Choosing server hardware

Consider the following factors when selecting compute server hardware:

  • Server density

    A measure of how many servers can fit into a given measure of physical space, such as a rack unit [U].

  • Resource capacity

    The number of CPU cores, how much RAM, or how much storage a given server delivers.

  • Expandability

    The number of additional resources you can add to a server before it reaches capacity.

  • Cost The relative cost of the hardware weighed against the total amount of capacity available on the hardware based on predetermined requirements.

Weigh these considerations against each other to determine the best design for the desired purpose. For example, increasing server density means sacrificing resource capacity or expandability. It also can decrease availability and increase the chance of noisy neighbor issues. Increasing resource capacity and expandability can increase cost but decrease server density. Decreasing cost often means decreasing supportability, availability, server density, resource capacity, and expandability.

Determine the requirements for the cloud prior to constructing the cloud, and plan for hardware lifecycles, and expansion and new features that may require different hardware.

If the cloud is initially built with near end of life, but cost effective hardware, then the performance and capacity demand of new workloads will drive the purchase of more modern hardware. With individual hardware components changing over time, you may prefer to manage configurations as stock keeping units (SKU)s. This method provides an enterprise with a standard configuration unit of compute (server) that can be placed in any IT service manager or vendor supplied ordering system that can be triggered manually or through advanced operational automations. This simplifies ordering, provisioning, and activating additional compute resources. For example, there are plug-ins for several commercial service management tools that enable integration with hardware APIs. These configure and activate new compute resources from standby hardware based on a standard configurations. Using this methodology, spare hardware can be ordered for a datacenter and provisioned based on capacity data derived from OpenStack Telemetry.

Compute capacity (CPU cores and RAM capacity) is a secondary consideration for selecting server hardware. The required server hardware must supply adequate CPU sockets, additional CPU cores, and adequate RA. For more information, see Choosing a CPU.

In compute server architecture design, you must also consider network and storage requirements. For more information on network considerations, see Network architecture.

Considerations when choosing hardware

Here are some other factors to consider when selecting hardware for your compute servers.

Instance density

More hosts are required to support the anticipated scale if the design architecture uses dual-socket hardware designs.

For a general purpose OpenStack cloud, sizing is an important consideration. The expected or anticipated number of instances that each hypervisor can host is a common meter used in sizing the deployment. The selected server hardware needs to support the expected or anticipated instance density.

Host density

Another option to address the higher host count is to use a quad-socket platform. Taking this approach decreases host density which also increases rack count. This configuration affects the number of power connections and also impacts network and cooling requirements.

Physical data centers have limited physical space, power, and cooling. The number of hosts (or hypervisors) that can be fitted into a given metric (rack, rack unit, or floor tile) is another important method of sizing. Floor weight is an often overlooked consideration.

The data center floor must be able to support the weight of the proposed number of hosts within a rack or set of racks. These factors need to be applied as part of the host density calculation and server hardware selection.

Power and cooling density

The power and cooling density requirements might be lower than with blade, sled, or 1U server designs due to lower host density (by using 2U, 3U or even 4U server designs). For data centers with older infrastructure, this might be a desirable feature.

Data centers have a specified amount of power fed to a given rack or set of racks. Older data centers may have power densities as low as 20A per rack, and current data centers can be designed to support power densities as high as 120A per rack. The selected server hardware must take power density into account.

Selecting hardware form factor

Consider the following in selecting server hardware form factor suited for your OpenStack design architecture:

  • Most blade servers can support dual-socket multi-core CPUs. To avoid this CPU limit, select full width or full height blades. Be aware, however, that this also decreases server density. For example, high density blade servers such as HP BladeSystem or Dell PowerEdge M1000e support up to 16 servers in only ten rack units. Using half-height blades is twice as dense as using full-height blades, which results in only eight servers per ten rack units.

  • 1U rack-mounted servers have the ability to offer greater server density than a blade server solution, but are often limited to dual-socket, multi-core CPU configurations. It is possible to place forty 1U servers in a rack, providing space for the top of rack (ToR) switches, compared to 32 full width blade servers.

    To obtain greater than dual-socket support in a 1U rack-mount form factor, customers need to buy their systems from Original Design Manufacturers (ODMs) or second-tier manufacturers.


    This may cause issues for organizations that have preferred vendor policies or concerns with support and hardware warranties of non-tier 1 vendors.

  • 2U rack-mounted servers provide quad-socket, multi-core CPU support, but with a corresponding decrease in server density (half the density that 1U rack-mounted servers offer).

  • Larger rack-mounted servers, such as 4U servers, often provide even greater CPU capacity, commonly supporting four or even eight CPU sockets. These servers have greater expandability, but such servers have much lower server density and are often more expensive.

  • Sled servers are rack-mounted servers that support multiple independent servers in a single 2U or 3U enclosure. These deliver higher density as compared to typical 1U or 2U rack-mounted servers. For example, many sled servers offer four independent dual-socket nodes in 2U for a total of eight CPU sockets in 2U.

Scaling your cloud

When designing a OpenStack cloud compute server architecture, you must decide whether you intend to scale up or scale out. Selecting a smaller number of larger hosts, or a larger number of smaller hosts, depends on a combination of factors: cost, power, cooling, physical rack and floor space, support-warranty, and manageability. Typically, the scale out model has been popular for OpenStack because it reduces the number of possible failure domains by spreading workloads across more infrastructure. However, the downside is the cost of additional servers and the datacenter resources needed to power, network, and cool the servers.