The exact number of servers in a rack is not a fixed value; it varies significantly depending on the physical size of the servers being installed.
Understanding Rack Capacity and Server Size
To determine how many servers can fit into a rack, you need to understand two key factors based on the provided information:
- Rack Height (in Units): According to the reference, a typical 7 ft rack has 40–42 units to work with. A unit, standardized as "U", is a measure of vertical space, equivalent to 1.75 inches.
- Server Height (in Units): Physical servers come in various sizes. The reference explicitly states, "A physical server might be 1U high or it might be 12U high, or more."
The number of servers is limited by the total available vertical space (in U) within the rack and the height (in U) of each server.
Calculating the Number of Servers
Since server sizes vary (e.g., 1U, 2U, 4U, 12U, etc.), the number of servers per rack is determined by dividing the total usable rack units by the server's height in units.
Number of Servers = Total Rack Units / Server Units
This means a rack can hold many small servers or fewer large servers.
Examples Based on Reference Information
Using the reference's information (40-42U rack space, 1U or 12U+ server size), here's the potential range:
- Maximum Servers: If using only 1U servers, you can fit the maximum number.
- 42 units / 1 unit/server = 42 servers
- Minimum Servers: If using large servers, such as 12U servers.
- 40 units / 12 units/server ≈ 3.33 servers. Since you can't have a fraction of a server, this would be a maximum of 3 servers if only 12U servers are used, potentially with leftover space (40 - 3*12 = 4 units remaining).
| Server Height (U) | Typical Rack Size (U) | Maximum Number of Servers |
|---|---|---|
| 1U | 40-42U | Up to 42 |
| 2U | 40-42U | Up to 21 |
| 4U | 40-42U | Up to 10 |
| 12U | 40-42U | Up to 3 |
| Larger (e.g., Blade Chassis) | 40-42U | Fewer than 3 |
Note: This table illustrates the maximum number of servers assuming only one size of server is used and excludes factors like power, cooling, or network cabling requirements which can also limit capacity.
Why There Isn't One Exact Answer
As the reference shows, the variability in server height (from 1U to 12U or more) directly impacts how many units each server consumes. Therefore, without knowing the specific mix of server sizes planned for a rack, it is impossible to provide a single, exact number of servers per rack. The number is a function of the equipment being deployed.
