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navi-offroute: HPA spec — fix NFS wording to virtiofs (PR #43 amend)

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mj 2026-05-27 16:23:42 -06:00
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backend/services/navi_offroute/HPA-SPEC.md
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@ -186,7 +186,8 @@ CREATE TABLE meta (key TEXT PRIMARY KEY, value TEXT); -- build provenance, mode
convention, region boundary polygon/bbox, and build timestamp/commit.
- **Idaho-size estimate:** ~96K chunks × 400 entrance pairs × 4 modes × ~50 bytes/row
≈ **~1.5 GB SQLite**. (Sparse: impassable pairs absent, so the real file is smaller.) This
confirms the storage analysis already approved.
confirms the storage analysis already approved. `/mnt/nav/hpa/<region>.db` lives on the same
fast virtiofs (host NVMe) as the DEM and the other readers — local-disk speed, not network.
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@ -319,7 +320,7 @@ Each phase ≤ its own diff budget, HALT-at-diff at every boundary.
| **Stale tiles after DEM / mode-profile change** | Precomputed costs no longer match the live cost model. | `meta.mode_profile_hash` (hash of `MODE_PROFILES`) + DEM version; mismatch → fall back + log a rebuild-required warning. |
| **Transition cells moving between chunks on a DEM regrid** | Parking/trailhead chunk membership could shift, breaking stored transition entrances. | Chunk-coord origin pinned at lat=0/lon=0 modulo 1.5 km (§3): chunk identity is deterministic and regrid-stable. |
| **Mode-switch edges across chunk borders** | A parking lot on a chunk boundary belongs to two chunks; a switch there must be reachable from both. | The transition cell is added as an entrance to **both** neighbouring chunks (overlap, §5). |
| **SQLite read latency at query time** | Per-chunk cost lookups over NFS could add latency. | Costs are scalar and few (≤400 rows/chunk/mode); index `idx_chunk_mode`; batch-load the covered chunks once per query; the DB is small and OS-page-cacheable. |
| **SQLite read latency at query time** | Per-chunk cost lookups over virtiofs (host NVMe) — sub-ms cold, single-µs cached. SQLite-over-virtiofs is local-disk fast, not network-bound. | Costs are scalar and few (≤400 rows/chunk/mode); index `idx_chunk_mode`; batch-load the covered chunks once per query; the DB is small and OS-page-cacheable. |
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