DDR4 Server Memory Solutions

DDR4 speed by PowerEdge generation

The server generation and the processor set how fast DDR4 runs; Dell's manuals list these speeds:

Intel sets each processor's ceiling by generation: DDR4-2666 on the 1st Gen Xeon Scalable Gold 6100 series, DDR4-2933 on the 2nd Gen Gold 6200 series and DDR4-3200 on the 3rd Gen Gold 6300 series, and on the R740, Dell names the CPU's and the module's top rated speeds among the factors that decide the memory bus speed. AMD platforms take DDR4 too: the PowerEdge R7525, built for AMD EPYC 7002 and 7003 processors, supports DDR4 at up to 3200 MT/s.

Reading a DDR4 server module label

A DDR4 module name gives its speed one of two ways. Micron labels its modules by speed grade, such as PC4-3200, and lists a DDR4-3200 RDIMM at 25.6 GB/s of bandwidth; the PC4-25600 form names that same module by its bandwidth in MB/s, which is the transfer rate times eight bytes per transfer. The rank and width code follows: in a code such as 2Rx4 the figure before the R counts the ranks, Micron marks the same dual-rank build DR, and the x4 or x8 is the width of each DRAM chip's data path.

Mixing DDR4 modules

Pick one type per server: the R730 and R740 manuals both rule out mixing RDIMMs with LRDIMMs. The R740 manual allows x4 and x8 modules together and up to two capacities in one system, and runs mixed speeds at the speed of the slowest module. On the R650, Dell lists DDR4 RDIMMs from 8GB to 64GB and LRDIMMs at 128GB and 256GB.

Server DDR4 vs desktop DDR4

Two-socket servers run registered DDR4, and Micron notes that registered modules cannot run on boards made for unbuffered memory. Entry single-socket servers are the exception: Dell's PowerEdge R250 and R350 take DDR4 UDIMMs at up to 3200 MT/s with a Xeon E-2300 processor.

DDR4, DDR3 or DDR5

16th-generation PowerEdge servers such as the R660 and R760 take DDR5 RDIMMs, per Dell's technical guides, so DDR4 does not fit them; see DDR5 memory. 12th-generation servers such as the R720 run DDR3; see DDR3 memory. Processors for DDR4 servers are on Xeon E5-2600 v3 and Xeon E5-2600 v4. For why servers use ECC, see our ECC vs non-ECC memory guide, and every generation is on server memory.

DDR4 server memory questions

What does PC4-25600 mean?

It is DDR4-3200 named by bandwidth instead of speed. A DDR4 module moves eight bytes per transfer, so 3,200 million transfers per second gives 25,600 MB/s, and Micron lists its DDR4-3200 RDIMMs at 25.6 GB/s while labeling the same speed grade PC4-3200. PC4-21300 is DDR4-2666 and PC4-19200 is DDR4-2400 by the same arithmetic.

What does 2Rx4 mean on a DDR4 RDIMM?

Two ranks built from x4 DRAM chips. The number before the R is the rank count, which Dell's R720 manual writes as 1R, 2R or 4R for single-, dual- or quad-rank modules, and the x-number is the data width of each chip. Dell's R740 manual lets x4 and x8 modules share a server, but no channel may hold more than two different rank counts.

Can I mix DDR4 speeds in one server?

Yes, but the whole set runs at the slowest speed. Dell's R740 manual has a mixed-speed set run at the rate of its slowest module, and the processor's own ceiling caps it further. Mixing types is a different matter: registered and load-reduced modules never share a server, and the R740 accepts at most two module capacities per system.