RDIMM or LRDIMM: which should I buy to add memory to my server?

1. 1.On Linux, list every slot's Memory Device record. This only reads the firmware's SMBIOS tables and changes nothing. Do it before you shop: the class you find is the class you buy.

   sudo dmidecode -t memory

   ReadingJudge each populated slot's Type Detail line on its own: the line may carry other flags as well, for example 'Synchronous'. A line containing 'LRDIMM' is an LRDIMM. A line containing 'Registered (Buffered)' and no 'LRDIMM' is an RDIMM. Rank is the per-module count to hold against your platform's table (Dell's R750 matrix has RDIMMs at 1R and 2R and LRDIMMs at 4R and 8R). Speed is the module's rating and Configured Memory Speed the speed it runs at today. Part Number is what you look up on the module maker's or server vendor's list. Locator tells you which slots are full, so you can count DIMMs per channel.
2. 2.For a one-line-per-field view across all slots, filter the same output.

   sudo dmidecode -t memory | grep -E 'Locator|Size|Type Detail|Rank|Speed|Part Number'

   Reading'No Module Installed' under Size is an empty slot. Read each populated slot's Type Detail line by the rule above. The system is mixed only when some populated slots read LRDIMM and others read 'Registered (Buffered)' without 'LRDIMM', which the Dell R740, Lenovo SR650 and Intel S2600WF documents forbid: go to p3 before buying anything.
3. 3.Do not take the DMI tables on trust. The dmidecode manual says the information in them is more often than not inaccurate, incomplete or simply wrong. Ask the BMC over Redfish as a second opinion: list the Memory collection for the MemoryId values, then read one module.

   curl -sk -u <user> https://<bmc>/redfish/v1/Systems/<ComputerSystemId>/Memory/<MemoryId>

   ReadingBaseModuleType reads 'RDIMM' or 'LRDIMM'. RankCount is the rank count, OperatingSpeedMhz the running speed beside AllowedSpeedsMHz, and PartNumber the module's part. We have not verified whether iDRAC, iLO or XCC fill BaseModuleType, and we found no Dell document stating what iDRAC shows for module class.
4. 4.On a PowerEdge R760 (the manual we read), press F2 > System Setup Main Menu > System BIOS > Memory Settings.

   Power on, press F2 > System Setup Main Menu > System BIOS > Memory Settings

   ReadingSystem Memory Size, System Memory Type and System Memory Speed. The R760 manual page we read describes those three fields and nothing on that screen that names RDIMM or LRDIMM, so use it to confirm the total and the running speed, and take the class from dmidecode or Redfish.

1. 1.Dell PowerEdge R740 (DDR4). Dell says the R740 takes both RDIMMs and LRDIMMs, and that RDIMMs and LRDIMMs must not be mixed. A channel holds up to two RDIMMs or two LRDIMMs whatever their rank count, 2933 MT/s is supported only at one DIMM per channel, and modules of different speeds run at the slowest one's speed. Within LRDIMMs, 64GB DDP modules must not be mixed with 128GB TSV/3DS modules. We could not open the R740's per-type capacity table, so this page gives no R740 ceiling for either class.
2. 2.Dell PowerEdge R750 (DDR4). Table 2 of the R750 manual lists RDIMMs at 8GB single-rank and 16, 32 and 64GB dual-rank, and LRDIMMs at 128GB quad-rank and 256GB octal-rank, all at 3200 MT/s at one and at two DIMMs per channel, across eight channels per processor with two sockets each. On this server a module above 64GB is an LRDIMM. The R750 guidelines allow only RDIMMs of different capacities to be mixed and support 1, 2, 4, 6, 8, 12 or 16 DIMMs per processor; unbalanced layouts lose performance and the system may not identify the modules. We found no sentence on mixing RDIMMs with LRDIMMs in the R750 manual pages we read.
3. 3.Lenovo ThinkSystem SR650 (DDR4). Lenovo's product guide lists RDIMMs, LRDIMMs and 3DS RDIMMs and says mixing those types is not supported. LRDIMMs are listed for 1st Gen Xeon SP processors only and reach 1.5TB; 3DS RDIMMs reach 3TB with either generation. So here the largest configuration is built from 3DS RDIMMs, not LRDIMMs: which class tops out highest is a property of the platform, not of the module type. Every DIMM runs at the lowest of its rated speed, the CPU's speed and the DIMMs-per-channel speed: 2933 MHz at one per channel, and at two per channel 2933 MHz for Performance+ DIMMs and 2666 MHz for the rest.
4. 4.DDR5 servers. Dell's R760 memory table lists DDR5 RDIMMs only, from 16GB single-rank to 256GB octa-rank (8TB with two processors and 256GB modules), with the 128GB 4R module marked end of life and no LRDIMM row. Dell calls 3DS the DRAM technology used to make the highest-capacity DIMMs and says 3DS and non-3DS RDIMMs must not be mixed. Lenovo's SR650 V3 guide says 'UDIMMs and LRDIMMs are not supported' and reaches 8TB with 32 256GB 8Rx4 3DS RDIMMs. Lenovo's memory summary lists LRDIMM only among its DDR4 parts, and Samsung's module page lists DDR5 RDIMMs up to 256GB and no LRDIMM product. We have not found a document stating that 3DS RDIMMs replaced LRDIMMs on DDR5; the point here is only that these platform documents list none.
5. 5.Check the speed you will get at your DIMMs-per-channel count. The step-down differs by platform: Dell's R750 matrix shows 3200 MT/s at both one and two per channel. Lenovo's SR650 V3 guide disagrees with itself. Its Memory options section gives, with 5th Gen Xeon, RDIMMs up to 5600 MHz and 3DS RDIMMs up to 5200 MHz at one per channel, RDIMMs or 3DS RDIMMs up to 4400 MHz at two per channel, and Performance+ RDIMMs up to 4800 MHz at two per channel (4th Gen: 4800 at one, 4400 at two); its Standard specifications row says up to 4800 MHz at two per channel without that qualifier. Plan on the lower figure until Lenovo settles it.
   ReadingAfter the upgrade, Configured Memory Speed in dmidecode, or OperatingSpeedMhz in Redfish, shows which figure your server chose.
6. 6.HPE ProLiant and AMD EPYC boards. We found no HPE document stating HPE's RDIMM and LRDIMM mixing rule, supported types or per-channel speeds (the QuickSpecs fetch timed out), and no AMD document stating which LRDIMM rank configurations EPYC 7002 or 7003 accept (the population guide fetch timed out). For those platforms this page gives no rule; use the server or board maker's own memory table and qualified list. In a Level1Techs thread about an ASRock Rack ROMED8-2T, replies leaned to RDIMM at one DIMM per channel and said LRDIMM mainly helps at high rank counts or 128GB-plus modules, which is a forum view. The EPYC reports in p3 are owners' experiences, not AMD's rule.

1. 1.If p1 found LRDIMMs in some populated slots and not in others, the classes are already mixed. Dell's R740 manual, Lenovo's SR650 guide and Intel's S2600WF rules forbid that, so take one class out before you buy anything, and buy only if the class that remains leaves too little capacity. What a mixed or failed upgrade does at POST and boot is covered on /questions/server-not-recognizing-ecc-ram, linked below.
   ReadingRe-run the filtered dmidecode command from p1 after any change: every populated slot's Type Detail line should name the same class.
2. 2.LRDIMM rank and channel trouble on EPYC boards, as reported. With eight dual-rank LRDIMMs on EPYC 7302 and 7252 (ASRock Rack EPYCD8-2T and ROMED8-2T), channels A and H stayed disabled wherever the modules sat; the poster said AMD lists only quad-rank LRDIMMs for EPYC 7002 but linked no AMD document. With eight SK Hynix HMAA8GL7MMR4N-UH LRDIMMs on a Supermicro H12SSL-NT, which channels failed changed with the CPU (EPYC 7502P against 7282), and swapping modules and cleaning contacts did not help; a reply suggested known-good RAM or a Milan CPU. Both threads are unresolved.
   ReadingNote which Locator names are missing in dmidecode, or which modules the BMC lists, and whether the gap moves when you swap the CPU or the modules.
3. 3.Check the board's own memory table before buying a large module. On a Supermicro H12SSL-i with an EPYC 7452, eight 16GB modules booted and eight 64GB modules did not; replies sent the owner to the manual's memory table and the qualified list, whose only 64GB entry was an RDIMM, and one advised RDIMMs over used LRDIMMs. No fix was confirmed.
4. 4.Allow for heat if you move to high-capacity LRDIMMs in a quiet case. One owner who swapped 64GB RDIMMs for 128GB LRDIMMs in a workstation case reported SK Hynix modules idling near 70C and passing 95C under load; a reply said DIMMs of that build need active cooling, and Samsung OEM modules ran cooler for him. That is one report, not a specification.

We have not read a primary source giving latency or bandwidth figures for one class against the other, so this page prints none. In one Level1Techs thread a reply posted a third-party benchmark chart and judged the difference small; that is a forum view. What the documents do set is speed per DIMMs-per-channel count, and on Dell's R750 both classes run at 3200 MT/s at one and two per channel.

The LRDIMM explainer at /server-ram/lrdimm covers the buffer and how an LRDIMM differs from a 3DS RDIMM; this page does not repeat it.

No. Dell's R750 lists 128GB as a quad-rank LRDIMM, Lenovo's SR650 offers 128GB 4Rx4 and 8Rx4 3DS RDIMMs, and Dell's R760 lists a 128GB 4R DDR5 RDIMM (marked end of life in that table). Take the class from the part number and your platform's table, never from the size.

On Intel's S2600ST boards a channel is capped at 8 logical and 10 physical ranks, and three quad-rank LRDIMMs in one channel must run in Rank Multiplication RM=2, which makes each look to the controller like a dual-rank DIMM with ranks twice as large. Intel also says LRDIMM Rank Multiplication and Direct Map modes must not be mixed. That is Intel-board documentation; the numbers do not carry over to other platforms.