diff --git a/CHANGELOG.md b/CHANGELOG.md
index d121187..bdedc4d 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -3,6 +3,31 @@
 All notable changes to **felhom-agent** are recorded here. Update on every code
 change that gets pushed.
 
+## v0.29.0 — OS / Docker-data storage split: golden + provision (2026-06-13)
+
+Phase 1 of the storage-split slice (Phase 2 = felhom-controller v0.58.0 prevention layer). The
+controller guest's OS rootfs and Docker data are carved onto separate `local-lvm` volumes for
+RESILIENCE — an isolated OS rootfs stays bootable + agent-recoverable if the Docker volume fills.
+
+- **`configs/build-golden.sh` — split baked in:** `--rootfs ${ROOTFS_STORAGE}:${OS_SIZE_GB}` (default
+  **32**, was hardcoded 8) **plus** `--mp0 ${ROOTFS_STORAGE}:${GOLDEN_DOCKER_GB},mp=/var/lib/docker,backup=1`
+  (default 16). The baked controller + infra images land on the data volume and travel inside the
+  golden archive (no empty-volume shadowing, no deploy-time pull). `backup=1` is MANDATORY — extra LXC
+  mountpoints default to `backup=0` = EXCLUDED from vzdump (spike B3), which would drop the images from
+  the archive entirely. The script now also bakes Docker **log rotation** into `daemon.json`
+  (`max-size 10m`, `max-file 3` — prevention layer 2D), asserts `/var/lib/docker` is a separate mount,
+  and **aborts if vzdump excludes mp0**.
+- **`internal/reconcile/bringup.go` — sized provision:** `GuestMount` gains `Backup` (emits `,backup=1`
+  — closes the spike-B3/B5 silent-DB-loss trap at the mount builder). `BringUpSpec` gains
+  `DataVolGrowGB` + `DataVolMount` (default `mp0`): provision GROWS the golden-carried Docker-data
+  volume online to the per-customer target (grow-only, spike B4) rather than attaching a fresh empty
+  volume that would shadow the baked images. Plus `RootfsGrowGB` for the OS rootfs.
+- **CLI seam:** `--selftest=bring-up|provision` gain `-rootfs-grow` / `-datavol-grow` / `-datavol-mount`
+  flags. Per-customer sizing source = flags now, the slice-10 hub storage manifest later.
+- **`RUNBOOK-provisioning-storage.md`** (new): the split provisioning procedure + fresh-PVE-install
+  thin-pool carving knobs (`hdsize`/`maxroot`/`maxvz`, spike B4) + the per-customer sizing seam.
+- Tests: `buildBringUpConfig` backup=1 emission; bring-up issues rootfs + data-volume resizes.
+
 ## (no version) — storage OS/data-split spike findings (2026-06-13)
 
 Investigation only — **no code changed**. Findings report: `REPORT-storage-split-spike.md` (gates the
diff --git a/RUNBOOK-provisioning-storage.md b/RUNBOOK-provisioning-storage.md
new file mode 100644
index 0000000..54ad330
--- /dev/null
+++ b/RUNBOOK-provisioning-storage.md
@@ -0,0 +1,89 @@
+# RUNBOOK — provisioning the OS / Docker-data storage split
+
+How the controller guest's **OS rootfs** and **Docker data** are carved onto separate `local-lvm`
+volumes, and how to size them per node/customer. Grounded by the storage-split spike
+(`REPORT-storage-split-spike.md`). Defaults target a ~512 GB SSD: **32 GB OS rootfs + 256 GB
+Docker-data**, rest of the thin pool free for other guests.
+
+## The design (locked)
+
+- **Split for resilience, not backup size.** An isolated OS rootfs stays bootable + agent-recoverable
+  if the Docker volume fills. Both volumes ride into PBS regardless (so the backup size is unchanged) —
+  the win is independent sizing/growth and a smaller, faster-to-restore OS rootfs.
+- **One Docker daemon, one data-root, one volume** at `/var/lib/docker` for ALL images + overlay
+  (controller, traefik, cloudflared, filebrowser, and customer apps). Infra is NOT relocated to the OS
+  partition — infra/user separation under one daemon isn't cleanly possible and isn't needed. Infra is
+  protected by the controller's **prevention layer** (reserved buffer + deploy gate + disk alerts +
+  baked log rotation), not by placement.
+- **`backup=1` on the Docker-data volume is non-negotiable.** Extra LXC mountpoints default to
+  `backup=0` = EXCLUDED from vzdump/PBS (spike B3). Without it, named-volume DBs fall out of the
+  snapshot AND the volume (with baked images) is excluded from the golden archive.
+
+## Golden build (per controller-image baseline)
+
+`configs/build-golden.sh` builds the template guest with the split baked in:
+
+```
+# defaults shown; override via env
+OS_SIZE_GB=32 GOLDEN_DOCKER_GB=16 \
+  ./build-golden.sh <VMID> <TEMPLATE> <ROOTFS_STORAGE> <ARCHIVE_STORAGE> <BRIDGE> <CONTROLLER_IMAGE>
+```
+
+- `--rootfs ${ROOTFS_STORAGE}:${OS_SIZE_GB}` + `--mp0 ${ROOTFS_STORAGE}:${GOLDEN_DOCKER_GB},mp=/var/lib/docker,backup=1`.
+- Keep `GOLDEN_DOCKER_GB` small (just the baked images + headroom) — provision grows it.
+- The script bakes `/etc/docker/daemon.json` log rotation (`max-size 10m`, `max-file 3`), verifies
+  `/var/lib/docker` is a separate mount, and **aborts if vzdump excludes mp0** (the B3 trap).
+
+## Provision (per customer guest)
+
+The bring-up restores the golden (carrying rootfs + the small mp0 Docker-data volume + baked images),
+then sizes per customer. The Docker-data volume is **grown** (not re-attached) so the baked images are
+never shadowed:
+
+```
+felhom-agent --selftest=provision \
+  -archive <golden-volid> -vmid <VMID> \
+  -customer-id <id> -hub-password <retrieval-passphrase> \
+  -rootfs-grow <N>   # GiB to add to the OS rootfs beyond the golden's 32 (0 = keep 32)
+  -datavol-grow <N>  # GiB to add to the Docker-data volume beyond the golden's 16 (e.g. 240 → 256 total)
+  -datavol-mount mp0 # the golden's Docker-data slot (default mp0)
+```
+
+In code: `reconcile.BringUpSpec.{RootfsGrowGB, DataVolGrowGB, DataVolMount}` — both grows are online
+(`pct resize`, spike B4), each its own task. `GuestMount.Backup` (→ `,backup=1`) is set on any
+data-bearing additive mount (`bringup.go` `buildConfigParams`). The bootstrap mount is `mp9`; USB/data
+drive enroll picks the lowest free slot (`freeMountSlot`), so it never collides with the docker mp0.
+
+**Per-customer sizing source:** flags today; the **slice-10 hub storage manifest** is the intended
+home (marked at `bringup.go` `GuestMount` / the `slice 10 wires the hub storage manifest` comment) —
+the controller already pulls customer-scoped config from the hub, so OS/data sizes belong there.
+
+### Default sizing guidance
+- ~512 GB SSD: 32 GB rootfs + 256 GB Docker-data (`-rootfs-grow 0 -datavol-grow 240`), leaving the rest
+  of the thin pool free for other guests.
+- Other SSDs: size the data volume to "most of the free thin pool minus a reserve for other guests."
+  `local-lvm` is thin-provisioned, so an over-allocation only consumes space as it's written.
+
+## Fresh PVE host install — thin-pool carving (host-level, do this at install)
+
+This sizes the **host's** `local` root vs the `local-lvm` thin pool (the pool the guests' volumes are
+carved from). It is NOT the guest split — do it on the Proxmox ISO installer's **Advanced LVM options**:
+
+- `hdsize` — total disk PVE uses (leave headroom on multi-disk nodes).
+- `maxroot` — cap on the host `/` (`pve/root`). Keep small (e.g. 20–40 GB) — the host OS is thin.
+- `maxvz` — cap on the `data` thin pool (`local-lvm`). Set to 0 / leave default to let the pool take the
+  rest, OR cap it to reserve unallocated VG space.
+- `minfree` — reserved free space in the VG (thin-pool safety).
+- `swapsize` — host swap.
+
+**Never repartition a live host root.** Per-guest carving (rootfs + Docker-data) is done at provision
+time via `pct` against the existing thin pool — no host repartition needed (the demo's pool already has
+~350 GB free).
+
+## In-place migration of an EXISTING guest (NOT used by this slice)
+
+This slice rebuilds clean (fresh re-provision), so the move-then-verify in-place migration is **not
+used here**. It remains documented in `REPORT-storage-split-spike.md` §B2 for a future in-place node
+upgrade: stop apps → stop docker → `rsync -aHAX --numeric-ids /var/lib/docker/ <new-vol>/` (check
+rsync's OWN exit code) → switch data-root → verify volumes+DBs+boot → only then reclaim. `backup=1` on
+the new mount and a CT restart to activate it are required (spike B3/B4).
diff --git a/cmd/felhom-agent/main.go b/cmd/felhom-agent/main.go
index a4635d2..e5ae491 100644
--- a/cmd/felhom-agent/main.go
+++ b/cmd/felhom-agent/main.go
@@ -43,7 +43,7 @@ import (
 
 // version is the agent version. Overridable at build time with
 // -ldflags "-X main.version=<v>"; defaults to the in-repo CHANGELOG version.
-var version = "0.28.0"
+var version = "0.29.0"
 
 func main() {
 	var (
@@ -55,6 +55,9 @@ func main() {
 		mode        string
 		hostname    string
 		keep        bool
+		rootfsGrow  int
+		dataVolGrow int
+		dataVolMount string
 		pbsStorage  string
 		paperkey    bool
 		offline     bool
@@ -79,6 +82,9 @@ func main() {
 	flag.StringVar(&mode, "mode", "provision", "for --selftest=bring-up: `provision` (golden, fresh identity) | `dr` (customer backup, preserve continuity)")
 	flag.StringVar(&hostname, "hostname", "", "for --selftest=bring-up provision: the hostname to set on the new guest")
 	flag.BoolVar(&keep, "keep", false, "for --selftest=bring-up: KEEP the guest instead of tearing it down at the end")
+	flag.IntVar(&rootfsGrow, "rootfs-grow", 0, "for --selftest=bring-up|provision: grow the OS rootfs by this many GiB after restore (0 = keep golden size)")
+	flag.IntVar(&dataVolGrow, "datavol-grow", 0, "for --selftest=bring-up|provision: grow the golden's Docker-data volume (mp0) by this many GiB (0 = keep golden size)")
+	flag.StringVar(&dataVolMount, "datavol-mount", "", "for --selftest=bring-up|provision: the mpN slot of the Docker-data volume to grow (default mp0)")
 	flag.StringVar(&pbsStorage, "storage", "", "for --selftest=escrow-create: the pbs storage whose key to escrow (default: escrow.pbs_storage_id)")
 	flag.BoolVar(&paperkey, "paperkey", false, "for --selftest=escrow-create: ALSO emit the raw-key paperkey (opt-in (a); single-factor, unrevocable)")
 	flag.BoolVar(&offline, "offline", false, "for --selftest=escrow-create: ALSO emit the R-wrapped offline copy to print (opt-in (b))")
@@ -133,11 +139,13 @@ func main() {
 	case "lanresolver":
 		os.Exit(runSelftestLANResolver(context.Background(), cfg, logger, vmid))
 	case "bring-up":
-		os.Exit(runSelftestBringUp(context.Background(), cfg, logger, mode, archive, vmid, hostname, keep))
+		os.Exit(runSelftestBringUp(context.Background(), cfg, logger, mode, archive, vmid, hostname, keep,
+			bringUpSizing{RootfsGrowGB: rootfsGrow, DataVolGrowGB: dataVolGrow, DataVolMount: dataVolMount}))
 	case "provision":
 		os.Exit(runSelftestProvision(context.Background(), cfg, logger, provisionArgs{
 			archive: archive, vmid: vmid, hostname: hostname,
 			customerID: custID, hubPassword: hubPassword,
+			sizing: bringUpSizing{RootfsGrowGB: rootfsGrow, DataVolGrowGB: dataVolGrow, DataVolMount: dataVolMount},
 		}))
 	case "escrow-create":
 		os.Exit(runSelftestEscrowCreate(context.Background(), cfg, logger, pbsStorage, paperkey, offline, upload, idBundlePath, directivePath))
@@ -950,7 +958,16 @@ func runSelftestRestoreTest(ctx context.Context, cfg config.Config, logger *slog
 // (a selftest must not leave a guest running) unless -keep. -mode picks provision (golden, fresh
 // identity) or dr (customer backup, preserve continuity). It first Recovers, so a leaked guest
 // from a prior crashed bring-up is reaped before this run.
-func runSelftestBringUp(ctx context.Context, cfg config.Config, logger *slog.Logger, mode, archive string, vmid int, hostname string, keep bool) int {
+// bringUpSizing carries the OS-rootfs / Docker-data sizing knobs from the CLI into a bring-up.
+// Defaults (0/"") keep the golden's baked sizes; the provisioning spec sources these per-customer
+// (flags now; the slice-10 hub storage manifest later — see bringup.go GuestMount comment).
+type bringUpSizing struct {
+	RootfsGrowGB  int
+	DataVolGrowGB int
+	DataVolMount  string
+}
+
+func runSelftestBringUp(ctx context.Context, cfg config.Config, logger *slog.Logger, mode, archive string, vmid int, hostname string, keep bool, sizing bringUpSizing) int {
 	if err := cfg.Validate(); err != nil {
 		fmt.Fprintln(os.Stderr, "selftest: proxmox not configured:", err)
 		return 1
@@ -1006,6 +1023,7 @@ func runSelftestBringUp(ctx context.Context, cfg config.Config, logger *slog.Log
 	spec := reconcile.BringUpSpec{
 		Mode: bmode, Archive: archive, VMID: vmid, RestoreStorage: cfg.Backup.RestoreStorage,
 		Hostname: hostname, KeepMAC: bmode == reconcile.ModeDRGuestLoss,
+		RootfsGrowGB: sizing.RootfsGrowGB, DataVolGrowGB: sizing.DataVolGrowGB, DataVolMount: sizing.DataVolMount,
 	}
 	fmt.Printf("  bringing up %s → vmid %d on %s …\n", archive, vmid, cfg.Backup.RestoreStorage)
 	res := engine.RunBringUp(ctx, spec)
@@ -1049,6 +1067,7 @@ type provisionArgs struct {
 	hostname    string
 	customerID  string // baked into bootstrap (the hub config-pull target)
 	hubPassword string // the customer's hub retrieval passphrase (SECRET) — baked into bootstrap
+	sizing      bringUpSizing // OS-rootfs / Docker-data sizing for the bring-up half
 }
 
 // sanitizeHostname makes s a DNS-safe LXC hostname (RFC 1123 label-ish): lowercase, any run of
@@ -1156,6 +1175,7 @@ func runSelftestProvision(ctx context.Context, cfg config.Config, logger *slog.L
 	res := engine.RunBringUp(ctx, reconcile.BringUpSpec{
 		Mode: reconcile.ModeProvision, Archive: a.archive, VMID: a.vmid,
 		RestoreStorage: cfg.Backup.RestoreStorage, Hostname: hostname,
+		RootfsGrowGB: a.sizing.RootfsGrowGB, DataVolGrowGB: a.sizing.DataVolGrowGB, DataVolMount: a.sizing.DataVolMount,
 	})
 	if res.Err != nil || !res.Pass {
 		fmt.Fprintf(os.Stderr, "  [FAIL] front-half bring-up (vmid %d): %v\n", a.vmid, res.Err)
diff --git a/configs/build-golden.sh b/configs/build-golden.sh
index 94d1be1..e8d6b77 100644
--- a/configs/build-golden.sh
+++ b/configs/build-golden.sh
@@ -23,6 +23,16 @@
 # Usage: build-golden.sh [VMID] [TEMPLATE_VOLID] [ROOTFS_STORAGE] [ARCHIVE_STORAGE] [BRIDGE] [CONTROLLER_IMAGE]
 # Build-time registry login for the controller pull (used ONCE inside the build guest, then logged
 # out — never baked): set REGISTRY_USER + REGISTRY_TOKEN in the environment.
+#
+# OS / Docker-data SPLIT (storage-split slice): the golden is built with a SMALL OS rootfs and a
+# SEPARATE Docker-data volume mounted at /var/lib/docker (mp0, backup=1). The baked controller +
+# infra images land on that volume and travel INSIDE the golden archive — so provisioned guests boot
+# from baked images with no registry pull. The split is for RESILIENCE: an isolated OS rootfs stays
+# bootable + agent-recoverable if the Docker volume fills (the controller's prevention layer keeps it
+# from filling). Sizes are env-overridable (OS_SIZE_GB / GOLDEN_DOCKER_GB); provision GROWS the data
+# volume to the per-customer target (bringup.go DataVolGrowGB). backup=1 is MANDATORY on the data mp:
+# without it vzdump EXCLUDES the volume (extra LXC mountpoints default backup=0 — storage-split B3),
+# so the archive would carry NO images and provisioned guests would boot imageless.
 set -euo pipefail
 
 VMID="${1:-9100}"
@@ -32,12 +42,17 @@ ARCHIVE_STORAGE="${4:-local}"
 BRIDGE="${5:-vmbr0}"
 CONTROLLER_IMAGE="${6:-gitea.dooplex.hu/admin/felhom-controller:0.43.0}"
 REGISTRY_HOST="${CONTROLLER_IMAGE%%/*}"
+# OS rootfs size (GiB) and the golden's Docker-data volume size (GiB). Keep GOLDEN_DOCKER_GB just
+# large enough for the baked images + headroom; provision grows it to the per-customer target.
+OS_SIZE_GB="${OS_SIZE_GB:-32}"
+GOLDEN_DOCKER_GB="${GOLDEN_DOCKER_GB:-16}"
 
-echo "[golden] creating build LXC $VMID (nesting=1,keyctl=1, unprivileged) …"
+echo "[golden] creating build LXC $VMID (nesting=1,keyctl=1, unprivileged; rootfs ${OS_SIZE_GB}G + Docker-data ${GOLDEN_DOCKER_GB}G @ /var/lib/docker, backup=1) …"
 pct create "$VMID" "$TEMPLATE" \
   --hostname felhom-golden --unprivileged 1 \
   --features nesting=1,keyctl=1 \
-  --rootfs "${ROOTFS_STORAGE}:8" --cores 2 --memory 2048 \
+  --rootfs "${ROOTFS_STORAGE}:${OS_SIZE_GB}" --cores 2 --memory 2048 \
+  --mp0 "${ROOTFS_STORAGE}:${GOLDEN_DOCKER_GB},mp=/var/lib/docker,backup=1" \
   --net0 "name=eth0,bridge=${BRIDGE},ip=dhcp" --onboot 0
 
 echo "[golden] starting + installing Docker (official repo, trixie channel) …"
@@ -59,8 +74,20 @@ pct exec "$VMID" -- bash -c '
   apt-get update -qq
   apt-get install -y -qq docker-ce docker-ce-cli containerd.io >/dev/null
 '
-echo "[golden] verifying Docker works in the build guest …"
-pct exec "$VMID" -- bash -c 'systemctl start docker; sleep 2; docker run --rm hello-world >/dev/null && echo "  docker OK ($(docker info 2>/dev/null | sed -n "s/.*Storage Driver: //p"))"'
+echo "[golden] baking Docker log rotation into daemon.json (prevention layer: kills unbounded container logs for every guest) …"
+# /var/lib/docker is the mp0 mount (mounted empty before docker installs), so data-root needs NO
+# override — the existing image pulls + this config land on the volume automatically. Only the
+# json-file log caps are set (the most common runaway). Every container inherits these defaults.
+pct exec "$VMID" -- bash -c 'mkdir -p /etc/docker; cat > /etc/docker/daemon.json <<JSON
+{
+  "log-driver": "json-file",
+  "log-opts": { "max-size": "10m", "max-file": "3" }
+}
+JSON'
+echo "[golden] verifying Docker works in the build guest (storage driver should be overlayfs on the ext4 data volume) …"
+pct exec "$VMID" -- bash -c 'systemctl start docker; sleep 2; docker run --rm hello-world >/dev/null && echo "  docker OK ($(docker info 2>/dev/null | sed -n "s/.*Storage Driver: //p"); data-root $(docker info 2>/dev/null | sed -n "s/.*Docker Root Dir: //p"))"'
+# Confirm /var/lib/docker is genuinely the dedicated volume, not the rootfs (catch a silent mp miss).
+pct exec "$VMID" -- bash -c 'findmnt -no SOURCE,FSTYPE /var/lib/docker | grep -q . && echo "  /var/lib/docker is a separate mount: $(findmnt -no SOURCE,FSTYPE /var/lib/docker)" || { echo "[golden] FATAL: /var/lib/docker is NOT a separate mount — the mp0 split did not take"; exit 1; }'
 
 echo "[golden] baking the in-guest controller image $CONTROLLER_IMAGE (no registry cred at deploy) …"
 # docker login is used ONCE here on the trusted build host, then logged out before archiving so
@@ -197,8 +224,17 @@ pct exec "$VMID" -- bash -c '
 
 echo "[golden] stop + archive …"
 pct stop "$VMID"
-vzdump "$VMID" --storage "$ARCHIVE_STORAGE" --mode stop --compress zstd
+# --mode stop with mp0 backup=1 → the Docker-data volume (with baked images) is INCLUDED. The log
+# below MUST show "including mount point mp0" — if it shows "excluding … (disabled)" the backup flag
+# was lost and the archive carries no images (storage-split B3 trap).
+vzdump "$VMID" --storage "$ARCHIVE_STORAGE" --mode stop --compress zstd 2>&1 | tee /tmp/golden-vzdump.log | grep -iE "including mount point|excluding|archive file size|Finished Backup" || true
+if grep -q "excluding volume mount point mp0" /tmp/golden-vzdump.log; then
+  echo "[golden] FATAL: mp0 (/var/lib/docker) was EXCLUDED from the archive — backup=1 was lost; the golden would carry no images. Aborting."
+  exit 1
+fi
+grep -q "including mount point mp0" /tmp/golden-vzdump.log \
+  || echo "[golden] WARN: could not confirm mp0 inclusion in the vzdump log — verify manually before using this archive."
 
 VOLID=$(pvesm list "$ARCHIVE_STORAGE" --content backup 2>/dev/null | awk -v v="$VMID" '$1 ~ ("vzdump-lxc-" v "-") {print $1}' | sort | tail -1)
-echo "[golden] DONE. golden archive volid: ${VOLID:-<check ${ARCHIVE_STORAGE} dump dir>}"
+echo "[golden] DONE. golden archive volid: ${VOLID:-<check ${ARCHIVE_STORAGE} dump dir>} (rootfs ${OS_SIZE_GB}G + Docker-data ${GOLDEN_DOCKER_GB}G, both in the archive)"
 echo "[golden] (the build guest $VMID is stopped; destroy it with: pct destroy $VMID --purge)"
diff --git a/internal/reconcile/bringup.go b/internal/reconcile/bringup.go
index f16c43d..60173eb 100644
--- a/internal/reconcile/bringup.go
+++ b/internal/reconcile/bringup.go
@@ -40,6 +40,9 @@ const (
 
 const bringUpKind = "bring_up"
 
+// DefaultDataVolMount is the mpN slot the golden bakes the Docker-data volume (/var/lib/docker) at.
+const DefaultDataVolMount = "mp0"
+
 // configLockMaxAttempts bounds the F4 config-lock retry. configLockBackoff is a package var so
 // tests can shrink it (the production value gives PVE time to release its async config lock).
 const configLockMaxAttempts = 5
@@ -52,6 +55,12 @@ type GuestMount struct {
 	Storage    string // PVE storage id (e.g. "local-lvm")
 	SizeGB     int    // new-volume size in GiB
 	MountPoint string // in-guest path (e.g. "/mnt/data")
+	// Backup includes this mountpoint in vzdump/PBS. MANDATORY for any data-bearing mount (DB
+	// volumes), because extra LXC mountpoints default to backup=0 = EXCLUDED from the snapshot
+	// (storage-split finding B3). The Docker-data volume normally rides in from the golden archive
+	// (already backup=1) and is grown via DataVolGrowGB rather than attached here, but any data
+	// mount attached through spec.Mounts MUST set this or its contents silently fall out of PBS.
+	Backup bool
 }
 
 // BringUpSpec is the input to one bring-up. The caller resolves it (the selftest, or slice-10
@@ -65,7 +74,14 @@ type BringUpSpec struct {
 	Cores          int           // 0 = leave as restored
 	MemoryMB       int           // 0 = leave as restored
 	RootfsGrowGB   int           // optional grow-only rootfs resize (0 = skip)
-	Mounts         []GuestMount  // additive mpN mounts (slice 7 may pass empty/test)
+	// DataVolGrowGB grows the golden-carried Docker-data volume (DataVolMount, default mp0) to the
+	// per-customer target. The golden ships a small data volume with the baked images; provision
+	// grows it online (grow-only, storage-split B4) rather than attaching a fresh empty volume that
+	// would shadow the baked images. 0 = skip (keep the golden's size).
+	DataVolGrowGB int
+	// DataVolMount is the mpN slot of the golden's Docker-data volume to grow; "" → DefaultDataVolMount ("mp0").
+	DataVolMount string
+	Mounts       []GuestMount  // additive mpN mounts (slice 7 may pass empty/test)
 	KeepMAC        bool          // DR knob: keep the archived MAC (true) unless a source may be live
 	BootTimeout    time.Duration // 0 → DefaultBootTimeout; bounds the link-up liveness wait
 }
@@ -213,6 +229,26 @@ func (e *Engine) runBringUp(ctx context.Context, spec BringUpSpec, res *BringUpR
 		}
 	}
 
+	// 4b. Grow the golden-carried Docker-data volume (mp0) to the per-customer target. Grow-only,
+	//     online (storage-split B4); its OWN call like the rootfs resize. The volume + baked images
+	//     came in with the restore, so we grow it rather than attach a fresh one that would shadow
+	//     the baked images.
+	if spec.DataVolGrowGB > 0 {
+		mount := spec.DataVolMount
+		if mount == "" {
+			mount = DefaultDataVolMount
+		}
+		dupid, err := e.api.ResizeLXC(ctx, spec.VMID, mount, fmt.Sprintf("+%dG", spec.DataVolGrowGB))
+		if err != nil {
+			res.Err = fmt.Errorf("reconcile: bring-up data-volume resize (%s): %w", mount, err)
+			return
+		}
+		if _, err := e.waitTask(ctx, dupid, proxmox.WaitOptions{}); err != nil {
+			res.Err = fmt.Errorf("reconcile: bring-up data-volume resize task (%s): %w", mount, err)
+			return
+		}
+	}
+
 	// Capture the post-reset MAC for the result (fresh for provision; archived for DR keep).
 	if cfg2, err := e.api.GuestConfig(ctx, spec.VMID); err == nil {
 		res.AssignedMAC = net0MAC(cfg2)
@@ -310,7 +346,13 @@ func buildBringUpConfig(spec BringUpSpec, cfg proxmox.GuestConfig) map[string]st
 		params["memory"] = strconv.Itoa(spec.MemoryMB)
 	}
 	for i, m := range spec.Mounts {
-		params[fmt.Sprintf("mp%d", i)] = fmt.Sprintf("%s:%d,mp=%s", m.Storage, m.SizeGB, m.MountPoint)
+		// backup=1 for data-bearing mounts: extra LXC mountpoints default to backup=0 = EXCLUDED
+		// from vzdump/PBS (storage-split B3), which would silently drop their DBs from the snapshot.
+		spec := fmt.Sprintf("%s:%d,mp=%s", m.Storage, m.SizeGB, m.MountPoint)
+		if m.Backup {
+			spec += ",backup=1"
+		}
+		params[fmt.Sprintf("mp%d", i)] = spec
 	}
 	return params
 }
diff --git a/internal/reconcile/bringup_test.go b/internal/reconcile/bringup_test.go
index f802dc9..0b96d8d 100644
--- a/internal/reconcile/bringup_test.go
+++ b/internal/reconcile/bringup_test.go
@@ -74,6 +74,58 @@ func TestRunBringUp_ProvisionHappyPath(t *testing.T) {
 	}
 }
 
+// A data-bearing additive mount must carry backup=1 (so its DBs stay in PBS — storage-split B3);
+// a non-backup mount must NOT. Pure-function check on buildBringUpConfig.
+func TestBuildBringUpConfig_BackupFlagOnDataMount(t *testing.T) {
+	params := buildBringUpConfig(BringUpSpec{
+		Mode: ModeProvision,
+		Mounts: []GuestMount{
+			{Storage: "local-lvm", SizeGB: 2, MountPoint: "/mnt/data", Backup: true},
+			{Storage: "local-lvm", SizeGB: 1, MountPoint: "/mnt/scratch"}, // no backup
+		},
+	}, scratchCfg())
+	if params["mp0"] != "local-lvm:2,mp=/mnt/data,backup=1" {
+		t.Errorf("data mount must carry backup=1: mp0=%q", params["mp0"])
+	}
+	if params["mp1"] != "local-lvm:1,mp=/mnt/scratch" {
+		t.Errorf("non-backup mount must NOT carry backup=1: mp1=%q", params["mp1"])
+	}
+}
+
+// The golden-carried Docker-data volume is grown via a SEPARATE resize on its mpN slot (B4),
+// alongside (but distinct from) the rootfs grow.
+func TestRunBringUp_StorageSplit_DataVolGrow(t *testing.T) {
+	const vmid = 8050
+	api := &fakeAPI{cfg: map[int]proxmox.GuestConfig{vmid: scratchCfg()}}
+	e, _, q := newEngine(t, api, EmptyProvider{})
+	defer q.Close()
+
+	res := e.RunBringUp(context.Background(), BringUpSpec{
+		Mode: ModeProvision, Archive: "local:backup/golden.tar.zst", VMID: vmid,
+		RestoreStorage: "local-lvm", Hostname: "felhom-prov-8050",
+		RootfsGrowGB: 8, DataVolGrowGB: 240, // grows mp0 (DefaultDataVolMount)
+	})
+	if res.Err != nil || !res.Pass {
+		t.Fatalf("provision must pass, got %+v", res)
+	}
+	// TWO resizes: rootfs +8G and the Docker-data volume mp0 +240G.
+	if len(api.resizes) != 2 {
+		t.Fatalf("expected rootfs + data-volume resizes, got %+v", api.resizes)
+	}
+	var sawRootfs, sawData bool
+	for _, r := range api.resizes {
+		if r.disk == "rootfs" && r.size == "+8G" {
+			sawRootfs = true
+		}
+		if r.disk == "mp0" && r.size == "+240G" {
+			sawData = true
+		}
+	}
+	if !sawRootfs || !sawData {
+		t.Errorf("want rootfs +8G AND mp0 +240G, got %+v", api.resizes)
+	}
+}
+
 func TestRunBringUp_CompensatingRollback(t *testing.T) {
 	const vmid = 8000
 	lockBackoffFast(t)