RDIMM vs LRDIMM: choosing server memory modules
Registered vs load-reduced DIMMs - capacity limits, ranks, speed penalties and mixing rules for HPE, Dell and Lenovo servers.
The short version
RDIMM (registered) buffers the address/command signals and is the default server module from 8 GB to 96 GB (DDR5). LRDIMM (load-reduced) also buffers the data lines, so the memory controller sees one electrical load per module regardless of rank count - that is how 128 GB and 256 GB modules and fully populated 2-DPC configurations keep running at rated speed.
| RDIMM | LRDIMM | |
|---|---|---|
| DDR4 capacities | 8 - 64 GB | 64 - 256 GB |
| DDR5 capacities | 16 - 96 GB (128 GB 2DPC) | 128 - 256 GB |
| Cost per GB | Lower | Higher (premium for density) |
| Latency | Lower | Slightly higher |
| Power | Lower | Higher per module |
| Mixing | Never mix RDIMM and LRDIMM in one system | |
Rules that save a support call
- Populate identical part numbers per channel; follow the server's population order (HPE and Dell print it on the chassis lid).
- On DDR5 Xeon 4th/5th Gen and EPYC 9004, 1 DIMM per channel (8 per CPU) runs at full 4800-5600 MT/s; 2 DIMMs per channel drops speed - use larger RDIMMs instead of filling all 16 slots when possible.
- Use LRDIMM only when you need more than ~1 TB per socket or the platform's RDIMM limit is reached.
- OEM-branded (HPE SmartMemory, Dell certified) and third-party (Kingston Server Premier, Samsung, Micron, SK hynix) modules both work; OEM modules unlock some firmware features (HPE Fast Fault Tolerance) and avoid warning messages.
Send us the server model and current memory configuration (from iLO/iDRAC) and we will quote compatible server RAM with the exact part numbers.
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Can I mix RDIMM and LRDIMM?
No. The server will refuse to boot or disable one type. Use one type throughout the system.
What is a rank?
A rank is a 64-bit (72-bit with ECC) block of memory chips on the module addressed together. Dual-rank modules usually perform slightly better than single-rank; quad- and octal-rank modules are LRDIMMs.
