Dell PowerEdge R640 Upgrade Guide: RAM and Storage

Lifecycle and support

Standard support has ended; paid extended support runs until April 30, 2027 (estimate).

Dates not listed haven't been published by the manufacturer or a reliable third party.

RAM: what fits and the limits

The R640 has 24 DIMM slots: 12 per processor, arranged as six channels per CPU with two slots per channel. In each channel the first socket has white release tabs and the second has black. It takes full-size DDR4 registered ECC memory only. Dell's documents never mention SO-DIMM, and no memory is soldered on.

You can use RDIMMs or LRDIMMs, but not both: they can't be mixed within a channel, memory controller, socket or across sockets. RDIMMs go up to 64 GB per module, about 1.5 TB across 24 slots. Dell's spec sheet and technical guide give LRDIMMs as 128 GB per module and 3 TB total. One Dell memory table lists 512 GB LRDIMMs and totals of 6 TB (one CPU) or 12 TB (two CPUs). The documents disagree, so treat 3 TB as the safe figure unless Dell confirms more for your BIOS and CPU.

Speed depends on your CPU. With 2nd Gen Xeon Scalable CPUs, dual-rank 16, 32 and 64 GB RDIMMs run at 2933 MT/s, while single-rank 8 GB RDIMMs and LRDIMMs run at 2666. First-gen Xeon Scalable CPUs are listed at 2666 for both RDIMM and LRDIMM. With only one CPU installed, just 12 slots are usable, so maximum capacity is halved (for example, 768 GB with 64 GB RDIMMs).

NVDIMM-N and Intel Optane persistent memory (PMem) are also supported, but they are specialist parts, not ordinary RAM. PMem needs 2nd Gen CPUs and specific modes. NVDIMM-N isn't supported with a single processor, and 8 GB RDIMMs and NVDIMM-N must not be mixed. Follow Dell's manual for those setups.

Buying tips

• Check which CPU generation your unit has before expecting 2933 MT/s.
• Buy registered ECC DDR4 RDIMMs, and don't mix in LRDIMMs.
• Confirm a second CPU is fitted before planning for more than 12 modules.
• Fill the slots in the order given in Dell's population guidelines. Ignoring them can cause a failed boot, a hang or reduced memory.
• Buy matching modules in sets rather than one-off odd sizes.

Storage: drive bays, NVMe, BOSS and microSD

The front bay layout depends on the backplane your server has. Dell lists options of up to 10 x 2.5-inch SAS/SATA (up to 8 of them NVMe-capable), up to 10 x 2.5-inch NVMe, or up to 4 x 3.5-inch SAS/SATA hard drives. Dell's technical guide also mentions 8-bay and other variants. We couldn't confirm whether the backplane can be changed after purchase, so check what your unit has before buying drives. Some units also have an optional rear cage for 2 x 2.5-inch drives, depending on CPU and riser configuration. It is not on every unit.

NVMe drives work only on NVMe-capable backplanes and run at PCIe Gen3 speed, so a Gen4 drive won't go faster. All drives need a Dell drive caddy. The bays are tool-less hot-swap. Dell's documents we used don't state drive thickness limits (such as 7 mm versus 15 mm), so check the owner's manual before buying.

Two boot-oriented options exist. The optional BOSS card holds 2 x M.2 SSDs in hardware RAID (offered in 240 GB and 480 GB), but the M.2 length and whether it is SATA or NVMe aren't stated in Dell's documents we used, so confirm before buying replacements. The internal dual microSD module (IDSDM/vFlash) is Dell-proprietary and meant for hypervisor or boot use. It takes two microSD cards of 16, 32 or 64 GB for IDSDM, or one 16 GB card for vFlash, and it is not for general storage. Its write-protect switch is on the module.

The largest capacity figures Dell quotes (such as 76.8 TB or 64 TB) reflect the biggest drives Dell listed, not a hard limit.

Buying tips

• Look at your front bays first: 2.5-inch and 3.5-inch units use different drives and caddies.
• Only buy NVMe drives if your backplane is NVMe-capable.
• Buy Dell caddies together with your drives if you don't already have them.
• Check drive thickness in Dell's manual before ordering.
• Treat microSD cards as boot media only, and use 16, 32 or 64 GB cards.

How to upgrade

Before you start

• Find out which CPU generation and how many CPUs your unit has, since this decides memory speed and how many DIMM slots work.
• Check your front backplane type (8 or 10 bays, 2.5-inch or 3.5-inch, NVMe-capable or not).
• Have Dell's owner's manual open for exact disassembly and population guidelines.
• Check your BIOS and firmware are up to date, and look at Dell's support page for your service tag.
• Have an anti-static wrist strap or mat ready, and make sure you have a full backup.

Steps

1. Back up your data
   Back up anything important and note your current RAID and BIOS settings before you change hardware.
2. Power off and unplug
   Shut the server down properly, then remove all power cables and wait for the unit to power down fully. Drives in the front bays are hot-swap, but DIMM slots are not hot-pluggable, so memory and CPU work needs the server fully off.
3. Ground yourself
   Use an anti-static strap or touch bare metal on the chassis before handling parts. Hold DIMMs and cards by their edges.
4. Open the case
   Follow the owner's manual for removing the top cover and any airflow shroud. Dell's manual has the exact procedure for your unit.
5. Install the part
   For memory, open the release tabs (white for the first socket in each channel, black for the second), seat the module in the slot Dell's population guidelines call for, and close the tabs. For drives, fit the drive into a Dell caddy and slide it into a bay. For the microSD module or BOSS card, follow the manual's instructions.
6. Close up and reconnect
   Refit any shroud and the top cover, then reconnect the power and start the server.
7. Check it is recognised
   For memory, press F2 at start-up, then go to System Setup > System BIOS > Memory Settings and confirm the capacity. For drives, check the BIOS, your RAID controller or your operating system to confirm the new drive appears.

Good to know

• Dell's documents we used give no CPU compatibility list, so check Dell's manual and your BIOS version before buying a faster processor.
• Dell's documents disagree on the maximum LRDIMM size: most say 128 GB per module (3 TB total), but one table lists 512 GB modules. Treat 3 TB as safe unless Dell confirms more for your unit.
• Dell's spec sheet lists only 2nd Gen Xeon Scalable CPUs, but its technical guide also covers first-gen CPUs (limited to 2666 MT/s). Check your unit's CPU generation before expecting 2933 MT/s or PMem support.
• Drive thickness limits aren't stated in the documents we used, and we couldn't confirm whether the front backplane can be swapped after purchase.
• The BOSS card's M.2 length and interface (SATA or NVMe) aren't given in the documents we used.
• Rear drive bays and some NVMe options depend on CPU and riser configuration and aren't on every unit.
• Dell publishes no model-wide end-of-sale or end-of-support dates, and third-party dates conflict. Check your service tag on Dell Support for your unit's support status.

Frequently asked questions

How much RAM can the Dell PowerEdge R640 take?

With two CPUs and all 24 slots filled, up to about 1.5 TB with 64 GB RDIMMs, or 3 TB with 128 GB LRDIMMs per Dell's spec sheet. One Dell table lists larger LRDIMMs, but Dell's documents disagree, so 3 TB is the safe figure. With one CPU only 12 slots work, which halves these totals.

What type of RAM does the R640 use?

DDR4 registered ECC memory (RDIMM or LRDIMM). Dell's documents don't mention SO-DIMM. NVDIMM-N and Optane persistent memory are also supported in specific configurations.

Can I mix RDIMMs and LRDIMMs?

No. They can't be mixed within a channel, memory controller or socket, or across sockets. 8 GB RDIMMs and NVDIMM-N also must not be mixed.

Does the R640 support NVMe SSDs?

Yes, but only on NVMe-capable front backplanes (and some rear-bay setups, depending on CPU and riser configuration). NVMe drives run at PCIe Gen3 speed. Drives need a Dell caddy.

What memory speed does the R640 run at?

Up to 2933 MT/s. That needs 2nd Gen Xeon Scalable CPUs and, for RDIMMs, dual-rank 16, 32 or 64 GB modules. 8 GB single-rank RDIMMs and LRDIMMs run at 2666 MT/s, and so do all modules with first-gen CPUs.

Can I use the microSD slot as extra storage?

No. The internal dual microSD module is Dell-proprietary and meant for hypervisor or boot use. It takes two 16, 32 or 64 GB cards for IDSDM, or one 16 GB card for vFlash.

Official documents and references

• PDF Dell PowerEdge R640 spec sheet
  Official memory, drive and BOSS specifications
• PDF Dell PowerEdge R640 technical guide
  Memory speed tables, drive options and controllers
• PDF Dell PowerEdge R640 installation and service manual (PDF)
  Disassembly steps and memory population guidelines
• PDF Dell EMC R640 installation and service manual (PDF)
  Copy of the manual with microSD module details
• Dell R640 memory specifications
  Module types, mixing rules and single-CPU limits
• Installing a memory module
  Install steps and BIOS memory check
• R640 system memory guidelines
  Population rules including NVDIMM-N and PMem
• PowerEdge rack and tower servers: specifications, documentation and videos
  Dell's index of hardware documentation