Server memory is chosen by five things: generation (DDR4 or DDR5), module type (RDIMM, LRDIMM or 3DS), capacity, rank organization (such as 2Rx4) and speed. Get those right for your server platform, populate the channels evenly, and the system runs at full bandwidth.
Memory generation
DDR4 and DDR5 modules are not interchangeable. They have different notch positions and different electrical designs. The processor platform decides which you need.
| Platform examples | Memory |
|---|---|
| Intel Xeon Scalable 1st to 3rd generation | DDR4 |
| AMD EPYC 7001, 7002, 7003 series | DDR4 |
| Intel Xeon Scalable 4th and 5th generation, Xeon 6 | DDR5 |
| AMD EPYC 9004 and 9005 series | DDR5 |
Browse DDR4 and DDR5 server memory.
Module types
RDIMM (registered)
A registered DIMM has a register (RCD) that buffers command and address signals between the memory controller and the DRAM chips. This lets a server support more modules per channel than unbuffered memory. RDIMMs are the standard choice for most servers.
LRDIMM (load-reduced)
An LRDIMM also buffers the data lines, which lowers the electrical load on the channel. That allows higher capacities per module and per channel. On DDR4, LRDIMMs were common at 64 GB and above. On DDR5, the RDIMM design handles high capacities well and LRDIMMs are rarely used.
3DS (3D stacked)
3DS modules stack several DRAM dies in one package using through-silicon vias. This gives very high capacities, such as 128 GB and 256 GB modules. 3DS modules can be RDIMMs or LRDIMMs and are labeled with ranks such as 2S2Rx4 or 8Rx4.
ECC
All server RDIMMs and LRDIMMs support ECC (error-correcting code). DDR5 modules also have on-die ECC inside each chip, but that is not a replacement for system-level ECC. Server DDR5 RDIMMs still carry extra chips for side-band ECC.
Ranks and organization
The label on a module shows its organization, for example 2Rx4 or 1Rx8.
- The rank count (1R, 2R, 4R, 8R) is the number of sets of DRAM chips the controller can address independently. More ranks usually mean more capacity per module.
- The data width (x4 or x8) is the width of each DRAM chip. x4 modules use more chips per rank and support stronger error correction features on many platforms, such as Chipkill-style correction.
Some platforms limit the total number of ranks per channel. Others run slower with more ranks installed. Check the server's memory population guide.
Speed
Module speed is shown as a data rate, such as DDR4-2933, DDR4-3200, DDR5-4800 or DDR5-5600. The memory actually runs at the lowest of three values:
- The processor's maximum supported memory speed.
- The module's rated speed.
- The speed the platform allows for that number of DIMMs per channel (DPC).
Many servers lower memory speed when two DIMMs per channel are installed. A faster module in a slower platform works, but runs at the platform's speed. A slower module than the platform supports pulls the whole system down to its speed.
Populating channels
Each processor has a fixed number of memory channels. Memory bandwidth scales with the number of channels in use.
| Processor family | Channels per socket |
|---|---|
| Intel Xeon Scalable 1st and 2nd generation | 6 (DDR4) |
| Intel Xeon Scalable 3rd generation (Ice Lake) | 8 (DDR4) |
| Intel Xeon Scalable 4th and 5th generation | 8 (DDR5) |
| AMD EPYC 7001 to 7003 | 8 (DDR4) |
| AMD EPYC 9004 and 9005 | 12 (DDR5) |
General rules that apply to most platforms:
- Populate every channel before adding a second DIMM to any channel. One module per channel across all channels gives full bandwidth.
- Keep it balanced. Install the same number of modules in each channel and the same configuration on each processor.
- Follow the slot order in the server's documentation. Slots are often color-coded or numbered by population order.
- Install memory for each processor. In a two-socket server, memory attached to an empty socket is not usable.
Mixing rules
- Do not mix RDIMMs and LRDIMMs in the same server.
- Do not mix 3DS and non-3DS modules on most platforms.
- Do not mix DDR4 and DDR5. They do not fit the same slots.
- Mixing capacities or ranks is allowed on some platforms with restrictions, and not allowed on others. On DDR5 platforms, many vendors require identical modules throughout.
- Mixing speeds works on most platforms, but all modules run at the slowest speed.
- Mixing x4 and x8 modules is restricted on many platforms.
When you expand an existing server, the simplest safe approach is to match the part number of the modules already installed.
OEM-branded vs generic modules
Server brands sell memory under their own part numbers, for example HPE SmartMemory, Dell memory SKUs and Lenovo option numbers. These modules are made by memory manufacturers such as Samsung, SK hynix and Micron, then validated and labeled by the server brand.
- OEM-branded modules match the server's support matrix. Some platforms show warnings, or limit features such as enhanced error reporting, with non-branded memory.
- Generic modules from the memory makers can work when they match type, rank, organization and speed exactly. They are common in white-box and Supermicro servers.
For servers under OEM support, branded memory avoids support questions. For out-of-warranty systems, matching generic modules can be a practical choice. See server part numbers explained for how option and spare numbers work.
Checklist for your request
- Server model and generation, and processor model.
- Memory part number already installed, if expanding.
- Target total capacity and number of modules.
- Module type, rank organization and speed, if known.
- Condition: new, refurbished or used. See new vs refurbished vs used.
Browse server memory or send your list as an RFQ. If you are not sure which module fits, give us the server model and we will match it.