DDR5 Memory

What has to match in a DDR5 set

DDR5 server memory is sold as ECC registered DIMMs, and three properties have to match across a set: capacity, organization (ranks and data width) and rated speed. The organization is written as a code such as 1Rx8 or 2Rx4, where the first figure counts the ranks and the x-number gives the data width of each DRAM chip.

DDR5 RDIMM, 3DS RDIMM and UDIMM

DDR5 comes in three module classes, and a server takes only the registered kinds:

Micron rules registered modules out of any system designed for UDIMMs.

On-die ECC is not server ECC

Every DDR5 chip corrects some errors inside itself, and that is a different job from the ECC a server relies on. Micron describes DDR5 on-die ECC as single-error correction that pairs 128 data bits with 8 parity bits inside each DRAM device and corrects the data during reads before it leaves the chip, which reduces the system's error-correction burden. The system-level correction still has to come from ECC modules, so a DDR5 desktop module with on-die ECC is not an ECC server module, and Dell's R760, for one, lists registered ECC DDR5 modules alone.

Which servers take DDR5 RDIMMs, and at what speed

DDR5 RDIMMs go in 4th and 5th Gen Intel Xeon Scalable servers, eight channels per socket, and AMD EPYC 9004 and 9005 servers, twelve channels per socket; in Dell's line-up that is the 16th generation. The server sets the running speed, and the module's rating is only its ceiling. Dell's technical guides for the R660 and R760 list RDIMMs at 4800 MT/s with 4th Gen Xeon Scalable processors at one DIMM per channel, and Dell's R760 specification sheet gives up to 5600 MT/s with 5th Gen processors. With two DIMMs in each channel, both generations drop to 4400 MT/s, and a DDR5-6400 module installed in these servers runs at the speed the processor supports.

Each processor has eight memory channels with two sockets per channel, which gives 16 sockets per processor and 32 per two-socket server. On the R760, Dell supports populating each processor with 1, 2, 4, 6, 8, 12 and 16 RDIMMs.

Mixing DDR5 modules

Dell's R760 installation and service manual sets the mixing rules plainly: all DIMMs must be DDR5, and modules of different capacities, x4 and x8 modules, and 3DS and non-3DS RDIMMs must not be mixed. Modules with different rated speeds can share a server, but every module then runs at the speed of the slowest one. In practice that means one organization per build: 128 GB RDIMMs are made both as 3DS and as non-3DS modules, which Dell does not allow together, and Dell's R660 and R760 memory tables list 32 GB only as a two-rank module. Earlier PowerEdge generations that take DDR4 are served by DDR4 Memory.

Ordering DDR5 from Enterasource

Every DDR5 module is tested before it ships. Tell us the server model, the processor and how many DIMMs you need, and we will quote modules that match on capacity, organization and speed. See the R760 buying guide, ECC vs non-ECC memory and every generation on server memory.

DDR5 RDIMM questions

Can I use DDR5 UDIMMs in a server that takes RDIMMs?

No. Unbuffered and registered DDR5 are separate classes: Micron describes UDIMMs as wired straight to the memory controller for desktops, and its note says a registered module will not work in a UDIMM system. Servers built for registered memory list RDIMMs only; Dell's R760 is specified for registered ECC DDR5 and nothing else. Match the module class, then capacity, organization and speed.

Does DDR5 on-die ECC replace ECC RDIMMs?

No. Micron describes on-die ECC as single-error correction inside each DRAM chip, 128 data bits with 8 parity bits, applied during reads before data leaves the device, and says it reduces the system's error-correction burden rather than removing it. It cannot detect double-bit errors on its own, so Micron lays it out to pass them on to system-level error correction, which a server gets from ECC RDIMMs.