memory: snapshot — R640 RAM harvest may zero the RAM line; CPU diff was missing

Operator has 2x Dell R640 at 768 GB each and asked whether the memory is
interchangeable with the R750xa. Both are DDR4 RDIMM platforms and Dell
does not vendor-lock DIMMs, so the answer is very likely yes.

- 16 slots in the R750xa x 32 GB = 512 GB, double the factory spec, and
  it deletes the 8x M04W6 purchase.
- Gating question is RDIMM vs LRDIMM. 24x 32 GB 2Rx4 RDIMM is the safe
  and most likely case; 12x 64 GB LRDIMM needs Ice Lake support checked.
- Cleanest harvest is to strip one R640 entirely and leave the other
  whole, rather than half-emptying both into unbalanced populations.

Separately, reading the factory CSV to answer this surfaced a gap in the
diff table: the reseller also swapped 2x Xeon Platinum 8362 (32C/64T,
265 W, DDR4-3200) for 2x Xeon Silver 4314 (16C/32T, 135 W, DDR4-2666).
The Silvers were recorded under "As bought" but never diffed, so the swap
went unremarked. Two consequences: the box cannot use the 3200 rating the
buy list was paying for, which makes 2666 R640 DIMMs a free lunch; and
the CPUs draw 260 W less, which the existing ~1,020 W power figure
already assumed correctly.

Memory-only; no version bump per the SemVer SKIP list.

persistent-memory.d/2026-09-01-ada-inference-server-r750xa.md

@@ -26,12 +26,27 @@ separate machine → [[2026-09-01-pfi-gx10-onboarding]].
 | **GPU cables** | `470-AEVS` 4-GPU kit: `4RW1P` `W4K7M` `XC48N` `6C77X` | **not listed** |
 | **Riser cables** | signal `CP67W` `CXYF8` `H4D7D` `N61TK`; power `HXJDR` `W1P56`; pad `C2JNP` | **not listed** |
 | **Fans** | 6× `FD00R` **Very High Performance** | **not listed** |
+| **CPU** | **2× Xeon Platinum 8362** 32C/64T, 265 W, **DDR4-3200** (`338-CCUS`/`W51V5`) | **2× Xeon Silver 4314** 16C/32T, 135 W, **DDR4-2666** |
 | **Controller** | PERC H345 | **HBA355i — an UPGRADE for us** |
 | **Drives** | 2× 480 GB | 2× 1.92 TB |
 
 ⚠ **"Not listed on the invoice" is not proof of removal.** Operator will pull an iDRAC
 inventory to settle it. Diff that against the CSV rather than the invoice.
 
+⚠ **The CPU row was MISSING from this table until 2026-09-01** — the Silver 4314s were
+recorded under "As bought" but never diffed against the factory CSV, so the swap went
+unremarked. It has two live consequences:
+
+- **Memory speed:** Silver 4314 caps at **DDR4-2666**; Platinum 8362 ran 3200. The box
+  **cannot use the 3200 rating** of the factory-spec `M04W6` parts on the buy list. Any
+  DDR4-2666 RDIMM performs identically in this chassis as configured.
+  ⚠ Verify the 2666 figure on Intel ARK before spending on it.
+- **Power, in our favour:** 270 W of CPU instead of 530 W. The ~1,020 W sustained figure
+  below already assumes the Silvers and is correct as written.
+
+Cores went 64C/128T → 32C/64T. For an inference/render box that is largely irrelevant; noted
+so nobody re-derives it as a surprise later.
+
 ## The GPU power chain — resolved, and my first answer was wrong
 
 **RTX 6000 Ada: 300 W, ACTIVE cooling, one 16-pin 12VHPWR (CEM5) connector** (nvidia.com).
@@ -116,11 +131,31 @@ circuit this lands on before racking, not after.**
 
 ## Buy list
 
-**Certain:**
+⚠ **THE RAM LINE MAY BE FREE — operator has 2x Dell R640 with 768 GB each (2026-09-01).**
+Both are DDR4 **RDIMM** platforms (factory `M04W6` = `DIMM,16GB,3200,2RX8,8G,DDR4,**R**`), so
+the part class is the same 288-pin 1.2 V DDR4 RDIMM and Dell does not vendor-lock DIMMs.
+Three things decide it, in order:
+
+1. **RDIMM vs LRDIMM.** 768 GB in an R640's 24 slots is almost certainly **24x 32 GB 2Rx4
+   RDIMM** — the safe case, drops straight in. If it is instead **12x 64 GB LRDIMM**, verify
+   Ice Lake-SP / R750xa LRDIMM support first; do NOT assume it.
+2. **Speed is a non-issue as configured.** R640 DIMMs are 2666 or 2933; the Silver 4314s cap
+   the box at **2666** regardless (see the CPU note above), so 2666 parts lose **nothing**
+   against the 3200-rated `M04W6`. ⚠ It WOULD become a cap if the Platinum 8362s were ever
+   restored — the factory CPUs ran 3200.
+3. **Slot arithmetic.** R750xa has **16** slots (8 channels/socket, 1 DPC); R640 has 24.
+   **16x 32 GB = 512 GB — double the factory 256 GB spec**, and it kills the 8x `M04W6` line
+   entirely. Cleanest harvest: **strip ONE R640 of all 24, use 16, keep 8 as spares**, leaving
+   the second R640 whole — rather than half-emptying both into unbalanced populations.
+
+⚠ **Confirm the actual DIMM part numbers from iDRAC or the DIMM labels before ordering
+anything.** And confirm the R640s are spares, not in service.
+
+**Certain (pending the R640 harvest above, which may delete the RAM line):**
 
 | item | part | qty |
 |---|---|---|
-| RDIMM 16 GB 3200 2Rx8 | **`M04W6`** | **8** → restores 256 GB, all 16 slots, all 8 channels/socket |
+| RDIMM 16 GB 3200 2Rx8 | **`M04W6`** | **8** → restores 256 GB. **Buy only if the R640 harvest fails**; 16x 32 GB from an R640 gives 512 GB free |
 | NVIDIA 12VHPWR adapter | **`930-00030-1546-000`** | **2** |
 | 2 TB SATA SSD (any; match/exceed the MX500s) | — | **2** → pool ~3.57 TiB at ~41%; without it the migration lands at ~84%. Pair the two NEW drives together, not one-each with a 1.92 |
 
@@ -128,7 +163,8 @@ circuit this lands on before racking, not after.**
 `XC48N`, `6C77X`, `CP67W`, `CXYF8`, `H4D7D`, `N61TK`, `HXJDR`, `W1P56`, `C2JNP`,
 `FD00R` ×6.
 
-**Spend order:** RAM → **2 SATA SSDs (now certain, capacity-driven)** → check PCIe slots →
+**Spend order:** **check the R640 DIMM part numbers FIRST (may zero the RAM line and double
+the RAM)** → RAM only if that fails → **SATA SSDs (certain, capacity-driven)** → check PCIe slots →
 only then consider filling the remaining bays. (The original order deprioritised drives on a
 *throughput* argument, which still holds; the drives are now on the certain list for
 **capacity**, which is a different axis.)