Twin server, also called 0.5U server, is a form factor that fits two compute nodes into 1U of data center rack space.
Rack-mount server form factors are typically defined by the vertical space they occupy, such as 1, 2 or 4U. In contrast, 0.5U servers are defined by the horizontal space on a rack that they occupy. The two compute nodes in a twin server reside side-by-side; each node contains the processor, memory, peripheral slots and network connections of a standard unit size server.
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Twin servers share a power supply to keep the power load near optimum conditions. The chassis design for twin servers can reduce costs for power supply, cooling and rack space costs when compared to more traditional form factors. Compute density is higher than comparable 1U servers and competes with some blade server designs and dual-core processor 1U rack servers.
0.5U servers are used for stateless cloud computing, high-performance computing and enterprise IT computing tasks that require high availability. The twin form factor is not suitable for in-memory database serving or other memory-intensive tasks because of the limited memory included in the architecture.
SuperMicro, the leading twin server manufacturer makes 1U twins as well as 2U "fat" twin servers that house 4 computing nodes.