Memory (RAM)

RDIMM vs LRDIMM vs UDIMM — Which Memory Type Do You Need?

RDIMM (Registered DIMM) — The standard for dual-socket enterprise servers. A register buffer between the memory chips and the memory controller reduces electrical load, enabling higher DIMM counts per channel. Used in Dell PowerEdge R640/R740/R650/R750/R660/R760 and HPE ProLiant DL360/DL380 Gen9-Gen10.

LRDIMM (Load-Reduced DIMM) — Higher capacity per module than RDIMM (128 GB, 256 GB). Uses a data buffer to further reduce load, allowing maximum memory configurations. Required for 3 TB+ configurations on 14G/15G servers and 8 TB on 16G servers. LRDIMM and RDIMM cannot be mixed in the same system.

UDIMM (Unbuffered DIMM) — Used in single-socket entry servers only (Dell R340, R350, R250). Lower capacity (max 16-32 GB per module) and fewer slots. Not compatible with dual-socket servers.

DDR4 vs DDR5 — Which Servers Use Which?

DDR4 powers Dell PowerEdge 13th, 14th, and 15th Generation servers (R630-R750 era) and HPE ProLiant Gen9 and Gen10 systems. Speeds range from 2133 MT/s to 3200 MT/s depending on server generation and population.

DDR5 powers Dell PowerEdge 16th Generation servers (R660, R760, R6615, R6625, R7615, R7625). DDR5 runs at 4800 MT/s with lower voltage (1.1V vs 1.2V). DDR4 and DDR5 are physically different and cannot be interchanged.

Memory Population Rules

Server memory performance depends on how DIMMs are installed, not just total capacity:

• Populate all channels equally — 6 DIMMs per CPU on 14G (6 channels), 8 per CPU on 15G/16G (8 channels)
• Do not mix capacities within a channel or across channels on the same CPU
• Do not mix RDIMM and LRDIMM in the same system
• 1 DPC (DIMM per channel) runs faster than 2 DPC — balance capacity vs speed based on workload

How to Identify Compatible Memory

1. Check your server generation — this determines DDR4 vs DDR5
2. Check DIMM slot count — 24 slots (14G), 32 slots (15G/16G), 4 slots (entry)
3. Match the type — RDIMM, LRDIMM, or UDIMM (do not mix)
4. Contact Enterasource if unsure — we verify compatibility before shipping

Frequently Asked Questions

What is the difference between RDIMM and LRDIMM?

RDIMM (Registered DIMM) uses a register buffer to reduce memory controller load, supporting higher DIMM counts per channel. LRDIMM (Load-Reduced DIMM) adds a data buffer for even lower load, enabling higher per-module capacities (128 GB and 256 GB modules). LRDIMM is required for maximum memory configurations — 3 TB+ on 14th/15th Gen and 8 TB on 16th Gen servers. RDIMM and LRDIMM cannot be mixed in the same system. For most deployments under 1 TB, RDIMM is more cost-effective.

Can I mix DDR4 and DDR5 memory in the same server?

No. DDR4 and DDR5 use different physical connectors and are not interchangeable. Dell PowerEdge 13th, 14th, and 15th Generation servers use DDR4. 16th Generation servers (R660, R760, and AMD-based R6615/R6625/R7615/R7625) use DDR5. Check your server generation before ordering memory modules.

How do memory population rules affect server performance?

Memory performance depends on how DIMMs are distributed across memory channels, not just total capacity. Populating all channels evenly (6 DIMMs per CPU on 14G, 8 per CPU on 15G/16G) maximizes memory bandwidth. Running 1 DIMM per channel (1 DPC) provides higher clock speeds than 2 DPC. Mixing DIMM capacities within a channel can cause the server to run all memory at the lowest common speed. For optimal performance, use identical modules across all populated slots.

Need help selecting the right memory? Browse compatible processors or complete server configurations built to your specifications.