Server memory
RDIMM vs LRDIMM for DDR5 servers: which do you need, and will it fit?
Buy RDIMM. Choose LRDIMM only when you need the largest module sizes your platform offers and the server explicitly supports it. Here is the difference, and how to know it fits.
VeriLicense · 5 min read
The short answer
Buy RDIMM. It is cheaper, lower-latency and supported by virtually every enterprise server. Choose LRDIMM only when you need the largest module sizes your platform offers and the server explicitly lists LRDIMM as supported.
If that settles it, you are done. If you might be in the LRDIMM camp, or you are not sure it will fit, read on.
What is the actual difference?
Both are registered ECC modules for servers. The difference is how much of the electrical signal they buffer.
RDIMM buffers the command, address and clock signals between the memory controller and the DRAM chips. That keeps the signal clean when you have multiple modules per channel. It is the standard enterprise module and it handles the vast majority of workloads.
LRDIMM adds data buffers on top, isolating the data lines as well. That reduces the load on the memory controller further, which lets the platform carry more capacity per channel. The cost is higher latency, a higher price and narrower availability.
Side by side
| RDIMM | LRDIMM | |
|---|---|---|
| Buffers | Command / address / clock | Plus the data lines |
| Latency | Lower | Slightly higher |
| Cost | Lower | Higher |
| Top capacity per module | Standard sizes | Highest your platform offers |
| Availability | Widely available | More limited |
| Right for most servers | Yes | No |
Which should I buy?
RDIMM, unless you are pushing the maximum memory your server can hold and RDIMM cannot reach the module size you need. Even large databases and in-memory workloads are usually fine on RDIMM. Do not pay the LRDIMM premium unless the capacity ceiling forces it.
Will LRDIMM fit my server?
Physically, yes — RDIMM and LRDIMM are both DDR5 and use the same slot. Functionally is the question that matters. Three rules:
- The server must list LRDIMM as supported for that generation. Check the QuickSpecs, datasheet or memory QVL.
- You cannot mix RDIMM and LRDIMM in the same server. All modules must be the same type, or it will fail to boot or run unstable.
- Maximum module sizes and supported speeds vary by platform and processor. The same server can take different capacities depending on the CPU fitted.
Can I move from RDIMM to LRDIMM later?
Only by replacing every module at once. You cannot mix the two types, so a switch means swapping the whole memory configuration, not adding to it. Decide before you populate the server.
The bottom line: buy RDIMM. Reach for LRDIMM only when you need the largest capacity your platform supports and the server lists it. DDR5 vs DDR4 here if you are also choosing a memory generation.
Common questions
- Will LRDIMM physically fit a DDR5 RDIMM slot?
- Yes. Both are DDR5 and use the same slot. But the server must list LRDIMM as supported for that generation, and you cannot mix the two types in one system.
- Is LRDIMM faster than RDIMM?
- No, it is slightly slower. LRDIMM has higher latency than RDIMM. The benefit is capacity, not performance. If raw memory performance matters, RDIMM is the better choice.
- Can I mix RDIMM and LRDIMM in the same server?
- No. All modules must be the same type. Mixing them will either stop the server booting or cause memory errors. Switching type means replacing every module at once.
- How do I know the maximum module size my server takes?
- Check the server's memory QVL or datasheet for that generation — supported sizes and speeds vary by processor. Or send us the model and we will confirm before you order.
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