diff --git a/documentation/architecture/07-backup-architecture.md b/documentation/architecture/07-backup-architecture.md
index ee2c898..eb06b23 100644
--- a/documentation/architecture/07-backup-architecture.md
+++ b/documentation/architecture/07-backup-architecture.md
@@ -322,6 +322,54 @@ backups/secondary/<stack>/
   class member that grows like a media library. The pre-push gate + notification is what makes
   that growth a conversation instead of a surprise.
 
+### 9.1 PBS whole-guest tier — measured capacity state (R-82 Phase 0, 2026-07-26)
+
+Recorded per the operator's 2026-07-26 ruling: **the datastore will be grown later; R-82 proceeds
+meanwhile.** These are measurements, not projections-of-record — re-measure before relying on them.
+Full method + evidence: `audits/SPIKE-r82-phase0-2026-07-26.md`.
+
+| Fact | Value | Source |
+|---|---|---|
+| `felhom-offsite` datastore total (ep0) | **37.2 GB** | ep0 `df` via the hub `usage` op (`scripts/felhom-tenantsync.sh` v1.2.0) |
+| Used | **10.8 GB (28.9%)** | same, 2026-07-26 12:02 UTC |
+| Hub alert thresholds | warn **80%** (29.8 GB) / crit **90%** | `monitor.PBSDRBoxChecker` |
+| Snapshots present | **1** — demo-felhom `ct/9201/2026-07-18T18:31:06Z`, 9.74 GB logical, `verify ok`. demo-hp namespace exists with **zero** | PBS API |
+| First-snapshot compression | ≈ **1:1** (9.74 GB logical ≈ 10.8 GB on disk) | source is already-compressed Docker layers |
+| Weekly incremental size | **UNMEASURED** — no guest has ever had a second PBS snapshot | measure at the first Slice-D weekly cycle |
+
+**Three facts that govern the cost model here — all differ from the restic tier above:**
+
+1. **No cross-customer dedup.** Backups are client-side encrypted with a per-customer key
+   (`encryption-key` in `storage.cfg`), so PBS derives chunk digests under that key and chunks
+   never dedup between tenants. Every customer's snapshots cost their full independent size. There
+   is no fleet-scale dividend — the opposite of the shared-model assumption in §9 above.
+2. **The datastore is small relative to the fleet.** At weekly `keep-last` retention the current
+   three boxes project to ≈15–21 GB (40–57%); each additional customer costs ≈5–10 GB, so the 80%
+   warn is reached at roughly the **second additional customer**. This is the open constraint the
+   operator deferred, not a solved problem.
+3. **PVE cannot see this datastore's fill.** `pvesm status felhom-pbs` reports **0/0/0 KiB while
+   active** — PBS answers the status call with HTTP 200 and zeroed usage because the per-customer
+   token holds `DatastoreBackup` on its namespace, not `Datastore.Audit` on the datastore root.
+   **Operators must read PBS fill from the hub's PBS-DR gauge, never from `pvesm`/the PVE UI.**
+   Cosmetic, not a fault: writes work (the 07-18 snapshot is owned by that exact token).
+
+**What the PBS tier does and does not carry** (confirmed live against `pct config 9201` and the
+agent's own `uncovered_volumes`): rootfs + `/var/lib/docker` (mp0, `backup=1`) + `/mnt/sys_drive`
+(mp1, `backup=1`) are **in**; `/mnt/felhom-drives` (mp8) and `/etc/felhom-bootstrap` (mp9) are bind
+mounts and are **out**. The §2 tier table's "bind-mounted drives out of reach" is correct.
+
+**Why weekly is sufficient (R-82 P0.1 verdict).** The only state exposed by a 7-day-old snapshot is
+the non-SMB half of `settings.json` — `storage_paths`, `app_backup` toggles, notification prefs,
+`password_hash`, `launcher_share_token` — all recoverable (drives are physical and re-enrollable;
+the password has a hub claim-reset path). Critically, `encryption.key` and the offbox credentials
+are **stable files unchanged since first boot**, so a week-old copy is byte-identical. Everything
+referentially coupled to app state rides the DAILY tiers.
+
+> ⚠️ **The verdict is conditional on Tier-3 offsite being enabled and healthy.** On a box without it
+> (drill-r50 reports `offsite: null`), PBS-weekly is the ONLY DR tier and the 7-day window would
+> then cover app data and definitions too. Such a box needs offsite enabled first, or a shorter PBS
+> cadence.
+
 ---
 
 ## 10. Decisions requiring Viktor confirmation (one-line vetoes)