RDIMM: Registered DIMM
An RDIMM is a server memory module with a register (buffer) between the memory controller and the DRAM chips for address and command signals. Buffering reduces electrical load on the controller, allowing more DIMMs per channel and greater stability than unbuffered UDIMMs.
The register on an RDIMM latches address, command, and clock signals before they reach the DRAM chips. This lowers the electrical load the CPU's memory controller sees, so a server can run more modules per channel at high, reliable speeds. Data still flows directly between the controller and the DRAM, so latency overhead is minimal.
RDIMMs almost always include ECC and are the standard choice for mainstream servers using Intel Xeon Scalable and AMD EPYC. They are not electrically compatible with UDIMMs, and a platform is built for one type or the other. Capacities scale from 8 GB up to 64 GB and beyond per module.
When configuring servers from China, match RDIMM speed to what the CPU actually supports, since memory runs at the slower of the module and controller ratings. Populate DIMMs in matched sets across channels, and do not mix RDIMM and LRDIMM in the same server.
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