 2U rugged rack servers. Weight varies greatly based on configuration.

3U vs. 4U vs. 5U rack server

Like the 1U and 2U rack servers, a 3U rack server will support less PCIe slots and storage options compared to the 4U or 5U rack server.

A 5U rack server can really pack a punch. For example, a Trenton Systems 5000 Series Rugged Server can support up to 18 PCIe Gen 3 slots natively, up to 48 front-access/hot-swap SATA 6 Gb/s drives, 4 internal, fixed 2.5" 6 Gb/s SATA drives and up to two SlimLine Optical drives. Pair this with PCIe expansion kit and/or JBOD, and you’ve got yourself quite the formidable machine.

Trenton Systems 3U Rack Server
Rugged Chassis Power Supply Size (W x H x D)
TRC3010 650W fixed or 1500W redundant 19" x 5.25" x 20"
48.3 cm x 13.3 cm x 50.8 cm
TRS3501 650W Fixed or 1500W Redundant 19" x 5.25" x 20"
48.3 cm x 13.3 cm x 50.8 cm

Trenton Systems 4U Rack Server
Rugged Chassis Power Supply Size (W x H x D)
THS4086 1200W (AC) redundant or 850W (DC) 19" x 7" x 16.5"
48.3 cm x 17.8 cm x 41.9 cm
THS4095 1200W fixed 19" x 7" x 26"
48.3 cm x 17.8 cm x 66 cm
TRS4019 875W Redundant, 200-240V AC 19" x 7" x 29"
48.3 cm x 17.8 cm x 73.7 cm

Trenton Systems 5U Rack Server
Rugged Chassis Power Supply Size (W x H x D)
THS5095 1200W Redundant 19" x 8.75" x 19.5"
48.3 cm x 22.2 cm x 49.5 cm
THS5087 1550W N+1 or 2430W N+1 19" x 8.75" x 23"
48.3 cm x 22.2 cm x 58.4 cm
THS5090 1000W Removable (2) 19" x 8.75" x 19.2"
48.3 cm x 22.2 cm x 48.8 cm
THS5091 800W Removable (4) 19" x 8.75" x 19.2"
48.3 cm x 22.2 cm x 66 cm
TSS5201 1550W N+1 Redundant 19" x 8.75" x 24.5"
48.3 cm x 22.2 cm x 62.2 cm

Tables: Power supply and dimensions for Trenton Systems' 3U, 4U and 5U rugged rack servers. Weight varies greatly based on configuration.


Trenton Systems can help you determine not only the size and type of rack server you need and how many, but we also offer integrated rack solutions for programs and applications needing multiple rack servers in a single rack.

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Topics:rackmount computersrack serversserver racks1U rack server5U rack serverWhat is a rack server?rack mount serversrack server dimensionsserver rack dimensions2U rack server3U rack server4U rack server


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