infrastructure-control.prose.md

@@ -364,6 +364,79 @@ For docker-vm specifically:
 - No PBS backup in 48h → fail
 ```
 
+### Backup Safety Preconditions (2026-09-15)
+
+#### Background & Rationale
+
+Two incidents from 2026-09-13/14 demonstrate that backup operations can catastrophically fail when storage conditions are not verified first:
+
+1. **acerpve thin-pool VM 101** (acerpve, 192.168.68.9, 2026-09-13): A snapshot-mode vzdump of VM 101 on acerpve filled the LVM thin pool. `dmsetup status pve-data-tpool` showed `thin-pool Error` (then `Fail`), the host root remounted `emergency_ro`, ordinary commands failed with I/O errors, LVM tools returned nothing and VM 101 (the RTX 3090 host) went unreachable while the host still answered ping and ssh. It happened TWICE in one day with different modes: snapshot at 13:39Z and a `--mode stop` cold run at 18:52Z. Both times a reboot rolled the failed transaction back and the pool returned rw (~30% data, ~1.2% metadata). Pool capacity was NOT the obvious explanation - ~816G with ~572G free - which is why the metadata/snapshot-pressure hypothesis stands unproven. A full or errored thin pool fails EVERY volume on the VG at once, including the host root.
+
+2. **amdpve 0700 tmpdir** (amdpve, 192.168.68.15, 2026-09-14): A custom vzdump `tmpdir` created with mode 0700 broke a whole night of container backups: `fstat "<dir>/vzdumptmp<n>_<ct>//." failed - EACCES`, because the archive step runs through an unprivileged user namespace and could not traverse a root-owned 0700 directory. Fixed with `chmod 1777` (match /var/tmp) and proved with a real backup.
+
+3. **acerpve GPU-host fact**: VM 101 (llm-gpu) and VM 103 (ocu-llm) are in NO scheduled job, so their only coverage is one-off runs - and for VM 101 that is deliberate until the thin-pool is understood.
+
+> ⚠️ **Hostname Resolution Warning (2026-09-15)**: The PVE node hostnames (acerpve, amdpve, minipve, storepve, ocupve) all resolve to the VPS (72.61.0.17, the Netbird VPS at srv1079750.hstgr.cloud) via the wildcard `*.dns.sysloggh.net` record, NOT to the actual nodes. So `ssh acerpve` lands on the VPS. **Nodes must be addressed by IP**: acerpve 192.168.68.9, amdpve 192.168.68.15, storepve 192.168.68.6, minipve 192.168.68.12, ocupve 192.168.68.5. Guest CTs are reached through their node (`pct exec`). Guest hostnames that resolve on the LAN (e.g. kagentz = 192.168.68.14) are fine. (The DNS address records are a separate decision — row: dag-daemon-node-hostnames-resolve-to-the-vps-20260915.)
+
+#### PREFLIGHT Preconditions (Before ANY snapshot-mode backup on thin-pool hosts)
+
+Before starting ANY snapshot-mode vzdump on a host whose storage is an LVM thin pool, the following checks MUST pass:
+
+```bash
+# Check 1: Pool headroom (PRIMARY - yields percentages directly)
+# Run on the NODE (e.g. ssh root@192.168.68.9 for acerpve) — NOT by bare hostname, see warning above
+lvs -o lv_name,data_percent,metadata_percent,lv_size pve/data
+# Example output (acerpve, 192.168.68.9):
+#   LV   Data%  Meta%  LSize
+#   data 29.95  1.22   <816.21g
+# Required thresholds (documented minimum):
+# data_percent < 90%  (80% recommended for safety margin)
+# metadata_percent < 70%  (metadata fills faster than data)
+
+# Check 2: Verify pool is not in error state (dmsetup shows the raw DM device)
+# Run on the NODE (e.g. ssh root@192.168.68.9 for acerpve) — NOT by bare hostname
+dmsetup status pve-data-tpool | grep -q "Error\|Fail" && exit 1
+# dmsetup status pve-data-tpool field order (verified on 192.168.68.9):
+#   $1=start  $2=length  $3="thin-pool"  $4=transaction-id
+#   $5=metadata_used/metadata_total (blocks)  $6=data_used/data_total (sectors)
+#   remaining fields are flags ("-", "rw", "discard_passdown", "queue_if_no_space", ...)
+# This is only used for the ERROR-STATE check; use the lvs command above for percentages.
+#   metadata_percent = $5 / ($5 split by /) [second number in pair]
+```
+
+**Minimum thresholds**: If either `data_percent >= 90%` or `metadata_percent >= 70%`, the backup MUST NOT start. State explicitly that these are hard stops, not warnings.
+
+**Why this is a precondition**: A full or errored thin pool fails EVERY volume on the VG at once, including the host root. This is not a soft failure - it takes down the entire Proxmox host.
+
+#### Staging Directory Requirement (2026-09-14 incident)
+
+Any custom vzdump `tmpdir` MUST be world-traversable and writable exactly like `/var/tmp` (mode 1777). The archive step of vzdump runs in an unprivileged user namespace and cannot traverse a root-owned 0700 directory.
+
+**Symptom to recognize**: `fstat "<dir>/vzdumptmp<n>_<ct>//." failed - EACCES` on every container in the backup run.
+
+**Fix**: `chmod 1777 <custom-tmpdir>` before starting vzdump.
+
+#### Task Start Rule for Truncating Shells
+
+When starting a backup task from a shell that may truncate output (e.g., pipes, `head`), always use:
+
+```bash
+pvesh create /storage/backup --output-format json -- ... | head -2
+# ❌ Can kill the backup task ("broken pipe" status)
+```
+
+Instead, capture JSON output without piping to truncating commands:
+
+```bash
+# Use --output-format json and capture to variable
+result=$(pvesh create /storage/backup --output-format json -- ...)
+# Then parse result if needed
+```
+
+#### GPU Host Backup Status (acerpve VM 101)
+
+VM 101 (llm-gpu) and VM 103 (ocu-llm) have NO scheduled backup job. Coverage is manual one-off runs only. This is intentional for VM 101 until the thin-pool failure mechanism is understood and documented.
+
 ## Section 5: Network Services — Monitoring
 
 ### 5.1 Service Inventory