new tasks, model pricing, auto quantisation, etc...

docs/issues/44-tracker-shard-source-partial-download.md

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+# US-044 — Tracker as model-file source; nodes download only their shard
+
+Status: planned
+Priority: High (blocks multi-machine big-model serving; pairs with US-042)
+Stage: Designed (grill remaining decisions before build)
+
+## Context
+
+Common deployment: the tracker and the first node share a machine that already
+holds the model files (e.g. `/run/media/popov/DATA/llm/safetensor`). When a
+second node joins with no model selected, the tracker assigns it the uncovered
+layer range — and today that node then downloads the **entire snapshot from
+HuggingFace**, even for a 20-layer shard of a 160 GB model.
+
+What exists already (build on it, don't duplicate):
+
+- Nodes serve their shard dir as a tar at `GET /v1/shards/download` with
+  checksum verification; `download_shard` tries assignment-provided `peers`
+  before HF (`downloader.py`). But it only matches **identical layer ranges**,
+  and the HF fallback runs `snapshot_download` of the whole repo.
+- The torch path (`--model-id`) bypasses `download_shard` entirely:
+  `TorchModelShard` → `from_pretrained` downloads **and loads into RAM** the
+  full model, then executes only the assigned layers. Sharding currently saves
+  compute, not memory or bandwidth.
+
+## Design
+
+1. **Tracker `--models-dir PATH`** (env `MESHNET_MODELS_DIR`). When set, the
+   tracker indexes HF-layout snapshots under it and advertises itself as a
+   model-file source in `/v1/nodes/assign` responses.
+2. **Layer-aware file selection.** For safetensors models, read
+   `model.safetensors.index.json` and map the assigned layer range → the
+   subset of weight files containing those layers, plus the always-needed
+   files (config, tokenizer, index, embeddings/head files for head/tail
+   shards). Serve exactly that subset (tar stream, per-file checksums).
+   GGUF (US-042): single file or naive byte-range — phase 2.
+3. **Node download order**: exact-shard peer (existing) → tracker/peer file
+   subset (new) → HF `snapshot_download` with `allow_patterns` for the same
+   subset (new — stop downloading the whole repo even from HF) → full snapshot
+   (last resort).
+4. **Partial LOAD (the hard half).** Downloading a subset is wasted unless the
+   node stops instantiating the full model: build the model skeleton on the
+   `meta` device, materialize only assigned layers (+embeddings/norm/head as
+   role requires) from the local files, leave the rest on meta. Without this,
+   an 80 GB machine can never hold a shard of a 160 GB model regardless of
+   how the bytes arrive. This is the acceptance bar for the issue.
+
+## Open questions (grill before building)
+
+- Trust: joining nodes fetch weights from the tracker/peers — checksum against
+  what root of trust? (HF etag/sha vs tracker-signed manifest.)
+- Disk layout: partial snapshots must not corrupt the HF cache dir; probably
+  a meshnet-owned layout keyed by repo+revision.
+- Serving cost: a 100 GB tar stream per joining node on the tracker box —
+  rate-limit/queue? LAN-only heuristic?
+
+## Acceptance criteria
+
+- Two-machine test: machine A (tracker + node, holds full snapshot) serves
+  layers 0–k; machine B joins with no model and receives **only** the files
+  for its assigned range from A — nothing fetched from HF
+- Machine B's resident memory scales with its shard size, not model size
+- Checksums verified end-to-end; corrupted transfer falls back cleanly
+- Single-node/full-model flows unchanged
+- `python -m pytest` passes from repo root