REPORT + CONTEXT: v0.157.0 live proof on the three test boxes; not vouched

Claude-Session: https://claude.ai/code/session_012qRErfCoiTkvDK9N5XHbzb

.claude/rules/unprompted-work.md

@@ -44,8 +44,12 @@ it **first** in the morning note. A decision you cannot write in that shape is o
    Throwaway apps only; the standing apps and `bentopdf` stay.
 5. **Evidence off the machine at the end of each phase**, before any revert (R-320).
 6. **One release per repo per session**, with a CHANGELOG entry (controller: with its `MinAgent`
-   line), REPORT overwritten, floor raised to deliver it. **No golden unless a drill or fresh install
+   line) and REPORT overwritten. **A controller release needs no golden** (operator ruling 2026-10-15
-   needs one** (the waiver, R-468). **No `--no-verify`.**
+   11:26, `09` §3 decision 250; decision 7 stands): push the image only when it is ready for EVERY box (the
+   household's update button offers the highest tag); raise the floor, MinAgent declared, only after it ran
+   on Tester 1, demo-hp and demo-felhom through the normal update path and the box walk's checks are green
+   on all three — not at night unless the night brief says so. **Agent releases still stop for the
+   operator's vouch.** The golden is baked weekly and vouched by the operator; CC never vouches. **No `--no-verify`.**
 7. **An enumerated gap becomes a row in the same session — or, if it is small, is fixed in it** (the size rule).
    Prose is not a record.
 8. **Hungarian text is searched with ASCII fragments**, with a positive and a negative control.

CHANGELOG.md

@@ -1,3 +1,60 @@
+## v0.157.0 — every storage place knows its physical disk(s); POST /disks/same-disk (2026-10-15)
+
+Design: felhom.eu `architecture/16-storage-physical-identity.md` §3, §4, §10 (operator rulings D1–D6, 2026-10-15). Spike:
+felhom.eu `audits/storage-identity-2026-10-15/`.
+
+- **Identity of a storage place** (`internal/storage/identity.go`): the physical disk(s) under it, from `/sys`, `/run/udev/data`
+  and `/proc/mounts` only — partition → disk, device-mapper / LVM thin → slaves, loop → backing file → its disk. Keys:
+  `eui:` (NVMe), `serial:<bus>:<model>:<serial>`, `wwn:` only without a serial and only with a vendor (OUI ≠ 000000).
+  `proven`, or `unknown` with one reason: `vm` (systemd-detect-virt), `network`, `usb_bridge` (a placeholder serial — the rule
+  is the design's, UNPROVEN on a real enclosure, R-1077), `weak_wwn`, `no_serial`. No block I/O.
+- **Reported:** `GET /disks` rows and the hub report's `storage_targets` gain `kind`, `physical_disks`, `identity`,
+  `identity_reason` (additive; the golden and the hub's copy change together). A dir storage is judged only on its OWN mount — an
+  absent drive is "not computed", never the root disk.
+- **`POST /disks/same-disk`** `{a, b}` (guest paths) → `{different, proven, reason, a, b}`. Maps `/mnt/felhom-drives/<name>` to
+  the host mount at exactly that path, and `/mnt/sys_drive`, `/var/lib/felhom`, `/var/lib/docker` to the guest's data volume's
+  storage. Anything else, or an absent drive → `unmapped`. A shared disk key is "same, proven" even when the disk is unknown.
+  Scoped like every `/disks` call.
+- Tests: fixture `/sys` + udev trees for every spike case (one disk LVM thin, two disks, two partitions, a loop on a file, NVMe eui,
+  vendor-less WWN, a VM, a USB placeholder serial, a network share) and the same-disk call (equal, different, VM, NAS, absent
+  drive, scope); red-proofed (the LVM "diamond" bug was seen failing first; the VM, WWN, own-mount and sys_drive rules broken on purpose).
+- `configs/` unchanged — no config-bundle job needed. **Not vouched** (the operator's act).
+
+## Unreleased (2026-10-15, afternoon) — the shared rule file; no code change
+
+- `.claude/rules/unprompted-work.md` §3 item 6: **a controller release needs no golden** — the floor carries it after the
+  three test boxes; agent releases still wait for the operator's vouch; the golden is weekly (operator ruling 2026-10-15
+  11:26, `09` §3 decision 250; the same morning's „bake first", decision 247, reversed). Same text in all five copies.
+
+## Unreleased (2026-10-15) — the shared rule file; no code change
+
+- `.claude/rules/unprompted-work.md` §3 item 6: **bake first** — a controller release is released only when its golden is
+  baked; bake → the operator vouches → the operator raises the floor; never ask for a floor for a version with no golden
+  (operator ruling 2026-10-15, `09` §3 decision 247, superseding decision 7). Same text in all five copies.
+
+## v0.156.1 — R-1008 by process liveness, not the clock; 0.156.0 superseded — do not vouch it (2026-10-14)
+
+Released by `scripts/release-agent.sh`: binary sha256 `83c7da27d457b63ba8a4cde5a8717b3e5f9db9f83a46e63f38902dc7471bffb9`, bundle
+`5a678def792c70ec1cbbd198c14542161fac91d11a69bd0ce23dd86469e96ec3` (tag `v0.156.1` = `7ae3081`). Step bundle `0.156.1-step1`
+sha256 `ac83ea41507616d4c1056cccfd35de1410ff01e0296f4e64ff925207a43a8490` (the 0.155.0 bundle with only `felhom-os-apply`
+replaced), published as package version `0.156.1-step1`. **Not vouched** (the operator's act). Delivery as 0.156.0: per box
+`agent_update 0.156.1` → `agent_config_update 0.156.1-step1` → `agent_config_update 0.156.1`.
+
+- **0.156.0 is superseded — never vouch it.** A background security review of the pushed commit flagged „fail-open state
+  drift": 0.156.0 called a listed vzdump task dead when its wall-clock start was before the kernel's `btime`. `btime` is
+  derived from the wall clock and moves when the clock is corrected after the boot (an NTP step after a power cut), so a
+  backup started between the boot and the correction could look „from before the boot" — and the stale-lock pass would
+  clear a LIVE backup's lock.
+- **Now:** `taskAlive` is Proxmox's own rule (`PVE::ProcFSTools::check_process_running`, read on demo-hp): the UPID's pid and
+  pstart against `/proc/<pid>/stat` — same start tick (field 22) and not a zombie. No clock is involved. Fails safe (running)
+  when the UPID does not parse, the stat cannot be read, or this agent cannot see root's processes at all (PID 1 not visible:
+  a `hidepid` /proc or `ProtectProc=` would hide every worker — demo-hp: `/proc` plain, `ProtectProc=default`).
+- Tests: worker gone, pid reused, zombie, live worker, a „clock step" (wall-clock start absurdly early, worker alive → still
+  running), doubt, hidden /proc — each red-proofed; 19 guards in all (`power-cut-fixes-2026-10-14/redproof.txt`).
+- R-1006 (the cleanup) is unchanged; its „before this boot" rule still reads `btime`, but a drifted `btime` cannot remove a
+  live partial: the Go side asks this liveness rule, and the root helper refuses while any vzdump runs or anything holds
+  the file open.
+
 ## v0.156.0 — after a power cut: the dead half-written backup is removed (R-1006); a backup task from before the boot is not „in flight" (R-1008) (2026-10-14)
 
 Released by `scripts/release-agent.sh`: binary sha256 `8db2278838dcb733d5201fba84b75a829b9adebdf899c89dd7de0939ec10d51a`, bundle

CONTEXT.md

@@ -1,5 +1,11 @@
 # CONTEXT — felhom-agent working state
 
+> **2026-10-15 (night) — v0.157.0 RELEASED, NOT VOUCHED (storage identity build A, felhom.eu `16` §13).** Every storage
+> place carries `kind`, `physical_disks`, `identity` (+ reason) in `GET /disks` and the hub report; `POST /disks/same-disk`
+> answers the controller (v0.317.0, MinAgent 0.157.0). Computed on every read, never cached. Delivered by signed jobs to
+> Tester 1, demo-hp, N100 only. Next: the operator vouches 0.157.0 → the global floor 0.317.0 / MinAgent 0.157.0. Build B
+> (partition candidates, the internal place) waits for a task file (R-1079).
+
 > **2026-10-08 (day) — UNRELEASED on main, ships with tomorrow's release (decision 178).** R-899: `POST /backup?trigger=manual`
 > runs no OS leg (`localapi/server.go`); after-boot kernel reports carry the saved block's ring (`kernelReportRing`). R-304:
 > `escrow.ErrRetainedUnchecked` → HTTP 424 `older_unchecked` when not every earlier package was tried (withheld, caps,

REPORT.md

@@ -1,38 +1,43 @@
-# REPORT — v0.156.1: after a power cut the box clears up (R-1006, R-1008) (2026-10-14)
+# REPORT — v0.157.0: every storage place knows its physical disk(s); POST /disks/same-disk (2026-10-15)
 
-Evidence: felhom.eu `documentation/audits/power-cut-fixes-2026-10-14/` — the spike (C1 the night's leftover, C2 a killed
+Written in ASD-STE100 Simplified Technical English. Design read first: felhom.eu `architecture/16-storage-physical-identity.md`
-worker, C3 a real power cut, C4 a full disk), the 17 red-proofs.
+(§3, §4, §8, §10; ACCEPTED 2026-10-15, §13 = what was built), `03-host-agent.md`. Task: storage identity build A (operator
-
+rulings D1–D6). Controller counterpart: felhom-controller v0.317.0 (MinAgent 0.157.0); hub v0.154.0 shows the fields.
-## What the spike showed
-
-- A power cut mid-backup leaves `vzdump-lxc-<vmid>-<date>.tar.dat` + `.tmp/` (root, in `/var/lib/vz/dump`); Proxmox never
-  lists it as a backup; nothing removes it; every later backup is skipped „not enough space".
-- The dead vzdump task stays in Proxmox's active list ~30–60 s after the boot — long enough for the agent's start-up pass
-  to call it „genuinely in flight" (R-1008). Short, so R-1008's harm is small.
-- A killed worker leaves `tar`/`zstd` writing into the `.tar.dat` for minutes, none named „vzdump" — the reason the root
-  helper checks open files, not only process names.
-- On „No space left" vzdump removes its own partial (both paths): the preflight's sizing stays. But a full host root breaks
-  LVM's own metadata writes (a half-removed snapshot; the janitor cannot clear it — new row).
-
-## 0.156.0 is superseded — do not vouch it
-
-A background security review of the pushed 0.156.0 flagged „fail-open state drift": its in-flight rule compared a task's
-wall-clock start with the kernel's `btime`, which moves when the clock is corrected after the boot (an NTP step after a
-power cut) — a live backup could then look „from before the boot" and its lock be cleared. 0.156.1 uses Proxmox's own
-liveness rule instead (the UPID's pid + pstart against `/proc/<pid>/stat` start tick and state; no clock), and fails safe
-when this agent cannot see root's processes (PID 1 not visible).
 
 ## What changed
 
-See `CHANGELOG.md` v0.156.0: `vzdumpInFlight` behind the one `BackupRunning`; `internal/vzpartial` at start-up and before
+- `internal/storage/identity.go`: the physical disk(s) under a place, from `/sys`, `/run/udev/data`, `/proc/mounts` only
-each backup; the root helper `felhom-vzdump-partial-clean` + one sudoers line + the bundle entry.
+  (no block I/O). `proven`, or `unknown` with `vm` / `network` / `usb_bridge` / `weak_wwn` / `no_serial`.
+- `GET /disks` rows and the hub report's `storage_targets` gain `kind`, `physical_disks`, `identity`, `identity_reason`.
+- `POST /disks/same-disk` `{a, b}` → `{different, proven, reason, a, b}`; scoped like every `/disks` call; a path the agent
+  did not mount itself is `unmapped`.
+- `configs/` unchanged.
 
-## Gates
+## Tests and red-proofs
 
-`go build ./... && go vet` clean; `go test ./... -count=1` rc 0; `agent_gates.py` all OK; `visudo -cf` parsed OK; the
+Fixture `/sys` + udev trees for every spike case and five same-disk handler tests. Seen failing first: the LVM thin
-config bundle tests (55) and the helper's tests (21) pass. 17 mutations, 17 red.
+„diamond" (tmeta/tdata under one pool) read as a failure — fixed by counting a seen node once. Broken on purpose and seen
+failing: the VM rule, the vendor-less WWN rule, the own-mount rule (an absent drive), the `sys_drive` mapping.
+`python3 scripts/agent_gates.py`: all OK.
 
-## Next
+## Release and delivery
 
-Released: tag `v0.156.0`, binary sha256 `8db22788…`, bundle `a2160521…`, step bundle `0.156.0-step1` `8be2bb8f…`. Then the operator's vouch + floor, then the live proof on demo-hp (a cut mid-backup) and
+- `scripts/release-agent.sh 0.157.0`: tag `v0.157.0`, sha256 `addd4041…0aef`, bundle `0604bc60…397a`.
-delivery to Tester 1 and the N100.
+- Delivered by signed `agent_update` jobs to Tester 1, demo-hp and the N100 (the three test boxes). **NOT VOUCHED** — the
+  operator's act. Until the vouch, the global floor cannot declare MinAgent 0.157.0.
+
+## Live proof (method: the agent's local API on each PVE host, with the guest's own token)
+
+| Box | `GET /disks` | same-disk: drive vs system data |
+|---|---|---|
+| demo-hp | `local`/`local-lvm` proven (SATA SSD), NVMe drive proven (`eui:`), PBS `network` | different, proven |
+| N100 | system proven (SATA SSD serial), USB drive proven (the bridge passes the real serial) | different, proven |
+| Tester 1 (VM) | every place `unknown/vm` | not proven (`vm`) |
+
+On all three: system data vs `/var/lib/docker` → same disk, proven; a drive name that is not mounted → `unmapped`. The hub
+stored the same fields (read from its database with the WAL). Evidence: felhom.eu `audits/storage-identity-buildA-2026-10-15/`.
+
+## Observations
+
+1. FILED: R-1083 — scratch box 9202 has no `local_api` in its `bootstrap.json`, so it cannot use any agent call.
+2. NOT-A-FINDING: `usb_bridge` is built from the design's rule and was not seen on a real enclosure; that is R-1077, already open.

REUSE.md

@@ -41,6 +41,7 @@
 | `storage.ResolveStorageDevice` | internal/storage/durable_device.go | `ResolveStorageDevice("uuid:<fs-uuid>") (dev, err)` | re-resolve enrolled STORAGE drives (remount) | `uuid:` scheme ONLY — distinct from the wipe schemes; never trust a remembered /dev node |
 | `deriveDurableID` | internal/storage/durableid.go | `deriveDurableID(typ, s, backingDevice, uuid) string` | storage-target durable id (DR re-attach key) | Deterministic per type; `uuid:` for usb/local-dir; PBS id includes `#<fingerprint>` |
 | `SystemDisks` / `isSystemBacked` / `RoleForStorage` / `RoleForRawDevice` | internal/storage/role.go | `RoleForRawDevice(device, sysDisks, sysKnown) DeviceRole` | protection-tier classification | Fails safe to `system` (most protected) on any ambiguity; role is AGENT-derived, never caller-supplied |
+| `storage.IdentityResolver` / `Relation` / `MountFor` | internal/storage/identity.go | `HostPath(path) PlaceIdentity`, `Device(dev) PlaceIdentity`, `Relation(a, b) (different, proven, reason)` | physical-disk identity of a storage place (felhom.eu `16-storage-physical-identity.md` §3, v0.157.0) | Reads /sys + /run/udev/data + /proc/mounts only (no block I/O). Judge a drive ONLY on its own mount (`localapi.ownMountIdentity`) — never the containing fs, or an absent drive reads as the root disk. A shared disk key is "same, proven" even when unknown. Pinned by identity_test.go (fixture trees) + samedisk_test.go |
 | `ValidateUUID/MountPath/BlockDevice/FSType/SMARTDevice/LVMName`, `UnitNameForMount` | internal/storage/validate.go | `Validate*(v) error` | EVERY arg that reaches a root shell-out | The security boundary; strict whitelists (no by-* symlinks, no dm, no traversal); `systemdEscapePath` computed in-process |
 | `ValidateNetworkMountSpec` | internal/storage/netmount.go | `ValidateNetworkMountSpec(spec) error` | NAS mount input boundary | Same discipline as validate.go; SMB requires a creds ref; mountpoint confined under `NetworkMountRoot` |
 
@@ -158,6 +159,7 @@
 | `proxmox.Runner` | internal/proxmox/privileged.go | `*ExecRunner` (direct/sudo) | `mockRunner` internal/proxmox/mock_test.go; runner fakes in storage tests |
 | `storage.HostOps` | internal/storage/hostops.go | `*SudoHostOps` (prod), `NoopHostOps` (degraded) | fakes in internal/storage/observe_test.go, watchdog_test.go |
 | `storage.HostReader` | internal/storage/hostread.go | `*ProcHostReader` | `fakeHostReader` internal/localapi/disks_test.go; internal/storage/role_test.go. v0.87.0: `BlockSlaves(name)` lists `/sys/block/<name>/slaves` (root-free) — backs the `SystemDisks` dm/md walk (`physicalDisksOf`/`walkSlaves`, role.go); per-branch conservatism: an unresolvable slave fails the WHOLE walk → all-system fail-safe. NEVER weaken the signature test `TestSystemDisks_WalkTopologies` (root-backing disk always in the system set). |
+| `storage.IdentityHost` | internal/storage/identity.go | `*SysfsIdentityHost` | fixture /sys + udev trees in identity_test.go (`newFx`); `fakeIdentHost` in internal/localapi/samedisk_test.go. A SEPARATE interface from HostReader on purpose — adding methods to HostReader would break every existing fake. |
 | `localapi.DiskOps` / `StorageGate` / `GuestAttacher` / `GuestLister` | internal/localapi/disks.go | `*storage.SudoHostOps`; `storageGateAdapter` (cmd/felhom-agent/main.go); `*GuestBinder`; `*proxmox.Client` | `fakeDiskOps`/`fakeGate`/`fakeGuestAttacher`/`fakeGuestList` internal/localapi/disks_test.go |
 | `lanresolver.hostRoot` + `dnsmasqUnitPaths` (data seam, R-317) | internal/lanresolver/lanresolver.go | prod `hostRoot = "/"`; probe = the `dnsmasq` package's systemd UNIT, never `/usr/sbin/dnsmasq` (owned by `dnsmasq-base`) | internal/lanresolver/ensure_dnsmasq_test.go — fixture root tree + recording `proxmox.Runner`; the REAL `os.Stat` probe and `EnsureDnsmasq` run. `TestEnsureDnsmasq_ProductionProbeIsTheUnit` pins the production wiring |
 | `localapi.GuestAPI` / `BackupService` / `BackupStore` / `TokenAuthority` | internal/localapi/server.go | `*proxmox.Client`, `*backup.BackupRunner`, `*backup.Store`, `*TokenStore` | `fakeGuests`/`fakeBackups`/`fakeStore` internal/localapi/server_test.go |

cmd/felhom-agent/main.go

@@ -777,6 +777,7 @@ func runDaemon(cfg config.Config, logger *slog.Logger, logRing *applog.Ring) int
 	hostReader := storage.NewProcHostReader()
 	hostOps := newHostOps(cfg, logger)
 	observer := storage.NewObserver(px, hostReader, hostOps, logger)
+	observer.SetIdentity(storage.NewIdentityResolver()) // `16` §3 physical-disk identity
 	// Backup + restore-test state store (slice 6): holds the latest backup-per-target +
 	// latest restore-test result; the collector reads it via the BackupReporter /
 	// RestoreTestReporter seams; the cadence scheduler writes it.
@@ -2000,6 +2001,7 @@ func buildLocalAPIServer(cfg config.Config, px *proxmox.Client, store *backup.St
 		DriveTargets:     driveTargets,                    // Impl-2a: registry+units drives for the /disks view (union w/ Observe storages)
 		Smart:            storage.NewSmartReader(hostOps), // v0.95.0 Fix B: SMART for the union-path drives
 		HostReader:       storage.NewProcHostReader(),     // Impl-2b: durableIDForMount raw-mount fallback + role gate
+		Identity:         storage.NewIdentityResolver(),   // v0.157.0: `16` §3 identity on /disks + POST /disks/same-disk
 		Tokens:           tokens,
 		BackupCadence:    cfg.Backup.BackupCadence(),
 		// Disk management (slice 8C): the privileged host surface + the data-bearing wipe gate.
@@ -2188,6 +2190,7 @@ func runSelftestHub(ctx context.Context, cfg config.Config, logger *slog.Logger)
 		return 1
 	}
 	observer := storage.NewObserver(px, storage.NewProcHostReader(), newHostOps(cfg, logger), logger)
+	observer.SetIdentity(storage.NewIdentityResolver()) // `16` §3 physical-disk identity
 	// Wire the LIVE PBS reporter here too (v0.39.0): selftest=hub is a separate one-shot process — no
 	// verify loop runs — so a nil reporter previously yielded pbs_snapshots:[] and an absent dr_recipe
 	// pbs coord. The live reporter lists snapshots directly (fresh store, last-known-good fallback) so
@@ -2244,6 +2247,7 @@ func runSelftestStorage(ctx context.Context, cfg config.Config, logger *slog.Log
 	hostReader := storage.NewProcHostReader()
 	hostOps := newHostOps(cfg, logger)
 	observer := storage.NewObserver(px, hostReader, hostOps, logger)
+	observer.SetIdentity(storage.NewIdentityResolver()) // `16` §3 physical-disk identity
 
 	octx, cancel := context.WithTimeout(ctx, 60*time.Second)
 	fmt.Printf("=== felhom-agent %s selftest=storage ===\n", version)

internal/hub/contract_test.go

@@ -39,11 +39,14 @@ func TestHostReport_ContractMatchesGolden(t *testing.T) {
 				State: StorageStateAttached, Reachable: true, ClassHint: "fast",
 				ThinPool: &ThinPoolFill{DataUsedFraction: 0.42},
 				Smart:    SmartSummary{Health: SmartUnknown},
+				// v0.157.0 (`16` §3): the identity keys ride on both rows so the key-set guard sees them.
+				Kind: "system", PhysicalDisks: []string{"serial:ata:SanDisk_X600:182195804614"}, Identity: "proven",
 			},
 			{
 				Name: "usb-backup", Type: StorageTypeUSB, DurableID: "uuid:x",
 				State: StorageStateAttached, Reachable: true, MountPath: "/mnt/usb-backup", TotalBytes: 2000000000000,
 				Smart: SmartSummary{Health: SmartUnknown},
+				Kind:  "disk", PhysicalDisks: []string{"serial:usb:External:000000000000"}, Identity: "unknown", IdentityReason: "usb_bridge",
 			},
 			// A pbs target so the recipe's pbs coord has a storage.cfg row to resolve its namespace
 			// from (R-106). Without one the fixture produces namespace_state=unknown while the golden

internal/hub/report.go

@@ -431,6 +431,15 @@ type StorageTarget struct {
 	// metadata fill is null until Phase B's privileged `lvs` read.
 	ThinPool *ThinPoolFill `json:"thin_pool,omitempty"`
 
+	// Physical-disk identity (felhom.eu architecture/16-storage-physical-identity.md §3, agent v0.157.0):
+	// the place's kind, the keys of the whole physical disks under it, and whether the agent can prove
+	// them (proven | unknown + identity_reason vm|network|usb_bridge|weak_wwn|no_serial). Agent-computed
+	// from /sys + udev only; empty when not computed (an absent device, an older agent). Additive.
+	Kind           string   `json:"kind,omitempty"`
+	PhysicalDisks  []string `json:"physical_disks,omitempty"`
+	Identity       string   `json:"identity,omitempty"`
+	IdentityReason string   `json:"identity_reason,omitempty"`
+
 	// Smart is the disk-health summary, populated in Phase B via smartctl (sudoers). In
 	// Phase A it is {health:"UNKNOWN", <counters null>} — the keys are committed now so the
 	// contract is stable across both phases.

internal/hub/testdata/host-report.golden.json

@@ -46,6 +46,9 @@
       "backing_device": "",
       "class_hint": "fast",
       "role": "",
+      "kind": "system",
+      "physical_disks": ["serial:ata:SanDisk_X600:182195804614"],
+      "identity": "proven",
       "thin_pool": { "data_used_fraction": 0.42, "metadata_used_fraction": null },
       "smart": {
         "health": "UNKNOWN",
@@ -74,6 +77,10 @@
       "backing_device": "/dev/sdb1",
       "class_hint": "slow",
       "role": "",
+      "kind": "disk",
+      "physical_disks": ["serial:usb:External:000000000000"],
+      "identity": "unknown",
+      "identity_reason": "usb_bridge",
       "smart": {
         "health": "UNKNOWN",
         "temperature_c": null,

internal/localapi/disks.go

@@ -188,6 +188,13 @@ type DiskInfo struct {
 	// device that exposes no SMART (USB bridge, unread) is ABSENT, not a misleading zero-value UNKNOWN;
 	// the controller feature-detects presence and renders "Nincs adat" when nil (never alarms).
 	Smart *hub.SmartSummary `json:"smart,omitempty"`
+	// Physical-disk identity (v0.157.0, felhom.eu architecture/16-storage-physical-identity.md §3) — the
+	// same agent-computed verdict the hub report carries. Empty when not computed (an absent drive, no
+	// resolver). Additive; an older controller ignores it.
+	Kind           string   `json:"kind,omitempty"`
+	PhysicalDisks  []string `json:"physical_disks,omitempty"`
+	Identity       string   `json:"identity,omitempty"`
+	IdentityReason string   `json:"identity_reason,omitempty"`
 }
 
 // handleDisks lists the host's drives + data-bearing flags (read-only/benign).
@@ -227,6 +234,8 @@ func (s *Server) handleDisks(w http.ResponseWriter, r *http.Request, vmid int) {
 			// really is the target. Reported here so the controller can name the drive in an
 			// absent-target alarm and hide the destructive controls the agent would refuse anyway.
 			BackupTarget: t.Name == primaryTargetID,
+			// `16` §3: copied from the observer, which computed it from /sys + udev.
+			Kind: t.Kind, PhysicalDisks: t.PhysicalDisks, Identity: t.Identity, IdentityReason: t.IdentityReason,
 		}
 		// Intermediary model: the stable in-guest path + whether felhom-data is bound under the parent.
 		// Only user-data /mnt/<name> drives have a guest path (system/backup mounts never cross in).
@@ -398,6 +407,10 @@ func (s *Server) handleDisks(w http.ResponseWriter, r *http.Request, vmid int) {
 						di.Smart = &sm
 					}
 				}
+				// `16` §3: a registry drive's identity, judged on its OWN mount only (never root's).
+				if p, kind, ok := s.ownMountIdentity(d.MountPath); ok {
+					di.Kind, di.PhysicalDisks, di.Identity, di.IdentityReason = kind, p.PhysicalDisks, p.Identity, p.Reason
+				}
 				// R-118: statfs ONLY while the drive's device is present. With the device gone the raw
 				// mountpoint reverts to a bare directory on the ROOT filesystem, and statfs would report
 				// pve-root's size as this drive's (measured: a 4 GB drive advertising 46 GiB). Same trap

internal/localapi/samedisk.go

@@ -0,0 +1,200 @@
+package localapi
+
+import (
+	"context"
+	"net/http"
+	"path"
+	"regexp"
+	"strings"
+
+	"gitea.dooplex.hu/admin/felhom-agent/internal/hub"
+	"gitea.dooplex.hu/admin/felhom-agent/internal/storage"
+)
+
+// POST /disks/same-disk (v0.157.0) — felhom.eu architecture/16-storage-physical-identity.md §4.
+//
+// The controller asks whether two of ITS paths sit on the same physical disk; the agent answers from its
+// own /sys + udev read, never from the caller's claim (`16` §10). A guest path is mapped only to a place
+// the agent itself made:
+//   - /mnt/felhom-drives/<name>[/…] → the host mount at exactly /mnt/felhom-drives/<name> (the drive's
+//     felhom-data bind, or a network share's mount). A path whose <name> is not mounted there (an absent
+//     drive: the bare directory on the root disk) is UNMAPPED — never "the system disk".
+//   - /mnt/sys_drive, /var/lib/felhom, /var/lib/docker [/…] → the guest's data volume (mp0, mp=/var/lib/felhom;
+//     or a legacy mp at that exact path) → the PVE storage holding it → that storage's identity.
+//   - anything else → unmapped.
+// Answer: {different, proven, reason, a, b}. Act on `proven` first: different+proven = a counted second
+// copy; same_disk+proven = not a second copy; proven=false = unknown (reason vm|network|usb_bridge|
+// weak_wwn|no_serial|unmapped). Pinned by samedisk_test.go.
+
+type sameDiskRequest struct {
+	VMID int    `json:"vmid,omitempty"`
+	A    string `json:"a"`
+	B    string `json:"b"`
+}
+
+// SamePlace is one side of the answer.
+type SamePlace struct {
+	Path          string   `json:"path"`
+	Mapped        bool     `json:"mapped"`
+	Kind          string   `json:"kind,omitempty"`
+	PhysicalDisks []string `json:"physical_disks,omitempty"`
+	Identity      string   `json:"identity,omitempty"`
+	Reason        string   `json:"reason,omitempty"`
+}
+
+// SameDiskResponse is the wire answer.
+type SameDiskResponse struct {
+	Different bool      `json:"different"`
+	Proven    bool      `json:"proven"`
+	Reason    string    `json:"reason,omitempty"`
+	A         SamePlace `json:"a"`
+	B         SamePlace `json:"b"`
+}
+
+// sysDataPaths are the guest paths that live on the guest's one data volume (golden v3: mp0 at
+// /var/lib/felhom, its docker/ and sys_drive/ folders bound in the guest — 07 §7.5.1).
+var sysDataPaths = []string{"/var/lib/felhom", "/mnt/sys_drive", "/var/lib/docker"}
+
+func (s *Server) handleSameDisk(w http.ResponseWriter, r *http.Request, vmid int) {
+	if s.ident == nil {
+		writeErr(w, http.StatusServiceUnavailable, "disk identity not configured on this host")
+		return
+	}
+	var req sameDiskRequest
+	if !decodeBody(w, r, &req) {
+		return
+	}
+	if !s.scopedFromBody(w, req.VMID, vmid, r.URL.Path) {
+		return
+	}
+	if !path.IsAbs(req.A) || !path.IsAbs(req.B) {
+		writeErr(w, http.StatusBadRequest, "a and b must be absolute guest paths")
+		return
+	}
+	a := s.mapGuestPlace(r.Context(), vmid, req.A)
+	b := s.mapGuestPlace(r.Context(), vmid, req.B)
+	resp := SameDiskResponse{A: a.view, B: b.view}
+	resp.Different, resp.Proven, resp.Reason = storage.Relation(a.id, b.id)
+	s.logger.Info("local-api: same-disk", "vmid", vmid, "a", req.A, "b", req.B,
+		"different", resp.Different, "proven", resp.Proven, "reason", resp.Reason)
+	writeOK(w, resp)
+}
+
+type mappedPlace struct {
+	view SamePlace
+	id   storage.PlaceIdentity
+}
+
+func (s *Server) mapGuestPlace(ctx context.Context, vmid int, guestPath string) mappedPlace {
+	p := path.Clean(guestPath)
+	out := mappedPlace{view: SamePlace{Path: p}}
+	set := func(kind string, id storage.PlaceIdentity) mappedPlace {
+		if !id.Computed() {
+			return out
+		}
+		out.id = id
+		out.view.Mapped, out.view.Kind = true, kind
+		out.view.PhysicalDisks, out.view.Identity, out.view.Reason = id.PhysicalDisks, id.Identity, id.Reason
+		return out
+	}
+	if strings.HasPrefix(p, StableParentDir+"/") {
+		name := strings.SplitN(strings.TrimPrefix(p, StableParentDir+"/"), "/", 2)[0]
+		if id, kind, ok := s.ownMountIdentity(StableParentDir + "/" + name); ok {
+			return set(kind, id)
+		}
+		return out
+	}
+	for _, root := range sysDataPaths {
+		if p != root && !strings.HasPrefix(p, root+"/") {
+			continue
+		}
+		id := s.guestDataVolumeIdentity(ctx, vmid, root)
+		return set(storage.PlaceKindSystem, id)
+	}
+	return out
+}
+
+// ownMountIdentity judges a HOST path only when it is ITSELF a mountpoint (an enrolled drive's raw mount
+// or its felhom-data bind, a network share). A path that is not its own mount — an absent drive, whose
+// directory now sits on the root disk — is not computed, never root's identity (the R-118 trap).
+func (s *Server) ownMountIdentity(hostPath string) (storage.PlaceIdentity, string, bool) {
+	if s.ident == nil || hostPath == "" {
+		return storage.PlaceIdentity{}, "", false
+	}
+	mounts, err := s.hostReader().Mounts()
+	if err != nil {
+		return storage.PlaceIdentity{}, "", false
+	}
+	m, ok := storage.MountFor(mounts, hostPath)
+	if !ok || path.Clean(m.MountPoint) != path.Clean(hostPath) {
+		return storage.PlaceIdentity{}, "", false
+	}
+	id := s.ident.HostPath(hostPath)
+	if !id.Computed() {
+		return id, "", false
+	}
+	kind := storage.PlaceKindDisk
+	switch {
+	case id.Reason == storage.ReasonNetwork:
+		kind = storage.PlaceKindNetwork
+	case strings.HasPrefix(m.Device, "/dev/") && !strings.HasPrefix(m.Device, "/dev/mapper/") && isPartitionPath(m.Device):
+		kind = storage.PlaceKindPartition
+	}
+	return id, kind, true
+}
+
+var rePartitionName = regexp.MustCompile(`^(nvme[0-9]+n[0-9]+p[0-9]+|(sd|vd|hd)[a-z]+[0-9]+)$`)
+
+func isPartitionPath(dev string) bool { return rePartitionName.MatchString(path.Base(dev)) }
+
+// guestDataVolumeIdentity finds the guest mountpoint that holds root (mp=/var/lib/felhom on golden v3, or a
+// legacy mp at that exact path) and returns the identity of the PVE storage that holds its volume.
+func (s *Server) guestDataVolumeIdentity(ctx context.Context, vmid int, root string) storage.PlaceIdentity {
+	if s.guests == nil || s.storage == nil {
+		return storage.PlaceIdentity{}
+	}
+	cfg, err := s.guests.GuestConfig(ctx, vmid)
+	if err != nil {
+		return storage.PlaceIdentity{}
+	}
+	storeID, fallback := "", ""
+	for _, spec := range cfg.MountPoints() {
+		st, mp, _ := parseMount(spec)
+		if strings.HasPrefix(st, "/") {
+			continue // a host bind (mp8, mp9), not a volume
+		}
+		switch mp {
+		case root:
+			storeID = st // a legacy mp at exactly this path wins
+		case "/var/lib/felhom":
+			fallback = st // golden v3: the one data volume
+		}
+	}
+	if storeID == "" {
+		storeID = fallback
+	}
+	if storeID == "" {
+		return storage.PlaceIdentity{}
+	}
+	targets, err := s.storage.Observe(ctx)
+	if err != nil {
+		return storage.PlaceIdentity{}
+	}
+	for _, t := range targets {
+		if t.Name == storeID {
+			return identityOfTarget(t)
+		}
+	}
+	return storage.PlaceIdentity{}
+}
+
+func identityOfTarget(t hub.StorageTarget) storage.PlaceIdentity {
+	if t.Identity == "" {
+		return storage.PlaceIdentity{}
+	}
+	disks := t.PhysicalDisks
+	if disks == nil {
+		disks = []string{}
+	}
+	return storage.PlaceIdentity{PhysicalDisks: disks, Identity: t.Identity, Reason: t.IdentityReason}
+}

internal/localapi/samedisk_test.go

@@ -0,0 +1,146 @@
+package localapi
+
+import (
+	"context"
+	"encoding/json"
+	"io"
+	"log/slog"
+	"net/http"
+	"testing"
+
+	"gitea.dooplex.hu/admin/felhom-agent/internal/hub"
+	"gitea.dooplex.hu/admin/felhom-agent/internal/storage"
+)
+
+// POST /disks/same-disk (`16` §4) — the controller's Tier-2 question, answered from the agent's own read.
+
+type fakeIdentHost struct {
+	mounts []storage.Mount
+	parent map[string]string
+	props  map[string]map[string]string
+	virt   string
+}
+
+func (f *fakeIdentHost) Mounts() ([]storage.Mount, error) { return f.mounts, nil }
+func (f *fakeIdentHost) KernelName(dev string) (string, bool) {
+	if len(dev) > 5 && dev[:5] == "/dev/" {
+		return dev[5:], true
+	}
+	return "", false
+}
+func (f *fakeIdentHost) Parent(n string) (string, bool)        { p, ok := f.parent[n]; return p, ok }
+func (f *fakeIdentHost) Slaves(string) []string                { return nil }
+func (f *fakeIdentHost) LoopBackingFile(string) (string, bool) { return "", false }
+func (f *fakeIdentHost) UdevProps(n string) map[string]string  { return f.props[n] }
+func (f *fakeIdentHost) Virt() string                          { return f.virt }
+
+// sameDiskRig: guest 8200 (token A) with mp0 on local-lvm (the system SSD, proven) + the given mounts.
+func sameDiskRig(t *testing.T, host *fakeIdentHost, localLVM hub.StorageTarget) http.Handler {
+	t.Helper()
+	fg := &fakeGuestsCfg{mounts: map[int]map[string]string{8200: {
+		"mp0": "local-lvm:vm-8200-disk-1,mp=/var/lib/felhom,backup=1,size=70G",
+		"mp8": "/mnt/felhom-drives,mp=/mnt/felhom-drives",
+	}}}
+	srv, err := NewServer(Options{
+		ListenAddr: "127.0.0.1:0", Guests: fg, Backups: &fakeBackups{}, Store: &fakeStore{},
+		Storage: fakeStorage{targets: []hub.StorageTarget{localLVM}}, Tokens: staticTokens{"A": 8200},
+		HostReader: fakeHostReader{mounts: host.mounts},
+		Identity:   &storage.IdentityResolver{Host: host},
+		Logger:     slog.New(slog.NewTextHandler(io.Discard, nil)),
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	srv.baseCtx = context.Background()
+	return srv.Handler()
+}
+
+var provenSSD = hub.StorageTarget{Name: "local-lvm", Type: hub.StorageTypeLVMThin, Kind: "system",
+	PhysicalDisks: []string{"serial:ata:SanDisk:1"}, Identity: "proven"}
+
+func baseHost() *fakeIdentHost {
+	return &fakeIdentHost{
+		virt:   "none",
+		parent: map[string]string{"sdc1": "sdc", "sdc2": "sdc"},
+		props: map[string]map[string]string{
+			"sdb": {"ID_BUS": "ata", "ID_MODEL": "TOSHIBA", "ID_SERIAL_SHORT": "65NOP3HDT"},
+			"sdc": {"ID_BUS": "usb", "ID_MODEL": "Portable", "ID_SERIAL_SHORT": "NAAB7C21"},
+		},
+		mounts: []storage.Mount{
+			{Device: "/dev/mapper/pve-root", MountPoint: "/", FSType: "ext4"},
+			{Device: "/dev/mapper/pve-root", MountPoint: "/mnt/felhom-drives", FSType: "ext4"},
+			{Device: "/dev/sdb", MountPoint: "/mnt/felhom-drives/hdd_1", FSType: "ext4"},
+			{Device: "/dev/sdc1", MountPoint: "/mnt/felhom-drives/a", FSType: "ext4"},
+			{Device: "/dev/sdc2", MountPoint: "/mnt/felhom-drives/b", FSType: "ext4"},
+			{Device: "//nas/films", MountPoint: "/mnt/felhom-drives/nas", FSType: "cifs"},
+		},
+	}
+}
+
+func sameDisk(t *testing.T, h http.Handler, a, b string) (SameDiskResponse, int) {
+	t.Helper()
+	body, _ := json.Marshal(map[string]string{"a": a, "b": b})
+	w := do(t, h, "POST", "/disks/same-disk", "A", string(body))
+	var env struct {
+		Data SameDiskResponse `json:"data"`
+	}
+	_ = json.Unmarshal(w.Body.Bytes(), &env)
+	return env.Data, w.Code
+}
+
+func TestSameDisk_DriveVsSystemData_DifferentProven(t *testing.T) {
+	h := sameDiskRig(t, baseHost(), provenSSD)
+	got, code := sameDisk(t, h, "/mnt/felhom-drives/hdd_1/felhom-data/paperless", "/mnt/sys_drive")
+	if code != http.StatusOK || !got.Different || !got.Proven || got.Reason != "" {
+		t.Fatalf("drive vs sys_drive: want different+proven, got %d %+v", code, got)
+	}
+	if got.B.Kind != "system" || got.A.Kind != "disk" {
+		t.Fatalf("kinds: got a=%q b=%q", got.A.Kind, got.B.Kind)
+	}
+}
+
+func TestSameDisk_TwoPartitionsOfOneDisk_SameProven(t *testing.T) {
+	h := sameDiskRig(t, baseHost(), provenSSD)
+	got, _ := sameDisk(t, h, "/mnt/felhom-drives/a", "/mnt/felhom-drives/b/x")
+	if got.Different || !got.Proven || got.Reason != "same_disk" || got.A.Kind != "partition" {
+		t.Fatalf("two partitions: want same+proven, got %+v", got)
+	}
+}
+
+func TestSameDisk_InsideAVM_NotProven(t *testing.T) {
+	host := baseHost()
+	host.virt = "kvm"
+	h := sameDiskRig(t, host, hub.StorageTarget{Name: "local-lvm", Identity: "unknown", IdentityReason: "vm",
+		PhysicalDisks: []string{"serial:scsi:QEMU_HARDDISK:drive-scsi0"}})
+	got, _ := sameDisk(t, h, "/mnt/felhom-drives/hdd_1", "/mnt/sys_drive")
+	if got.Proven || got.Reason != "vm" {
+		t.Fatalf("inside a VM: want not proven (vm), got %+v", got)
+	}
+}
+
+func TestSameDisk_NetworkAndUnmapped(t *testing.T) {
+	h := sameDiskRig(t, baseHost(), provenSSD)
+	if got, _ := sameDisk(t, h, "/mnt/felhom-drives/nas", "/mnt/sys_drive"); got.Proven || got.Reason != "network" {
+		t.Fatalf("a NAS: want not proven (network), got %+v", got)
+	}
+	// An ABSENT drive: its directory sits on the root disk — must be unmapped, never "the system disk".
+	if got, _ := sameDisk(t, h, "/mnt/felhom-drives/gone", "/mnt/sys_drive"); got.Proven || got.Reason != "unmapped" || got.A.Mapped {
+		t.Fatalf("an absent drive: want unmapped, got %+v", got)
+	}
+	if got, _ := sameDisk(t, h, "/opt/elsewhere", "/mnt/sys_drive"); got.Proven || got.Reason != "unmapped" {
+		t.Fatalf("an unknown guest path: want unmapped, got %+v", got)
+	}
+}
+
+func TestSameDisk_ScopedAndValidated(t *testing.T) {
+	h := sameDiskRig(t, baseHost(), provenSSD)
+	if w := do(t, h, "POST", "/disks/same-disk", "A", `{"vmid":9300,"a":"/mnt/sys_drive","b":"/mnt/sys_drive"}`); w.Code != http.StatusForbidden {
+		t.Fatalf("another guest's vmid: got %d want 403", w.Code)
+	}
+	if w := do(t, h, "POST", "/disks/same-disk", "A", `{"a":"relative","b":"/mnt/sys_drive"}`); w.Code != http.StatusBadRequest {
+		t.Fatalf("a relative path: got %d want 400", w.Code)
+	}
+	if w := do(t, h, "POST", "/disks/same-disk", "", `{"a":"/x","b":"/y"}`); w.Code != http.StatusUnauthorized {
+		t.Fatalf("no token: got %d want 401", w.Code)
+	}
+}

internal/localapi/server.go

@@ -220,6 +220,10 @@ type Options struct {
 	// ROLE (it backs SystemDisks for the eject role-gate + the /disks role hints). OPTIONAL — when nil
 	// it defaults to the production *storage.ProcHostReader. Injectable so the role-gate is testable.
 	HostReader storage.HostReader
+	// Identity computes physical-disk identity (felhom.eu architecture/16-storage-physical-identity.md §3)
+	// for the /disks rows and POST /disks/same-disk. OPTIONAL — when nil, /disks carries no identity and
+	// same-disk answers 503 "not configured" (the controller then keeps its old st_dev rule).
+	Identity *storage.IdentityResolver
 	// HostMetrics serves GET /host/metrics (slice 9) — host-wide health (cpu%/mem/load/uptime/
 	// cpu-temp) + per-storage capacity, host-wide and token-authed (one-customer-per-host). When
 	// nil the endpoint reports "not configured" (host still reports/reconciles).
@@ -326,6 +330,7 @@ type Server struct {
 	// from GuestPowerTick, which the ticker calls serially.
 	guestPowerSweeps int
 	host             storage.HostReader        // role classification source (optional; defaults to ProcHostReader)
+	ident            *storage.IdentityResolver // `16` §3 physical-disk identity (optional)
 
 	hostMetrics  HostMetricsProvider // slice 9 (optional)
 	hostID       string              // slice 10B: for the data-bearing-format pending-op hint
@@ -480,6 +485,7 @@ func NewServer(o Options) (*Server, error) {
 		formatJobs:      o.FormatJobs,
 		staleLock:       o.StaleLock,
 		host:            o.HostReader,
+		ident:           o.Identity,
 		hostMetrics:     o.HostMetrics,
 		hostID:          o.HostID,
 		agentVersion:    o.AgentVersion,
@@ -546,6 +552,7 @@ func (s *Server) Handler() http.Handler {
 	// Disk management (slice 8C) — self-scoped; format routes through the data-bearing classifier+gate.
 	mux.HandleFunc("GET /disks", s.withGuest(s.handleDisks))
 	mux.HandleFunc("GET /disks/candidates", s.withGuest(s.handleDiskCandidates))
+	mux.HandleFunc("POST /disks/same-disk", s.withGuest(s.handleSameDisk)) // `16` §4 (v0.157.0)
 	mux.HandleFunc("POST /disks/assign", s.withGuest(s.handleDiskAssign))
 	mux.HandleFunc("POST /disks/eject", s.withGuest(s.handleDiskEject))
 	mux.HandleFunc("POST /disks/decommission", s.withGuest(s.handleDiskDecommission))

internal/storage/identity.go

@@ -0,0 +1,445 @@
+package storage
+
+import (
+	"bufio"
+	"context"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"sort"
+	"strings"
+	"sync"
+	"time"
+)
+
+// Physical-disk identity of a storage place — felhom.eu architecture/16-storage-physical-identity.md §3.
+//
+// A storage place (a PVE storage, an enrolled drive, the guest's data volume, a network share) is
+// reduced to the PHYSICAL whole disks under it — partition → parent disk, device-mapper / LVM thin →
+// sysfs slaves (recursive), loop → its backing file → the filesystem holding that file — and each disk
+// is named by a key the agent trusts: `eui:` for NVMe, `serial:<bus>:<model>:<serial>` from udev,
+// `wwn:` only when no serial exists and the WWN carries a vendor (OUI ≠ 000000).
+//
+// The verdict is AGENT-COMPUTED evidence (`16` §10): it is read from /sys, /run/udev/data and
+// /proc/mounts only, never from a caller's claim, so a compromised controller cannot make two places
+// "different disks". `proven` means every disk under the place has a key the agent trusts; anything
+// else is `unknown` with ONE reason, chosen by precedence (vm > network > usb_bridge > weak_wwn >
+// no_serial). Nothing is cached: every call reads the current topology, so a disk swapped in the same
+// bay is re-judged on the next read (the "right before each Tier-2 run" rule is the caller calling).
+//
+// Pinned by identity_test.go (fixture /sys + udev trees, one per §3 case) — the spike that proved the
+// walk on real devices: felhom.eu documentation/audits/storage-identity-2026-10-15/.
+
+const (
+	IdentityProven  = "proven"
+	IdentityUnknown = "unknown"
+
+	ReasonVM        = "vm"
+	ReasonNetwork   = "network"
+	ReasonUSBBridge = "usb_bridge"
+	ReasonWeakWWN   = "weak_wwn"
+	ReasonNoSerial  = "no_serial"
+
+	PlaceKindSystem    = "system"
+	PlaceKindInternal  = "internal"
+	PlaceKindDisk      = "disk"
+	PlaceKindPartition = "partition"
+	PlaceKindNetwork   = "network"
+)
+
+// reasonRank orders the unknown reasons: the first one a place hits, by this precedence, is reported.
+var reasonRank = map[string]int{ReasonVM: 0, ReasonNetwork: 1, ReasonUSBBridge: 2, ReasonWeakWWN: 3, ReasonNoSerial: 4}
+
+// PlaceIdentity is what a storage place reports.
+type PlaceIdentity struct {
+	PhysicalDisks []string // disk keys, sorted, deduplicated
+	Identity      string   // proven | unknown | "" (not computed — e.g. the device is absent)
+	Reason        string   // set only when Identity == unknown
+}
+
+// Computed reports whether a verdict was reached at all.
+func (p PlaceIdentity) Computed() bool { return p.Identity != "" }
+
+// IdentityHost is the read surface the resolver needs. SysfsIdentityHost is production; tests use the
+// same struct pointed at fixture trees.
+type IdentityHost interface {
+	Mounts() ([]Mount, error)
+	// KernelName maps a /dev path (symlinks allowed: /dev/mapper/x, /dev/pve/y) to its kernel block name.
+	KernelName(dev string) (string, bool)
+	// Parent returns the parent disk's kernel name when name is a partition.
+	Parent(name string) (string, bool)
+	// Slaves lists /sys/class/block/<name>/slaves.
+	Slaves(name string) []string
+	// LoopBackingFile returns a loop device's backing file.
+	LoopBackingFile(name string) (string, bool)
+	// UdevProps returns the udev E: properties of a block device.
+	UdevProps(name string) map[string]string
+	// Virt returns the VM technology ("none" on bare metal).
+	Virt() string
+}
+
+// SysfsIdentityHost reads the real host. Every path is a field so tests can point it at fixtures.
+type SysfsIdentityHost struct {
+	SysClass   string                       // default /sys/class/block
+	UdevData   string                       // default /run/udev/data
+	ProcMounts string                       // default /proc/mounts
+	DevResolve func(string) (string, error) // default filepath.EvalSymlinks
+	VirtFn     func() string                // default `systemd-detect-virt --vm`, read once
+
+	virtOnce sync.Once
+	virt     string
+}
+
+// NewSysfsIdentityHost returns the production reader.
+func NewSysfsIdentityHost() *SysfsIdentityHost { return &SysfsIdentityHost{} }
+
+func (h *SysfsIdentityHost) sysClass() string {
+	if h.SysClass != "" {
+		return h.SysClass
+	}
+	return "/sys/class/block"
+}
+
+func (h *SysfsIdentityHost) Mounts() ([]Mount, error) {
+	return (&ProcHostReader{ProcMounts: h.ProcMounts}).Mounts()
+}
+
+func (h *SysfsIdentityHost) KernelName(dev string) (string, bool) {
+	if !strings.HasPrefix(dev, "/dev/") {
+		return "", false
+	}
+	resolve := h.DevResolve
+	if resolve == nil {
+		resolve = filepath.EvalSymlinks
+	}
+	real := dev
+	if r, err := resolve(dev); err == nil {
+		real = r
+	}
+	name := filepath.Base(real)
+	if _, err := os.Stat(filepath.Join(h.sysClass(), name)); err != nil {
+		return "", false
+	}
+	return name, true
+}
+
+func (h *SysfsIdentityHost) Parent(name string) (string, bool) {
+	p := filepath.Join(h.sysClass(), name)
+	if _, err := os.Stat(filepath.Join(p, "partition")); err != nil {
+		return "", false
+	}
+	real, err := filepath.EvalSymlinks(p)
+	if err != nil {
+		return "", false
+	}
+	return filepath.Base(filepath.Dir(real)), true
+}
+
+func (h *SysfsIdentityHost) Slaves(name string) []string {
+	entries, err := os.ReadDir(filepath.Join(h.sysClass(), name, "slaves"))
+	if err != nil {
+		return nil
+	}
+	out := make([]string, 0, len(entries))
+	for _, e := range entries {
+		out = append(out, e.Name())
+	}
+	sort.Strings(out)
+	return out
+}
+
+func (h *SysfsIdentityHost) LoopBackingFile(name string) (string, bool) {
+	if !strings.HasPrefix(name, "loop") {
+		return "", false
+	}
+	b, err := os.ReadFile(filepath.Join(h.sysClass(), name, "loop", "backing_file"))
+	if err != nil {
+		return "", false
+	}
+	f := strings.TrimSpace(string(b))
+	return f, f != ""
+}
+
+func (h *SysfsIdentityHost) UdevProps(name string) map[string]string {
+	out := map[string]string{}
+	devB, err := os.ReadFile(filepath.Join(h.sysClass(), name, "dev"))
+	if err != nil {
+		return out
+	}
+	dir := h.UdevData
+	if dir == "" {
+		dir = "/run/udev/data"
+	}
+	f, err := os.Open(filepath.Join(dir, "b"+strings.TrimSpace(string(devB))))
+	if err != nil {
+		return out
+	}
+	defer f.Close()
+	sc := bufio.NewScanner(f)
+	for sc.Scan() {
+		line := sc.Text()
+		if !strings.HasPrefix(line, "E:") {
+			continue
+		}
+		k, v, ok := strings.Cut(line[2:], "=")
+		if ok {
+			out[k] = v
+		}
+	}
+	return out
+}
+
+func (h *SysfsIdentityHost) Virt() string {
+	h.virtOnce.Do(func() {
+		if h.VirtFn != nil {
+			h.virt = h.VirtFn()
+			return
+		}
+		ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+		defer cancel()
+		// --vm: a container is not a reason (the agent runs on the host); exit 1 + "none" on metal.
+		out, _ := exec.CommandContext(ctx, "systemd-detect-virt", "--vm").Output()
+		h.virt = strings.TrimSpace(string(out))
+		if h.virt == "" {
+			h.virt = "unknown"
+		}
+	})
+	return h.virt
+}
+
+// IdentityResolver turns devices and host paths into PlaceIdentity.
+type IdentityResolver struct {
+	Host IdentityHost
+}
+
+// NewIdentityResolver returns a resolver over the real host.
+func NewIdentityResolver() *IdentityResolver {
+	return &IdentityResolver{Host: NewSysfsIdentityHost()}
+}
+
+var networkFSTypes = map[string]bool{
+	"nfs": true, "nfs4": true, "cifs": true, "smb3": true, "fuse.sshfs": true, "9p": true, "autofs": true, "ceph": true, "glusterfs": true,
+}
+
+// NetworkPlace is the fixed verdict for a network share or a remote store.
+func NetworkPlace() PlaceIdentity {
+	return PlaceIdentity{PhysicalDisks: []string{}, Identity: IdentityUnknown, Reason: ReasonNetwork}
+}
+
+// MountFor returns the mount that holds path (longest mountpoint prefix; the LAST entry wins on an equal
+// mountpoint, which is the one stacked on top — an automount's real filesystem over its autofs trigger).
+func MountFor(mounts []Mount, path string) (Mount, bool) {
+	p := filepath.Clean(path)
+	var best Mount
+	found := false
+	for _, m := range mounts {
+		mp := filepath.Clean(m.MountPoint)
+		if p != mp && !strings.HasPrefix(p, strings.TrimSuffix(mp, "/")+"/") {
+			continue
+		}
+		if !found || len(mp) >= len(filepath.Clean(best.MountPoint)) {
+			best, found = m, true
+		}
+	}
+	return best, found
+}
+
+// HostPath resolves the filesystem holding a HOST path.
+func (r *IdentityResolver) HostPath(path string) PlaceIdentity {
+	return r.hostPath(path, 0)
+}
+
+func (r *IdentityResolver) hostPath(path string, depth int) PlaceIdentity {
+	if r == nil || r.Host == nil || depth > 4 {
+		return PlaceIdentity{}
+	}
+	mounts, err := r.Host.Mounts()
+	if err != nil {
+		return PlaceIdentity{}
+	}
+	m, ok := MountFor(mounts, path)
+	if !ok {
+		return PlaceIdentity{}
+	}
+	if networkFSTypes[m.FSType] || !strings.HasPrefix(m.Device, "/dev/") {
+		return NetworkPlace()
+	}
+	return r.device(m.Device, depth)
+}
+
+// Device resolves a block device (a /dev path, symlinks allowed).
+func (r *IdentityResolver) Device(dev string) PlaceIdentity {
+	return r.device(dev, 0)
+}
+
+func (r *IdentityResolver) device(dev string, depth int) PlaceIdentity {
+	if r == nil || r.Host == nil || dev == "" {
+		return PlaceIdentity{}
+	}
+	name, ok := r.Host.KernelName(dev)
+	if !ok {
+		return PlaceIdentity{}
+	}
+	keys := map[string]bool{}
+	reason := ""
+	if !r.walk(name, depth, map[string]bool{}, keys, &reason) {
+		return PlaceIdentity{}
+	}
+	out := PlaceIdentity{PhysicalDisks: sortedKeys(keys), Identity: IdentityProven}
+	if reason != "" {
+		out.Identity, out.Reason = IdentityUnknown, reason
+	}
+	return out
+}
+
+// walk adds the keys of every physical disk under name; ok=false when a branch cannot be grounded.
+func (r *IdentityResolver) walk(name string, depth int, seen, keys map[string]bool, reason *string) bool {
+	if seen[name] {
+		return true // a diamond (a thin pool's tmeta and tdata on one partition): already counted
+	}
+	if len(seen) > 64 {
+		return false
+	}
+	seen[name] = true
+	if parent, ok := r.Host.Parent(name); ok {
+		return r.walk(parent, depth, seen, keys, reason)
+	}
+	if file, ok := r.Host.LoopBackingFile(name); ok {
+		p := r.hostPath(file, depth+1)
+		if !p.Computed() {
+			return false
+		}
+		for _, k := range p.PhysicalDisks {
+			keys[k] = true
+		}
+		noteReason(reason, p.Reason)
+		return true
+	}
+	if slaves := r.Host.Slaves(name); len(slaves) > 0 {
+		for _, s := range slaves {
+			if !r.walk(s, depth, seen, keys, reason) {
+				return false
+			}
+		}
+		return true
+	}
+	if strings.HasPrefix(name, "dm-") || strings.HasPrefix(name, "md") || strings.HasPrefix(name, "loop") {
+		return false // a virtual device with nothing beneath it cannot be grounded
+	}
+	key, why := r.diskKey(name)
+	keys[key] = true
+	noteReason(reason, why)
+	return true
+}
+
+func noteReason(cur *string, why string) {
+	if why == "" {
+		return
+	}
+	if *cur == "" || reasonRank[why] < reasonRank[*cur] {
+		*cur = why
+	}
+}
+
+// diskKey names one whole disk and says why it is not trusted ("" when it is).
+func (r *IdentityResolver) diskKey(name string) (key, reason string) {
+	p := r.Host.UdevProps(name)
+	serial := strings.TrimSpace(p["ID_SERIAL_SHORT"])
+	model := strings.TrimSpace(p["ID_MODEL"])
+	bus := strings.TrimSpace(p["ID_BUS"])
+	wwn := strings.ToLower(strings.TrimSpace(p["ID_WWN"]))
+	if bus == "" && strings.HasPrefix(name, "nvme") {
+		bus = "nvme"
+	}
+	usb := bus == "usb" || strings.Contains(p["DEVPATH"], "/usb")
+	switch {
+	case strings.HasPrefix(name, "nvme") && (strings.HasPrefix(wwn, "eui.") || strings.HasPrefix(wwn, "nvme.")):
+		key = "eui:" + strings.TrimPrefix(strings.TrimPrefix(wwn, "eui."), "nvme.")
+	case serial != "":
+		key = "serial:" + bus + ":" + model + ":" + serial
+		if usb && FakeBridgeSerial(serial) {
+			reason = ReasonUSBBridge
+		}
+	case wwn != "" && WWNHasVendor(wwn):
+		key = "wwn:" + wwn
+	case wwn != "":
+		key, reason = "wwn:"+wwn, ReasonWeakWWN
+	default:
+		key, reason = "dev:"+name, ReasonNoSerial
+	}
+	if v := r.Host.Virt(); v != "none" {
+		reason = ReasonVM // inside a VM a virtual disk says nothing about the host disk (spike cases 5–6)
+	}
+	return key, reason
+}
+
+// WWNHasVendor reports whether an NAA-5/6 WWN carries a registered vendor (OUI ≠ 000000). Other NAA
+// formats are taken as they are. Measured: the N100's AirDisk SSD reports 0x5000000123456f1f.
+func WWNHasVendor(wwn string) bool {
+	h := strings.TrimPrefix(strings.ToLower(wwn), "0x")
+	if len(h) < 7 {
+		return false
+	}
+	if h[0] == '5' || h[0] == '6' {
+		return h[1:7] != "000000"
+	}
+	return true
+}
+
+// FakeBridgeSerial reports a serial that a USB bridge reports for any disk: too short, one repeated
+// character, or a known placeholder. UNPROVEN on a real enclosure (R-1077) — the rule is the design's.
+func FakeBridgeSerial(s string) bool {
+	s = strings.ToUpper(strings.TrimSpace(s))
+	if len(s) < 6 {
+		return true
+	}
+	if strings.Count(s, s[:1]) == len(s) {
+		return true
+	}
+	switch s {
+	case "123456789", "1234567890", "123456789ABC", "0123456789ABCDEF", "1234567890ABCDEF", "DEADBEEF":
+		return true
+	}
+	return false
+}
+
+func sortedKeys(m map[string]bool) []string {
+	out := make([]string, 0, len(m))
+	for k := range m {
+		out = append(out, k)
+	}
+	sort.Strings(out)
+	return out
+}
+
+// Relation compares two places (`16` §4). A SHARED disk key is a positive fact, so two places on one disk
+// are "same, proven" even when that disk is unknown (inside a VM, two parts of one virtual disk). Otherwise
+// proven is true only when both verdicts are proven; different then means "no disk in common". When proven
+// is false, different only says the known keys do not overlap — callers must act on proven first.
+// reason: "" (proven, different), "same_disk", "unmapped" (a side has no verdict), or the unknown reason.
+func Relation(a, b PlaceIdentity) (different, proven bool, reason string) {
+	if !a.Computed() || !b.Computed() {
+		return false, false, "unmapped"
+	}
+	set := map[string]bool{}
+	for _, k := range a.PhysicalDisks {
+		set[k] = true
+	}
+	shared := false
+	for _, k := range b.PhysicalDisks {
+		if set[k] {
+			shared = true
+		}
+	}
+	if shared {
+		return false, true, "same_disk"
+	}
+	if a.Identity == IdentityProven && b.Identity == IdentityProven {
+		return true, true, ""
+	}
+	why := a.Reason
+	if why == "" || (b.Reason != "" && reasonRank[b.Reason] < reasonRank[why]) {
+		why = b.Reason
+	}
+	return true, false, why
+}

internal/storage/identity_test.go

@@ -0,0 +1,257 @@
+package storage
+
+import (
+	"os"
+	"path/filepath"
+	"reflect"
+	"strconv"
+	"strings"
+	"testing"
+)
+
+// Fixture /sys/class/block + /run/udev/data + /proc/mounts trees, one per `16` §3 case. The tree mimics the
+// kernel: /sys/devices/.../<disk>/<part> real dirs (a partition carries a "partition" file), /sys/class/block
+// symlinks, a "dev" file per node, udev "E:" lines keyed by major:minor. The cases are the ones the spike
+// measured on real devices (felhom.eu documentation/audits/storage-identity-2026-10-15/).
+
+type fx struct {
+	t      *testing.T
+	root   string
+	minor  int
+	mounts []string
+	links  map[string]string // /dev path -> kernel name
+	virt   string
+}
+
+func newFx(t *testing.T) *fx {
+	t.Helper()
+	r := t.TempDir()
+	for _, d := range []string{"sys/class/block", "sys/devices", "udev"} {
+		if err := os.MkdirAll(filepath.Join(r, d), 0o755); err != nil {
+			t.Fatal(err)
+		}
+	}
+	return &fx{t: t, root: r, links: map[string]string{}, virt: "none"}
+}
+
+func (f *fx) write(p, s string) {
+	f.t.Helper()
+	if err := os.MkdirAll(filepath.Dir(p), 0o755); err != nil {
+		f.t.Fatal(err)
+	}
+	if err := os.WriteFile(p, []byte(s), 0o644); err != nil {
+		f.t.Fatal(err)
+	}
+}
+
+// node makes a block node. parent "" → a top-level device. udev lines are "KEY=VAL".
+func (f *fx) node(name, parent string, udev ...string) {
+	f.t.Helper()
+	real := filepath.Join(f.root, "sys/devices", name)
+	if parent != "" {
+		pr, _ := filepath.EvalSymlinks(filepath.Join(f.root, "sys/class/block", parent))
+		real = filepath.Join(pr, name)
+		f.write(filepath.Join(real, "partition"), "1\n")
+	}
+	f.minor++
+	dev := "8:" + strconv.Itoa(f.minor)
+	f.write(filepath.Join(real, "dev"), dev+"\n")
+	if err := os.Symlink(real, filepath.Join(f.root, "sys/class/block", name)); err != nil {
+		f.t.Fatal(err)
+	}
+	var b strings.Builder
+	for _, u := range udev {
+		b.WriteString("E:" + u + "\n")
+	}
+	f.write(filepath.Join(f.root, "udev", "b"+dev), b.String())
+	f.links["/dev/"+name] = name
+}
+
+func (f *fx) slaves(name string, slaves ...string) {
+	f.t.Helper()
+	real, _ := filepath.EvalSymlinks(filepath.Join(f.root, "sys/class/block", name))
+	for _, s := range slaves {
+		if err := os.MkdirAll(filepath.Join(real, "slaves", s), 0o755); err != nil {
+			f.t.Fatal(err)
+		}
+	}
+}
+
+func (f *fx) loop(name, file string) {
+	real, _ := filepath.EvalSymlinks(filepath.Join(f.root, "sys/class/block", name))
+	f.write(filepath.Join(real, "loop", "backing_file"), file+"\n")
+}
+
+func (f *fx) mount(dev, mp, fstype string) {
+	f.mounts = append(f.mounts, dev+" "+mp+" "+fstype+" rw 0 0")
+}
+
+func (f *fx) resolver() *IdentityResolver {
+	f.write(filepath.Join(f.root, "mounts"), strings.Join(f.mounts, "\n")+"\n")
+	return &IdentityResolver{Host: &SysfsIdentityHost{
+		SysClass:   filepath.Join(f.root, "sys/class/block"),
+		UdevData:   filepath.Join(f.root, "udev"),
+		ProcMounts: filepath.Join(f.root, "mounts"),
+		DevResolve: func(p string) (string, error) {
+			if n, ok := f.links[p]; ok {
+				return "/dev/" + n, nil
+			}
+			return p, nil
+		},
+		VirtFn: func() string { return f.virt },
+	}}
+}
+
+var sanDisk = []string{"ID_BUS=ata", "ID_MODEL=SanDisk_X600", "ID_SERIAL_SHORT=182195804614", "ID_WWN=0x5001b448b60a9d6e"}
+
+// oneDiskLVM builds the PVE default layout on one SATA SSD: sda3 → pve-root, pve-data tmeta/tdata → tpool →
+// a guest volume (the shape of all three test boxes).
+func oneDiskLVM(f *fx) {
+	f.node("sda", "", sanDisk...)
+	f.node("sda3", "sda")
+	for _, d := range []string{"dm-1", "dm-2", "dm-3", "dm-4", "dm-6"} {
+		f.node(d, "")
+	}
+	f.slaves("dm-1", "sda3")
+	f.slaves("dm-2", "sda3")
+	f.slaves("dm-3", "sda3")
+	f.slaves("dm-4", "dm-2", "dm-3")
+	f.slaves("dm-6", "dm-4")
+	f.links["/dev/mapper/pve-root"] = "dm-1"
+	f.links["/dev/mapper/pve-data-tpool"] = "dm-4"
+	f.links["/dev/pve/vm-9201-disk-1"] = "dm-6"
+	f.mount("/dev/mapper/pve-root", "/", "ext4")
+}
+
+func TestIdentity_OneDiskLVM_AllPlacesSameProvenDisk(t *testing.T) {
+	f := newFx(t)
+	oneDiskLVM(f)
+	r := f.resolver()
+	want := []string{"serial:ata:SanDisk_X600:182195804614"}
+	for _, p := range []PlaceIdentity{r.HostPath("/var/lib/vz"), r.Device("/dev/mapper/pve-data-tpool"), r.Device("/dev/pve/vm-9201-disk-1")} {
+		if p.Identity != IdentityProven || !reflect.DeepEqual(p.PhysicalDisks, want) {
+			t.Fatalf("want proven %v, got %+v", want, p)
+		}
+	}
+	diff, proven, why := Relation(r.HostPath("/var/lib/vz"), r.Device("/dev/pve/vm-9201-disk-1"))
+	if diff || !proven || why != "same_disk" {
+		t.Fatalf("root vs guest volume: want same+proven, got diff=%v proven=%v why=%q", diff, proven, why)
+	}
+}
+
+func TestIdentity_TwoDisks_ProvenDifferent(t *testing.T) {
+	f := newFx(t)
+	oneDiskLVM(f)
+	f.node("sdb", "", "ID_BUS=ata", "ID_MODEL=TOSHIBA_MQ04ABF100", "ID_SERIAL_SHORT=65NOP3HDT", "ID_WWN=0x5000039ddb108568", "DEVPATH=/devices/pci0000:00/0000:00:14.0/usb1/1-3/block/sdb")
+	f.mount("/dev/sdb", "/mnt/hdd_1", "ext4")
+	f.mount("/dev/sdb", "/mnt/felhom-drives/hdd_1", "ext4") // the felhom-data bind shows the source device
+	r := f.resolver()
+	drive := r.HostPath("/mnt/felhom-drives/hdd_1/userdata")
+	if drive.Identity != IdentityProven || drive.PhysicalDisks[0] != "serial:ata:TOSHIBA_MQ04ABF100:65NOP3HDT" {
+		t.Fatalf("USB drive passing its identity: want proven, got %+v", drive)
+	}
+	diff, proven, why := Relation(r.Device("/dev/pve/vm-9201-disk-1"), drive)
+	if !diff || !proven || why != "" {
+		t.Fatalf("system vs drive: want different+proven, got %v %v %q", diff, proven, why)
+	}
+}
+
+func TestIdentity_TwoPartitionsOneDisk_Same(t *testing.T) {
+	f := newFx(t)
+	f.node("sdc", "", "ID_BUS=usb", "ID_MODEL=Portable", "ID_SERIAL_SHORT=NAAB7C21")
+	f.node("sdc1", "sdc")
+	f.node("sdc2", "sdc")
+	f.mount("/dev/sdc1", "/mnt/a", "ext4")
+	f.mount("/dev/sdc2", "/mnt/b", "ext4")
+	r := f.resolver()
+	diff, proven, why := Relation(r.HostPath("/mnt/a"), r.HostPath("/mnt/b"))
+	if diff || !proven || why != "same_disk" {
+		t.Fatalf("two partitions of one disk: want same+proven, got %v %v %q", diff, proven, why)
+	}
+}
+
+func TestIdentity_LoopPartitionsOnAFile_ResolveToTheFilesDisk(t *testing.T) {
+	f := newFx(t)
+	f.node("nvme0n1", "", "ID_MODEL=KXG50PNV1T02", "ID_SERIAL_SHORT=58BS11AFT8MQ", "ID_WWN=eui.000000000000001000080d020046c07b")
+	f.mount("/dev/nvme0n1", "/mnt/hdd_1", "ext4")
+	f.node("loop2", "")
+	f.loop("loop2", "/mnt/hdd_1/scratch.img")
+	f.node("loop2p1", "loop2") // the partition step must run BEFORE the loop step (the spike's probe bug)
+	f.mount("/dev/loop2p1", "/tmp/scrA", "ext4")
+	r := f.resolver()
+	p := r.HostPath("/tmp/scrA")
+	if p.Identity != IdentityProven || !reflect.DeepEqual(p.PhysicalDisks, []string{"eui:000000000000001000080d020046c07b"}) {
+		t.Fatalf("loop partition: want the NVMe's eui, got %+v", p)
+	}
+}
+
+func TestIdentity_WWNWithoutVendor(t *testing.T) {
+	f := newFx(t)
+	f.node("sda", "", "ID_BUS=ata", "ID_MODEL=AirDisk_512GB_SSD", "ID_SERIAL_SHORT=QDF922W009654S30EX", "ID_WWN=0x5000000123456f1f")
+	f.node("sdb", "", "ID_BUS=ata", "ID_MODEL=Cheap", "ID_WWN=0x5000000123456f1f")
+	r := f.resolver()
+	if p := r.Device("/dev/sda"); p.Identity != IdentityProven || p.PhysicalDisks[0] != "serial:ata:AirDisk_512GB_SSD:QDF922W009654S30EX" {
+		t.Fatalf("a real serial wins over a vendor-less WWN: got %+v", p)
+	}
+	if p := r.Device("/dev/sdb"); p.Identity != IdentityUnknown || p.Reason != ReasonWeakWWN {
+		t.Fatalf("no serial + vendor-less WWN: want unknown/weak_wwn, got %+v", p)
+	}
+}
+
+func TestIdentity_NoSerial(t *testing.T) {
+	f := newFx(t)
+	f.node("vdb", "", "ID_MODEL=")
+	if p := f.resolver().Device("/dev/vdb"); p.Identity != IdentityUnknown || p.Reason != ReasonNoSerial {
+		t.Fatalf("want unknown/no_serial, got %+v", p)
+	}
+}
+
+func TestIdentity_InsideAVM_EverythingUnknown(t *testing.T) {
+	f := newFx(t)
+	oneDiskLVM(f)
+	f.virt = "kvm"
+	f.node("sdb", "", "ID_BUS=scsi", "ID_MODEL=QEMU_HARDDISK", "ID_SERIAL_SHORT=drive-scsi1")
+	f.mount("/dev/sdb", "/mnt/felhom-drives/adatlemez", "ext4")
+	r := f.resolver()
+	a, b := r.Device("/dev/pve/vm-9201-disk-1"), r.HostPath("/mnt/felhom-drives/adatlemez")
+	for _, p := range []PlaceIdentity{a, b} {
+		if p.Identity != IdentityUnknown || p.Reason != ReasonVM {
+			t.Fatalf("inside a VM: want unknown/vm, got %+v", p)
+		}
+	}
+	diff, proven, why := Relation(a, b)
+	if proven || why != ReasonVM || !diff {
+		t.Fatalf("two virtual disks: want not proven (vm), got %v %v %q", diff, proven, why)
+	}
+}
+
+func TestIdentity_USBBridgeSerial(t *testing.T) {
+	f := newFx(t)
+	f.node("sdd", "", "ID_BUS=usb", "ID_MODEL=External_USB3.0", "ID_SERIAL_SHORT=000000000000")
+	f.node("sde", "", "ID_BUS=usb", "ID_MODEL=External_USB3.0", "ID_SERIAL_SHORT=20231106A1B2")
+	r := f.resolver()
+	if p := r.Device("/dev/sdd"); p.Identity != IdentityUnknown || p.Reason != ReasonUSBBridge {
+		t.Fatalf("a bridge placeholder serial: want unknown/usb_bridge, got %+v", p)
+	}
+	if p := r.Device("/dev/sde"); p.Identity != IdentityProven {
+		t.Fatalf("a USB disk with a plausible serial: want proven, got %+v", p)
+	}
+}
+
+func TestIdentity_NetworkShareAndUnmapped(t *testing.T) {
+	f := newFx(t)
+	oneDiskLVM(f)
+	f.mount("//192.168.0.123/TesztFilmek", "/mnt/felhom-drives/omv-filmek", "cifs")
+	r := f.resolver()
+	n := r.HostPath("/mnt/felhom-drives/omv-filmek/x")
+	if n.Identity != IdentityUnknown || n.Reason != ReasonNetwork || len(n.PhysicalDisks) != 0 {
+		t.Fatalf("network share: want unknown/network with no disks, got %+v", n)
+	}
+	_, proven, why := Relation(r.HostPath("/"), n)
+	if proven || why != ReasonNetwork {
+		t.Fatalf("system vs network: want not proven (network), got %v %q", proven, why)
+	}
+	if _, proven, why := Relation(PlaceIdentity{}, n); proven || why != "unmapped" {
+		t.Fatalf("an uncomputed side: want unmapped, got %v %q", proven, why)
+	}
+}

internal/storage/observe.go

@@ -40,6 +40,10 @@ type Observer struct {
 	// R-672 (v0.133.0): a thin pool crossing thinPoolAlarmFraction (data OR metadata) requests an
 	// out-of-band host report at once, so the hub's storage-fill alarm sees it in seconds instead of at
 	// the next 15-minute report. Rising edge per pool; re-armed below thinPoolRearmFraction.
+	// ident (agent v0.157.0, `16` §3) computes each target's physical-disk identity. nil → the fields stay
+	// empty (the pre-0.157 shape), which every consumer reads as "not computed".
+	ident *IdentityResolver
+
 	highMu     sync.Mutex
 	high       map[string]bool
 	onThinHigh func()
@@ -50,6 +54,51 @@ const (
 	thinPoolRearmFraction = 0.85
 )
 
+// SetIdentity wires the physical-disk identity resolver (main: NewIdentityResolver()).
+func (o *Observer) SetIdentity(r *IdentityResolver) { o.ident = r }
+
+// placeIdentity fills a target's identity fields (`16` §3). Network and remote stores are fixed
+// "unknown/network"; a dir storage is judged only on its OWN mount (the same rule as backing_device —
+// never the containing filesystem, so an absent drive is "not computed", never the root disk); the
+// builtin local and an lvmthin pool are the system disk(s) they sit on.
+func (o *Observer) placeIdentity(tgt *hub.StorageTarget, s proxmox.Storage) {
+	if o.ident == nil {
+		return
+	}
+	var p PlaceIdentity
+	switch tgt.Type {
+	case hub.StorageTypeNFS, hub.StorageTypeCIFS, hub.StorageTypePBS:
+		tgt.Kind = PlaceKindNetwork
+		p = NetworkPlace()
+	case hub.StorageTypeLocal:
+		tgt.Kind = PlaceKindSystem
+		if s.Path != "" {
+			p = o.ident.HostPath(s.Path)
+		}
+	case hub.StorageTypeLVMThin:
+		tgt.Kind = PlaceKindSystem
+		if s.VGName != "" && s.ThinPool != "" {
+			p = o.ident.Device("/dev/mapper/" + dmEscape(s.VGName) + "-" + dmEscape(s.ThinPool) + "-tpool")
+		}
+	default:
+		if tgt.BackingDevice == "" {
+			return // not its own mount (absent, or a dir on root): not computed
+		}
+		tgt.Kind = PlaceKindDisk
+		if _, known := wholeDiskOf(tgt.BackingDevice); known && !reWholeDisk.MatchString(tgt.BackingDevice) {
+			tgt.Kind = PlaceKindPartition
+		}
+		p = o.ident.Device(tgt.BackingDevice)
+	}
+	if !p.Computed() {
+		return
+	}
+	tgt.PhysicalDisks, tgt.Identity, tgt.IdentityReason = p.PhysicalDisks, p.Identity, p.Reason
+}
+
+// dmEscape doubles dashes the way device-mapper names an LV (pve + vm-1-disk-0 → pve-vm--1--disk--0).
+func dmEscape(s string) string { return strings.ReplaceAll(s, "-", "--") }
+
 // SetThinHighTrigger wires the out-of-band report request (main: the storage trigger channel).
 func (o *Observer) SetThinHighTrigger(f func()) { o.onThinHigh = f }
 
@@ -303,6 +352,9 @@ func (o *Observer) build(s proxmox.Storage, mounts []Mount) observed {
 		o.noteThinFill(s.Storage+"/data", frac)
 	}
 
+	// Physical-disk identity (`16` §3) — after typ/backing are settled; never from a caller's claim.
+	o.placeIdentity(&tgt, s)
+
 	// SMART-only device hint (v0.95.0): a dir-storage that lives INSIDE a shared filesystem (the
 	// builtin `local` on the LVM root) has empty backing by design, yet we can still read the PHYSICAL
 	// disk's SMART by resolving its containing filesystem. Gated to catDir + no own backing + reachable,

internal/storage/observe_identity_test.go

@@ -0,0 +1,73 @@
+package storage
+
+import (
+	"context"
+	"reflect"
+	"testing"
+
+	"gitea.dooplex.hu/admin/felhom-agent/internal/proxmox"
+)
+
+// The report rows carry the `16` §3 identity per storage type: local + lvmthin → the system disk; a dir
+// on its own partition → that disk, kind "partition"; nfs/pbs → network; an UNMOUNTED dir → not computed
+// (never the root disk — the same rule as backing_device).
+func TestObserve_FillsPhysicalDiskIdentity(t *testing.T) {
+	f := newFx(t)
+	oneDiskLVM(f)
+	f.node("sdb", "", "ID_BUS=ata", "ID_MODEL=TOSHIBA_MQ04ABF100", "ID_SERIAL_SHORT=65NOP3HDT")
+	f.node("sdb1", "sdb")
+	f.mount("/dev/sdb1", "/mnt/usb-backup", "ext4")
+	res := f.resolver()
+
+	api := &fakeStorageAPI{
+		node: "demo",
+		cluster: []proxmox.Storage{
+			{Storage: "local", Type: "dir", Path: "/var/lib/vz"},
+			{Storage: "local-lvm", Type: "lvmthin", VGName: "pve", ThinPool: "data"},
+			{Storage: "usb-backup", Type: "dir", Path: "/mnt/usb-backup"},
+			{Storage: "gone", Type: "dir", Path: "/mnt/gone"},
+			{Storage: "nfs-arch", Type: "nfs", Server: "10.0.0.5", Export: "/x"},
+		},
+		nodeSt: []proxmox.Storage{
+			{Storage: "local", Type: "dir", Path: "/var/lib/vz", Active: 1},
+			{Storage: "local-lvm", Type: "lvmthin", Active: 1},
+			{Storage: "usb-backup", Type: "dir", Path: "/mnt/usb-backup", Active: 1},
+			{Storage: "gone", Type: "dir", Path: "/mnt/gone", Active: 1},
+			{Storage: "nfs-arch", Type: "nfs", Active: 1},
+		},
+	}
+	host := &fakeHostReader{
+		mounts: []Mount{
+			{Device: "/dev/mapper/pve-root", MountPoint: "/", FSType: "ext4"},
+			{Device: "/dev/sdb1", MountPoint: "/mnt/usb-backup", FSType: "ext4"},
+		},
+		exists: map[string]bool{"/dev/sdb1": true, "/dev/mapper/pve-root": true},
+	}
+	ob := NewObserver(api, host, nil, quietLogger())
+	ob.SetIdentity(res)
+	got, err := ob.Observe(context.Background())
+	if err != nil {
+		t.Fatal(err)
+	}
+	m := byName(got)
+	ssd := []string{"serial:ata:SanDisk_X600:182195804614"}
+	for _, n := range []string{"local", "local-lvm"} {
+		if tg := m[n]; tg.Kind != "system" || tg.Identity != "proven" || !reflect.DeepEqual(tg.PhysicalDisks, ssd) {
+			t.Errorf("%s: want system/proven on the SSD, got kind=%q id=%q disks=%v", n, tg.Kind, tg.Identity, tg.PhysicalDisks)
+		}
+	}
+	if tg := m["usb-backup"]; tg.Kind != "partition" || tg.Identity != "proven" || tg.PhysicalDisks[0] != "serial:ata:TOSHIBA_MQ04ABF100:65NOP3HDT" {
+		t.Errorf("usb-backup: got %+v", tg)
+	}
+	if tg := m["nfs-arch"]; tg.Kind != "network" || tg.IdentityReason != "network" {
+		t.Errorf("nfs: got kind=%q reason=%q", tg.Kind, tg.IdentityReason)
+	}
+	if tg := m["gone"]; tg.Identity != "" || len(tg.PhysicalDisks) != 0 {
+		t.Errorf("an unmounted dir must not be computed (never root's disk), got %+v", tg)
+	}
+	// Without a resolver the fields stay empty (the pre-0.157 shape).
+	plain, _ := NewObserver(api, host, nil, quietLogger()).Observe(context.Background())
+	if tg := byName(plain)["local"]; tg.Identity != "" || tg.Kind != "" {
+		t.Errorf("no resolver: want empty identity, got %+v", tg)
+	}
+}