memory: snapshot — Ada sizing corrected; prune and drives are orthogonal

comfy-dev's disk-vs-catalog diff and a recount of my own figures both
landed on this thread. Two numbers were wrong and both were headed for
the operator's sizing conversation.

- My ~90% was a double-count. I read ALLOC 1.45T while their pull was
  running and then added the full ~112 GB on top; most of it was already
  in that reading. "Onboarded" is not "landed". Settled payload is
  ~1.47 TiB and the as-bought mirror lands at 84%, not 90%.
- comfy-dev's "pruning gets us nearer 45%" is the striped figure. On the
  as-bought pair mirrored, deleting all ~215 GiB of unreferenced weights
  still lands at 72%, with ~140 GiB of runway on a store that took on
  ~100 GiB in one day. The constraint is vdev layout, not payload — a
  1.75 TiB pool stays 1.75 TiB whatever goes in it.
- So the prune audit and the drive purchase are independent decisions
  and neither gates the cutover. Presenting them to the operator that
  way rather than as a trade.

Also recorded: pool arithmetic (1.75 / 3.49 / 3.57 TiB), the mirror-vdev
smallest-member gotcha if the new drives get paired one-each with the
1.92s, comfy-dev's 34 GiB of uncatalogued LTX 2.5, and an open question
back to them on whether the H3 encoder's nvfp4 pin was set under a
Blackwell assumption that sm_89 does not satisfy.

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

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

@@ -75,17 +75,26 @@ both cheaper and likely faster than four more SATA drives.
 
 ⚠ **CAPACITY, not throughput, is the binding constraint — the as-bought drives are SMALLER
 than the pool they receive from** (measured 2026-09-01). ComfyUI's `/storetank` on irv-ml1 is
-a 2x 2 TB mirror: SIZE **1.81 TiB**, ALLOC **1.45 TiB**, CAP **80%**, and growing as
-comfy-dev's ~112 GB batch lands (-> ~1.56 TiB). The R750xa's 2x **1.92 TB** mirrored is only
-**~1.74 TiB** — the migration would arrive at **~90% full** with zero growth room, past the
-~80% line where ZFS allocation degrades. Compression buys nothing: safetensors measured
+a 2x 2 TB mirror: SIZE **1.81 TiB**, ALLOC **1.45 TiB**, CAP **80%**. Settled payload
+**~1.47 TiB**. The R750xa's 2x **1.92 TB** mirrored is only **~1.75 TiB** — smaller than the
+source pool — so the migration arrives at **~84% full** with no growth room, past the ~80%
+line where ZFS allocation degrades. Compression buys nothing: safetensors measured
 `compressratio 1.00x`, `logicalused == used`.
 
 **-> Add 2x 2 TB SATA SSD to the buy list.** Six of eight bays free, HBA355i has the ports;
-two mirror vdevs striped = **~3.49 TiB at ~45%**, redundancy intact. Cheapest line on this
-buy list and it does not gate the cutover window. The no-spend alternative is striping the
-as-bought pair (same ~3.49 TiB, **no redundancy**) — only acceptable if irv-ml1 retains its
-copy, which makes retain a requirement rather than a recommendation.
+two mirror vdevs = **~3.57 TiB at ~41%**, redundancy intact. Cheapest line on this buy list
+and it does not gate the cutover window. The no-spend alternative is striping the as-bought
+pair (~3.49 TiB, **no redundancy**) — only acceptable if irv-ml1 retains its copy, which makes
+retain a requirement rather than a recommendation.
+
+⚠ **Pruning does NOT substitute for the drives.** comfy-dev found ~215 GiB of unreferenced
+weights; deleting every byte still lands the as-bought *mirror* at **72%**. The constraint is
+**vdev layout**, not payload — a 1.75 TiB pool stays 1.75 TiB whatever goes in it. The prune
+audit and the drive purchase are independent decisions.
+
+⚠ **Build-time: pair like with like.** A mirror vdev caps at its smallest member; pairing each
+new 2 TB with an existing 1.92 TB caps both vdevs at 1.92 TB and wastes ~150 GiB. Correct:
+the two 1.92s as one vdev, the two new drives as the other.
 → [[2026-09-01-ada-migration-branch-a]]
 
 **UNCHECKED, and it may moot the whole bay question:** free PCIe slots. Riser Config 0 is
@@ -113,7 +122,7 @@ circuit this lands on before racking, not after.**
 |---|---|---|
 | RDIMM 16 GB 3200 2Rx8 | **`M04W6`** | **8** → restores 256 GB, all 16 slots, all 8 channels/socket |
 | NVIDIA 12VHPWR adapter | **`930-00030-1546-000`** | **2** |
-| 2 TB SATA SSD (any; match/exceed the MX500s) | — | **2** → pool ~3.49 TiB at ~45%; without it the migration lands at ~90% |
+| 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 |
 
 **Only if the iDRAC inventory shows them absent:** `12XPY`, `9TR6X`, `4RW1P`, `W4K7M`,
 `XC48N`, `6C77X`, `CP67W`, `CXYF8`, `H4D7D`, `N61TK`, `HXJDR`, `W1P56`, `C2JNP`,