# 07 — Backup architecture: tiers × classes × targets

> **Status: DRAFT — awaiting Viktor's review (async one-line vetoes on the §10 list).**
> Written 2026-07-14 per the architecture-doc-first gate (Viktor ruling #5). Every claim below was
> verified against live Gitea at commit **felhom-controller `95f3180` (v0.132.0)**, felhom.eu
> `deacee11`, catalog `21e8df1`, agent `c040c18` (v0.88.0), hub v0.54.0. Line numbers are
> landmarks — reconfirm before editing.
>
> Inputs: `SPIKE-backup-classification-2026-07-14.md` (`790ec84`), `CAMPAIGN-6C-2026-07-14.md`
> (`deacee11`, F-6C-1), Viktor's locked rulings of 2026-07-14. This document DECIDES; it does not
> survey. Where a decision rests on an unproven restic mechanism, it is marked **SPIKE** and listed
> in §6.4 — per the spike-first gate, those spikes precede the Task 3a spec.

---

## 0. Verified baseline — what the code does today (source, not memory)

| Surface | Behavior | Evidence (v0.132.0) |
|---|---|---|
| Recovery unit (tier-1) | `compose/` (docker-compose.yml, `.felhom.yml`, secret-stripped app.yaml) + enumerated `db-dumps/*.sql` + `volume-dumps/*.tar` + `manifest.json` (SchemaVersion **1**). No userdata, no HDD appdata. | `recovery_unit.go:91-106,131` |
| Offsite (tier-3) | One `restic backup <unitdir> --tag felhom-offbox --tag <stack>` per toggled app; unit dir discovered across nsRoots, newest-manifest tiebreak. Retention `forget --keep-daily 7 --keep-weekly 4 --keep-monthly 6 --prune`, **default grouping = host+paths**. | `offbox.go:504-552` (discover), `:575` (backup), `:590-597` (forget) |
| Offsite quota | Soft gate on `RepoSizeBytes` from `restic stats --json` with **no mode flag → default restore-size over all snapshots**; ≥100% refuses new backups (prune still runs), ≥80% warns. | `offbox.go:407-415,630-645,700-708` |
| Offsite restore | `restic restore latest --tag <stack> --target <DataDir>/offbox-restore/<app>` — scratch, non-destructive, whole snapshot. **DataDir default = `/opt/docker/felhom-controller/data` — the guest rootfs.** | `web/offbox_handlers.go:236-246`, `config/config.go:310` |
| Tier-2 | rsync mirror of unit → `backups/secondary/<stack>/recovery-unit` + resolver-driven appdata dir → `.../appdata` (flat; **refuses N>1**). `rsync -a --delete`. Target pick: pinned > other data drive > SSD (headroom-gated); **zero fs-type awareness** (F-6C-1). Size for headroom = `dirSizeBytes(unit)+dirSizeBytes(appdata)`. | `tier2.go:170-249` (run), `:92-155` (select), `:437-445` (rsync), `:198` (size) |
| Tier-2 restore | Missing-files-only merge into live appdata: `rsync -a --ignore-existing`; refuses N>1. | `tier2_restore.go:15-44,127-137` |
| Manual `.fab` | Config + DB dump + volume tars + `ExportDataMounts` = `${HDD_PATH}` binds ∪ the **whole userdata root** as one tar (SQ6 over-capture). | `stacks/delete.go:575`, `appexport/export.go` (v0.130.0) |
| Classification (INERT) | `stacks.Manager.ClassifiedBinds(name)` → `([]ClassifiedBind, hasClassification)`; whole-block-reject at `LoadMetadata`; two-level default (explicit > `:ro`-reader > writable-mandatory; **no block → legacy**). Wired seam: `StackDataProvider.GetStackClassifiedBinds` (adapter main.go ~L1106). 13 catalog apps carry blocks (`21e8df1`). | `metadata.go:285-294`, `appbackup/classify.go:97,165` |
| Network storage metadata | `settings.StoragePath.Kind == "network"` + `IsNetwork()`; `Protocol` nfs\|smb. Drive lifecycle already refuses network paths. | `settings.go:181-219,1057` |

Two **new findings** from this verification pass (not in the spike or 6C):

- **F-A1 (HIGH for 3a): the offsite restore scratch dir is a rootfs-filler once snapshots carry
  userdata.** `offbox-restore/<app>` lives under `cfg.Paths.DataDir`, default
  `/opt/docker/felhom-controller/data` on the ~8 GB guest rootfs. Today snapshots are unit-sized
  (MB–1 GB); with mandatory userdata (nextcloud's data dir, immich's upload library) a
  `restore latest` can fill the rootfs and take the guest down. §7 decides the fix.
- **F-A2 (MEDIUM for 3a): quota accounting mode is wrong-by-design for the new shape.**
  `restic stats` default restore-size aggregates across **all retained snapshots** (~17 per app at
  full retention). Exact cross-snapshot counting semantics are undocumented enough to matter:
  if a 50 GB mandatory library counts once per snapshot, the gate trips at ~17× reality. §9 decides
  the candidate fix (`--mode raw-data`); **SPIKE SP-1** proves it.
- Minor: `tier2.go:167` (RunTier2 doc comment) still claims "recovery unit + **userdata**" — the
  last surviving F-S1 stale comment (fourth stale-comment instance this arc). 3b makes it true.

---

## 1. The class model (normative recap)

A class belongs to a **bind (host path)**, not to an app (SQ2 rule, locked):

- **mandatory** — referentially COUPLED to app state (DB rows reference the content; SQ3: restore
  without it = broken-not-empty). Never deselectable at any tier that carries userdata.
- **optional** — DECOUPLED but precious (hand-curated, not re-acquirable). Customer-selectable.
- **excluded** — DECOUPLED bulk/transient/re-derivable. Never captured automatically; `.fab` opt-in
  only, behind the two-number warning.
- **legacy** (origin, not a class) — app has no `backup:` block → every tier behaves byte-identically
  to v0.132.0. The safety net fails toward *today's cost*, never toward a quota blow-up (SQ5).

Roots: `${HDD_PATH}` (`hdd:` list; appdata stores) and `${USERDATA_PATH}` (`userdata:` list;
browsable tree). RelPaths are `${VAR}`-relative and hddPath-invariant (survive migration flips —
the Task 1 lesson). Resolution to absolute paths happens at capture time against the app's live
`HDD_PATH` (Model A: HDD_PATH == nsRoot).

---

## 2. The decision table: tier × class × target

| Tier | legacy (no block) | mandatory | optional | excluded | Allowed targets | Copy mechanism | Restore path |
|---|---|---|---|---|---|---|---|
| **Tier-1** recovery unit | config + DB dumps + volume tars | *unchanged* — the unit never carries userdata; classes ride inside it via the captured `.felhom.yml` | — | — | app's own drive (`backups/primary/<app>`) | direct writes | `RestoreApp` (existing) |
| **Tier-2** cross-drive | unit + resolver-appdata (byte-identical to v0.131.0) | unit + mandatory binds | + optional binds | never | **real local drives only** (network paths excluded per F-6C-1 ruling — both auto and pinned); SSD fallback = unit + mandatory **iff** headroom fits, optional skipped with honest reason | `rsync -a --delete` per capture-set path | missing-only merge (`--ignore-existing`), per-path from the new layout (§8) |
| **Tier-3** offsite restic | unit only (today's shape) | unit + mandatory binds — **not deselectable** | never | never | Hetzner Storage Box (SFTP) | one multi-path restic snapshot per app per run (§6) | staged scratch on a data drive → missing-only merge to live (§7) |
| **Manual `.fab`** | v0.130.0 full-root capture | locked-in (not deselectable) | checkbox, **pre-selected** | opt-in, behind the two-number size warning + FileBrowser pointer | download / chosen drive | tar, **exclusion-scoped** root (SQ5 verdict; manifest v1 unchanged) | existing import (old controllers import new bundles correctly) |
| **PBS** whole-guest | rootfs + docker volumes; bind-mounted drives out of reach | *unchanged* | | | PBS on DooPlex | vzdump | whole-guest restore |

Row-level decisions folded in:

1. **Classified apps' tier-2 appdata leg becomes fully class-driven** — per-bind paths, not the
   whole appdata dir. Consequence: paperless-ngx's tier-2 copy *shrinks* (`appdata/paperless/media`
   mandatory in, `appdata/paperless/export` excluded out) — correct per the model, and the N>1
   refusal disappears for classified apps because per-bind capture never needed the single-dir
   assumption. The F-S2 resolver remains **only** for legacy apps.
2. **The SSD fallback is a state-only tier.** Its existing Hungarian reason already says so
   ("csak az adatbázis/konfiguráció fér a belső SSD-re"); the engine now enforces it: unit +
   mandatory if `tier2FitsHeadroom` passes for that reduced set; optional never goes to the SSD.
3. **Offsite never carries optional.** Optional is the customer's local-copy tier; offsite cost is
   shared-model quota. (Viktor ruling #1, restated for the matrix.)
4. **Undeployed apps** (unit discovered on a drive, stack not deployed): offsite pushes the
   **legacy unit-only shape** + WARN. Rationale: mandatory-path resolution requires the live
   HDD_PATH; guessing it from a stripped app.yaml risks capturing a stale or foreign tree. No
   protection regression vs today.
5. **Declared-but-absent mandatory path** (compose/classes declare it, dir missing on disk): the
   F-S2 pattern — skip that path, **loud WARN**, capture the rest. The stat-filter before the
   restic invocation is MANDATORY and is the ONLY detection point: restic 0.14.0 (the pinned
   production binary) does NOT error on a nonexistent source path — it skips with a stderr
   warning, exits 0, and silently writes a partial snapshot
   (SPIKE-restic-snapshot-shape-2026-07-14.md, SP-3.4). 3a must never rely on a nonzero restic
   exit to catch a missing mandatory path; the controller stat-filters to detect the absence
   itself and raises the WARN from its own check. (If the container's restic is ever bumped,
   SP-3.4 must be re-run — later releases changed this exit behavior.)

---

## 3. Capture-set computation (Task 3-core)

One pure function, leaf package `appbackup`, consumed by all three engines (as-built, v0.133.0):

```
ComputeCaptureSet(binds []ClassifiedBind, hasClassification bool,
                  tier CaptureTier, hddPath string) CaptureSet
  → CaptureSet{HasClassification bool; Paths []CapturePath; Skipped []SkippedPath}
```

- Resolves each bind's `(Root, RelPath)` against `hddPath` (HDD) / `hddPath+"/userdata"`
  (USERDATA) into absolute paths (slash algebra — `path.Join`, never `filepath`); filters by the
  tier column of §2 (`TierOffsite` = mandatory only; `TierSecondary` = mandatory + optional;
  excluded is silently dropped). Each `CapturePath` carries `{Abs, Root, RelPath, Class}` (the
  relpath is what the tier-2 layout and restore need).
- The former `UnitOnly` field is replaced by **`HasClassification`** (it mirrors the classifier's
  existing bool); engines derive unit-only as `!HasClassification || len(Paths)==0`. When
  `hasClassification == false` the function short-circuits to `{HasClassification:false}` — nothing
  resolved, `Paths`/`Skipped` nil — so the legacy path stays byte-identical and testable (the SQ5
  cost-regression guard: an unmigrated app never resolves a bind into an automatic tier).
- **`Skipped`** carries would-be captures the tier filter selected but that are structurally unsafe,
  each with an English `Reason`, so the pure function stays log-free (engines log a skipped
  **mandatory** as a loud capture GAP). Three structural guards, evaluated after the tier filter:
  traversal (a `..` path segment or an absolute RelPath — **load-bearing because the compose parser
  path.Cleans but does NOT reject `..`, and `ValidateBackupSpec` only vets *spec* entries, so an
  unlisted writable `${HDD_PATH}/../x` bind arrives classed mandatory**), bare HDD drive-root
  (RelPath `""` — would nest `<hddPath>/backups` into the capture), and the reserved `backups/`
  zone; a bare **userdata** root (`<hddPath>/userdata`) is allowed (it does not nest `backups/`).
- After filtering+guards: an equal-`Abs` collision from two binds collapses to one entry with
  **mandatory winning** over optional (mandatory semantics never degrade); then **containment
  dedup** drops any path whose ancestor is already in the set (keep the ancestor); output is sorted
  by `Abs` (deterministic despite map/slice input).
- `CrossAppOverlaps(map[app]CaptureSet) []Overlap` is shipped here as a **pure** function (exact-Abs
  match across ≥2 apps' `Paths`; cross-app *containment* is legitimate per §4 and is NOT an overlap).
  Its WARN **wiring** lands in 3a/3b, not in 3-core — no log call sites exist yet.
- Pure: no FS, no docker, no logging (the Tasks 1/2 leaf-package discipline). The stat-filter
  (decision 2.5) lives in the engines, not here.
- Companion red-proofs: (a) sonarr with no block must produce unit-only (`HasClassification:false`,
  empty `Paths`) for offsite (the SQ5 cost-regression guard — the test that fails if the two-level
  default is miswired); (b) immich's explicit-optional `:ro` external library must appear in
  tier-2's set and NOT in offsite's; (c) paperless's `export` must appear in no automatic tier.

The engines consume the existing wired seam `GetStackClassifiedBinds` (main.go ~L1106) — no new
seam. Per the F-S3 lesson, 3-core still ships an end-to-end wiring test through a real Manager.

---

## 4. Cross-app dedup and writing authority

**Capture attribution rule (refined).** SQ2's "the writing app is the classification authority"
covers apps that write a path — but immich's external library (`media/photos:ro`, explicit
*optional*) is populated by the **customer via FileBrowser**, not by any app. The generalized rule:
**capture attribution = the app whose classified capture set contains the path.** Writing authority
remains the *catalog* rule for who gets to class a path non-excluded.

**Dedup decision: the capture engine dedups nothing across apps.** Ground truth from the SQ2
inventory: the overlap set for non-excluded classes is **empty today** — radarr+sonarr's shared
`downloads/` is excluded on both; the streamers' whole-`media` binds are RO-excluded readers;
calibre-web's mandatory `media/books` merely sits *inside* those excluded reader binds
(containment, not conflict). Engineering cross-app dedup for an empty case buys risk, not value.
Instead:

1. **Catalog convention (normative):** at most one app may class a given host path non-excluded.
   Two writers to one path is a catalog bug.
2. **Advisory guard (3-core, log-only):** a cheap cross-app pass at capture time WARNs if two
   deployed apps' sets contain the same absolute path — the convention's tripwire, no behavior.
3. Cost note if the convention is ever violated: offsite double-capture is nearly free in real repo
   bytes (restic content-defined chunking dedups across snapshots); tier-2 pays disk twice. The WARN
   makes it visible before it costs.

**Restore side.** Per-app restore restores *that app's* captured paths to their recorded relpaths —
attribution is positional, so the containment case is trivially correct: `media/books` restores
under calibre-web's set whether or not plex exists, and restoring plex restores nothing of
`media/` (it captured nothing). A path captured by exactly one app is restored by exactly that app.
The missing-only merge (§7) makes overlapping restores idempotent even in the violation case.

---

## 5. Ownership/permission fidelity per mechanism, and the NAS-as-target stance

The motivating failure is F-6C-1: `rsync -a` (`-o -g`) → chown on a root_squash NFS export →
exit 23 → the whole tier-2 run fails. Fidelity matters because userdata carries the setgid
convention (`SharedContentGID=1000`, mode 2775 — `appbackup/userdata.go:19-24`) and appdata stores
carry app-specific uids; a wrong-owner restore into a uid-sensitive app is a *silently broken*
restore.

| Mechanism | uid/gid | mode incl. setgid | xattrs | Fails on root_squash target? | Verdict |
|---|---|---|---|---|---|
| `rsync -a --delete` (tier-2) | preserved (`-og`) | preserved (`-p`) | no (`-X` not set — acceptable, nothing in the tree needs xattrs) | **yes** (the F-6C-1 chown) | keep for local drives; never point it at network fs |
| restic backup/restore (offsite) | stored in metadata; restored when run as root | preserved | yes | no (metadata lives in the repo, not on the target fs) | correct for the enlarged offsite shape as-is |
| tar (`.fab`) | preserved in archive; `--same-owner` applies on root extract | preserved | GNU tar: with flags | no | correct; Task 4 verifies the extract flags at spec time |

**NAS-as-tier-2-target stance (ruling #4, recorded):** excluded for now — auto AND pinned — with an
honest Hungarian card reason. Mechanism: filter `sp.IsNetwork()` in `selectTier2Target` (both the
pinned-validity check and the auto loop) — the metadata already exists (`settings.go:219`); no
fs-probing. Dropping `-o/-g` was rejected because it converts a loud failure into a silently
wrong-owner restore. **Revisit condition:** a tar-based tier-2 leg (ownership inside the archive)
would make NAS viable; that is a separate future task with its own restore round-trip proof, not a
Task 3 rider.

---

## 6. Offsite snapshot shape

**Decision: one multi-path snapshot per app per run.**
`restic backup <unitdir> <mandatory-abs-1> … --tag felhom-offbox --tag <stack>`.

Why this shape over two-snapshots-per-app (unit vs userdata split):

- Preserves the invariant every consumer relies on today: *latest `--tag <stack>` = one consistent
  app-level point* (`offbox.go:732` restore, SnapshotCount, the hub report).
- restic chunk-dedup makes unchanged media across daily runs cheap; the incremental cost of a run
  is the delta, not the library.
- The snapshot's own `paths` metadata (visible in `snapshots --json`) records exactly what was
  captured — the restore side introspects shape from there, so **no recovery-unit manifest change
  and no SchemaVersion bump** (spike rec #6 considered; answered: the unit's content and shape are
  unchanged, classes already ride in the captured `.felhom.yml`, and snapshot-paths metadata is a
  better shape record than a manifest field that can drift from it).

**Consequences that must be proven before the 3a spec (spike-first gate — these are unvalidated
restic mechanisms):**

- **SP-1 — quota stats semantics.** Measure `restic stats` default (restore-size, all snapshots)
  vs `--mode raw-data` on a repo holding N retained snapshots of a large path. Expected: raw-data ≈
  actual Storage Box disk; restore-size multiplies. Decides §9's accounting switch.
- **SP-2 — retention grouping across a shape change.** Default `forget` grouping is host+**paths**;
  the first enlarged push changes the path set → old-shape snapshots strand in their own group and
  its keep-daily/weekly/monthly floor retains them **forever** (no new snapshots ever age them
  out) — zombie quota cost. Candidate fix: `--group-by host,tags` (the `<stack>` tag groups old and
  new shape together so old shape ages out naturally). Prove: seed old-shape snapshots, push
  new-shape, run forget with the flag, verify old-shape aging.
- **SP-3 — multi-path restore shape.** Verify the `--target` tree layout for a multi-path snapshot
  (absolute source paths reconstructed under target), and that `--include <unitpath>` gives a
  unit-only selective restore. Feeds §7.

All three fit one nested-VM spike session against a scratch restic repo (no Hetzner needed —
a local SFTP or even a local repo reproduces the semantics).

---

## 7. Offsite restore path (3a first-class scope)

Today's restore is dump-to-scratch-and-inspect. With userdata in snapshots it needs three changes:

1. **Scratch relocation + headroom gate (fixes F-A1).** The scratch target moves from
   `DataDir/offbox-restore` (rootfs) to a data-drive location
   (`<nsRoot>/backups/offsite-restore/<app>` on the app's drive, or the tier-2 target-pick logic
   reused to find room). Before restoring, read the snapshot's logical size (restic
   `stats <snap>` / `snapshots --json`) and refuse with an honest Hungarian reason if the target
   lacks headroom — the `SizeUnknown`-never-renders-as-fits guard philosophy applies.
2. **Selective restore.** Default operator action restores the **unit only** (`--include` the unit
   path — SP-3 proves the flag shape); "full restore" (unit + userdata) is an explicit second
   action showing the size first. Keeps the daily case cheap, makes the big case deliberate.
3. **Restore-to-live = staged missing-only merge.** For the SQ3 acceptance ("immich restorable
   end-to-end from offsite alone"): restore to scratch → place mandatory paths into their live
   locations with the `tier2_restore.go` merge pattern (`rsync -a --ignore-existing`,
   never-delete) → then the existing unit restore (`RestoreApp`) brings the app up. Data before
   app start — the DB references the paths, so the paths must exist when the app first scans.
   Full-overwrite restore stays out of scope (PBS + DR tier own catastrophic recovery).

---

## 8. Tier-2 destination layout rework + migration

**New layout (v2), relpath-mirroring:**

```
backups/secondary/<stack>/
  .felhom-tier2-layout        # marker: "2"
  recovery-unit/              # unchanged leg
  hdd/<relpath>/              # e.g. hdd/appdata/paperless/media/
  userdata/<relpath>/         # e.g. userdata/media/books/
```

- Relpath mirroring represents N>1 appdata dirs and nested binds natively (the v0.131.0 refusal is
  lifted structurally, not by special-casing) and makes restore position-derivable: dest relpath →
  live path under the app's current HDD_PATH — hddPath-invariant like the classifier itself.
- **Legacy apps use the same layout**: the resolver's appdata dir(s) map to
  `hdd/appdata/<name>/`. Capture SET stays byte-identical (the SQ5 promise governs footprint and
  cost, not the internal arrangement of a derived copy); one layout means one restore reader.
- **Migration = rebuild, not preserve.** Tier-2 copies are fully derived from live data. On the
  first v2 run per app: marker absent → delete the old flat `appdata/` (and the old
  `recovery-unit/` stays — it's layout-identical) → mirror fresh into v2 → write the marker last
  ("floor field LAST" family: the marker asserts a complete layout, so it is written only after all
  legs succeed).
- **Reconcile step (answers the v0.131.0 deferred pruning question):** after mirroring, dest
  subdirs under `hdd/`/`userdata/` not present in the current capture set are removed — a bind
  removed from the compose (or re-classed excluded) stops occupying the secondary drive within one
  run.
- **Restore reads v2 only.** Marker absent → honest refusal: "futtass előbb egy másodlagos
  mentést" — acceptable because tier-2 restore is missing-files recovery, and the source of truth
  (live data) still exists in that scenario; pre-migration copies stay on disk untouched until the
  first v2 run replaces them.
- Headroom math: `unitSize` becomes unit + Σ(capture-set path sizes) via the existing `du -sb`
  loop; the SSD fallback computes it over the reduced (unit+mandatory) set per §2.2.

---

## 9. Quota & cost model

- **Accounting switch (pending SP-1): quota compares `restic stats --mode raw-data` — actual
  deduped+compressed repo bytes, i.e. what the customer's Storage Box really fills** — instead of
  today's all-snapshots restore-size. Without this, ~17 retained snapshots of a mandatory library
  would multiply the measured usage and permanently trip the ≥100% gate. The existing
  stale-but-safe last-known-value behavior stays.
- **Pre-push gate (ruling #2 mechanized).** Before an app's **first enlarged push** (and whenever
  its mandatory-set size estimate grows materially), compute the mandatory-set size with the
  existing per-mount `du` loop (`estimate.go` reuse — zero new measurement plumbing, SQ4). If
  last-known repo size + estimate crosses the quota: **the enlarged push is blocked and the
  customer is notified with the two numbers** — but the **unit-only push continues** (blocking it
  too would *regress* existing protection; the gate governs the enlargement, not the tier).
  The notification is the "size surfacing before the first enlarged push" from the ruling.
- The existing ≥80% warn / ≥100% refuse-new-backups run gate stays as the backstop, re-based on
  raw-data numbers.
- **Tier-2 cost:** headroom per §8; the two-number estimate (state+coupled vs +bulk) reuses the
  classified `du` split for the `.fab` warning (Task 4) and the tier-2 config panel display.
- Hetzner reality check: BX11 = 1 TB shared-model. Mandatory ≈ DB-coupled stores (immich uploads,
  nextcloud data, paperless documents) — bounded per SQ4's model, but immich uploads are the one
  class member that grows like a media library. The pre-push gate + notification is what makes
  that growth a conversation instead of a surprise.

---

## 10. Decisions requiring Viktor confirmation (one-line vetoes)

The locked rulings are not re-litigated; these are **consequences the doc surfaced**:

1. **Blocked-enlargement semantics (§9):** when the quota gate blocks an app's enlarged push, its
   unit-only push CONTINUES (no protection regression). Confirm. <--- Confirmed
2. **`.fab` optional default = pre-selected** (spike rec #2 wording; ruling #1 said "checkboxes"
   without a default). Confirm pre-selected. <-- Conrifmed
3. **Offsite restore default = unit-only; full (unit+userdata) is an explicit second action with
   the size shown first (§7.2).** Confirm. <-- Conrifmed
4. **Undeployed apps stay legacy unit-only offsite + WARN (§2.4).** Confirm. <-- Conrifmed
5. **Tier-2 v2 layout migration = delete-and-rebuild of the derived secondary copy (§8),** old
   copies replaced on the first v2 run per app. Confirm. <-- Conrifmed
6. **Quota accounting switches to raw-data bytes (§9), pending SP-1.** Confirm direction. <-- Conrifmed
7. **SSD fallback becomes explicitly state-only (unit + mandatory-if-fits, never optional)
   (§2.2).** Confirm. <-- Conrifmed

## 11. Explicitly out of scope (noted, not decided here)

- Cluster-mode / HA implications (Peti's two-node roadmap item): tier-2 target selection and
  drive discovery assume a single host's drive set; agent-follows-guest will revisit.
- `.fab` exclusion-scoping implementation details (Task 4 owns them; the SQ5 verdict — root tar
  minus excluded subtrees, manifest v1 unchanged — stands and is assumed by §2's `.fab` row).
- NAS as a tier-2 target via a tar-based leg (§5 revisit condition).
- Hub-side surfaces: nothing here forces a hub field; if the 3a report shape adds one (e.g.
  enlarged-push-blocked state), the spec flags it explicitly.

## 12. Observations (documented, not acted on)

- `tier2.go:167` RunTier2 doc comment still says "recovery unit + userdata" (stale F-S1 family) —
  3b corrects it as a side effect of making it true.
- `offboxRecordStats` reuses one probe-timeout context (`sctx`) for both `snapshots` and `stats`
  (`offbox.go:686-702`) — with a large repo the second call inherits whatever budget remains; worth
  a per-call timeout when 3a touches the file. Not a bug today.
- The offsite restore handler backgrounds a 30-minute context (`offbox_handlers.go:230-233` per the
  F4 fix); enlarged restores may need a bigger ceiling — spec-time decision in 3a, sized by SP-3
  timing evidence.

## Appendix — verified symbols Task 3 builds on

| Symbol | File (landmark) | Note |
|---|---|---|
| `Manager.ClassifiedBinds` | `stacks/metadata.go:285` | the seam; validated via `LoadMetadata` |
| `appbackup.ClassifyBinds` / `ValidateBackupSpec` | `appbackup/classify.go:165/:97` | two-level default; whole-block-reject |
| `appbackup.AppDataDirNames` | `appbackup/paths.go:90` | legacy resolver (stays for no-block apps) |
| `StackDataProvider.GetStackClassifiedBinds` | adapter `main.go:~1106` | wired end-to-end (Task 2) |
| `Manager.RunTier2` / `selectTier2Target` / `rsyncMirror` | `tier2.go:170/:92/:438` | engine to rework; `-a --delete` |
| `settings.StoragePath.IsNetwork` | `settings/settings.go:219` | the F-6C-1 filter, metadata-only |
| `Manager.RunOffboxBackup` / `runOffboxInternal` / `discoverOffboxUnit` | `offbox.go:376/:559/:508` | push engine; unit discovery |
| `Manager.RestoreOffbox` | `offbox.go:722` | restore-to-scratch; §7 rework |
| `offboxQuotaState` / `offboxRecordStats` | `offbox.go:632/:685` | quota gate; stats-mode switch |
| `restoreTier2Files` merge pattern | `tier2_restore.go:127-137` | `--ignore-existing` never-delete merge |
| `Exporter.EstimateExport` / `duBytes` | `appexport/estimate.go:33/:126` | the two-number split reuse |
| `RecoveryManifest` (SchemaVersion 1) | `recovery_unit.go:31,131` | unchanged by this design |

**Do NOT reuse** `rsyncMirror` for any restore leg (`--delete` — the named trap); restore merges
use the `--ignore-existing` pattern only.