diff --git a/scripts/setup/machine-setup/lib/disk.sh b/scripts/setup/machine-setup/lib/disk.sh
new file mode 100644
index 00000000..286cccfc
--- /dev/null
+++ b/scripts/setup/machine-setup/lib/disk.sh
@@ -0,0 +1,182 @@
+#!/bin/bash
+# =============================================================================
+# machine-setup — using the whole disk
+# =============================================================================
+#
+# Definitions only, like the other lib/ files.
+#
+# ── The problem this exists for ──
+#
+# Ubuntu Server's installer, left on its defaults, creates an LVM logical volume
+# at a fixed size and leaves the rest of the disk as free extents in the volume
+# group. On a 2TB drive you get a root filesystem of around 100GB and no
+# indication anything is wrong: `lsblk` shows the whole disk, `df` shows 100G,
+# and the two are never seen side by side until the day it fills.
+#
+# The same shape turns up two other ways:
+#
+#   a virtual disk grown at the hypervisor or provider, where the partition still
+#   ends where it used to
+#
+#   a partition that was resized without the filesystem inside it being told
+#
+# Three layers, and any one of them can be the short one:
+#
+#   disk        the physical or virtual device
+#   container   the partition, or the logical volume
+#   filesystem  what df reports
+#
+# So all three are measured and reported together. Seeing them in one place is
+# most of the value; the fix is usually two commands once you know which layer is
+# short.
+#
+# ── Only ever grows ──
+#
+# Nothing here shrinks anything, and nothing here creates or deletes a partition.
+# ext4, xfs and btrfs all grow while mounted, so there is no unmount and no
+# reboot, and a failure part-way leaves a smaller filesystem on a larger
+# container — which is exactly the state it started in.
+
+[[ -n "${MACHINE_SETUP_DISK_LOADED:-}" ]] && return 0
+MACHINE_SETUP_DISK_LOADED=1
+
+# -----------------------------------------------------------------------------
+# What is where
+# -----------------------------------------------------------------------------
+
+root_device() { findmnt -no SOURCE / 2>/dev/null; }
+root_fstype() { findmnt -no FSTYPE / 2>/dev/null; }
+
+# Size of a block device in bytes.
+dev_bytes() { lsblk -bndo SIZE "$1" 2>/dev/null || echo 0; }
+
+# Bytes, formatted the way df and lsblk format them.
+human_bytes() { numfmt --to=iec --suffix=B --format='%.1f' "$1" 2>/dev/null || echo "${1}B"; }
+
+# Is the root filesystem on a logical volume?
+root_is_lvm() { [[ "$(lsblk -ndo TYPE "$(root_device)" 2>/dev/null)" == "lvm" ]]; }
+
+# The whole disk a device ultimately sits on: /dev/sda1 -> /dev/sda, and through
+# LVM as well, since PKNAME walks one level at a time.
+parent_disk() {
+  local dev="$1" name
+  while true; do
+    name="$(lsblk -ndo PKNAME "$dev" 2>/dev/null)"
+    [[ -z "$name" ]] && break
+    dev="/dev/${name}"
+  done
+  echo "$dev"
+}
+
+# The partition immediately below a device — for LVM, the one holding the PV.
+backing_partition() {
+  local dev="$1" name
+  while [[ "$(lsblk -ndo TYPE "$dev" 2>/dev/null)" != "part" ]]; do
+    name="$(lsblk -ndo PKNAME "$dev" 2>/dev/null)"
+    [[ -z "$name" ]] && return 1
+    dev="/dev/${name}"
+  done
+  echo "$dev"
+}
+
+# Split /dev/sda1 into "/dev/sda 1" — growpart wants them as separate arguments.
+# The digits come off the end because that is where a partition number is, on
+# /dev/sda1 and /dev/nvme0n1p2 alike.
+partition_parts() {
+  local part="$1" num disk
+  num="${part##*[!0-9]}"
+  disk="${part%"$num"}"
+  disk="${disk%p}" # nvme0n1p2 -> nvme0n1
+  echo "$disk $num"
+}
+
+# -----------------------------------------------------------------------------
+# Sizes of the three layers
+# -----------------------------------------------------------------------------
+
+# What the filesystem itself believes it is, which is the number df reports and
+# the only one of the three that is asked of the filesystem rather than the
+# kernel's block layer.
+fs_bytes() {
+  local dev="$1"
+  case "$(root_fstype)" in
+    ext2 | ext3 | ext4)
+      local count size
+      count="$(tune2fs -l "$dev" 2>/dev/null | awk -F: '/^Block count:/ { gsub(/ /, "", $2); print $2 }')"
+      size="$(tune2fs -l "$dev" 2>/dev/null | awk -F: '/^Block size:/ { gsub(/ /, "", $2); print $2 }')"
+      [[ -n "$count" && -n "$size" ]] && echo $((count * size)) || echo 0
+      ;;
+    xfs | btrfs)
+      # Both report through the mount rather than the device.
+      echo $(($(findmnt -bno SIZE / 2>/dev/null || echo 0)))
+      ;;
+    *) echo 0 ;;
+  esac
+}
+
+# Unallocated extents in the volume group behind root. This is the Ubuntu
+# installer case, and the one that is invisible without asking LVM directly.
+vg_free_bytes() {
+  local vg
+  command -v vgs &>/dev/null || {
+    echo 0
+    return
+  }
+  vg="$(lvs --noheadings -o vg_name "$(root_device)" 2>/dev/null | tr -d ' ')"
+  [[ -z "$vg" ]] && {
+    echo 0
+    return
+  }
+  vgs --noheadings --nosuffix --units b -o vg_free "$vg" 2>/dev/null | tr -d ' ' || echo 0
+}
+
+# -----------------------------------------------------------------------------
+# Can anything be reclaimed?
+# -----------------------------------------------------------------------------
+
+# growpart answers this better than arithmetic on sector counts: it exits 0 when
+# it would change something and 1 with NOCHANGE when the partition already
+# reaches the end of the disk. Needs cloud-guest-utils, which is not installed by
+# default on every image.
+partition_can_grow() {
+  local part="$1" disk num
+  command -v growpart &>/dev/null || return 1
+  read -r disk num <<<"$(partition_parts "$part")"
+  growpart --dry-run "$disk" "$num" &>/dev/null
+}
+
+ensure_growpart() {
+  command -v growpart &>/dev/null && return 0
+  info "     installing cloud-guest-utils, which provides growpart"
+  pkg_install_now cloud-guest-utils
+}
+
+# -----------------------------------------------------------------------------
+# Growing
+# -----------------------------------------------------------------------------
+
+grow_partition() {
+  local part="$1" disk num
+  read -r disk num <<<"$(partition_parts "$part")"
+  growpart "$disk" "$num"
+}
+
+# Tell LVM the partition under the physical volume got bigger.
+grow_pv() { pvresize "$1"; }
+
+# Take every free extent in the volume group.
+grow_lv() { lvextend -l +100%FREE "$(root_device)"; }
+
+# Grow the filesystem into whatever room it now has. All three do this online, so
+# the root filesystem is grown while it is mounted and in use.
+grow_fs() {
+  case "$(root_fstype)" in
+    ext2 | ext3 | ext4) resize2fs "$(root_device)" ;;
+    xfs) xfs_growfs / ;;
+    btrfs) btrfs filesystem resize max / ;;
+    *)
+      warn "do not know how to grow a $(root_fstype) filesystem"
+      return 1
+      ;;
+  esac
+}
diff --git a/scripts/setup/machine-setup/machine-setup.sh b/scripts/setup/machine-setup/machine-setup.sh
index 6d982006..2dab1855 100755
--- a/scripts/setup/machine-setup/machine-setup.sh
+++ b/scripts/setup/machine-setup/machine-setup.sh
@@ -24,6 +24,8 @@ source "$SCRIPT_DIR/lib/packages.sh"
 source "$SCRIPT_DIR/lib/tools.sh"
 # shellcheck source=lib/system.sh
 source "$SCRIPT_DIR/lib/system.sh"
+# shellcheck source=lib/disk.sh
+source "$SCRIPT_DIR/lib/disk.sh"
 
 # Trap errors with context. Installed here rather than in lib/base.sh, because
 # that file is definitions only and a trap is a side effect on whoever sources it.
@@ -85,7 +87,102 @@ info "Refreshing the package index..."
 pkg_refresh >/dev/null
 
 # =============================================================================
-# 2. System update
+# 2. Disk space
+# =============================================================================
+#
+# First of the sections that change anything, because everything after it sizes
+# itself from free disk — the swapfile and the ballast both.
+
+step "Disk space"
+if ! skip; then
+  ROOT_DEV="$(root_device)"
+  ROOT_DISK="$(parent_disk "$ROOT_DEV")"
+  ROOT_PART="$(backing_partition "$ROOT_DEV" || true)"
+
+  DISK_B="$(dev_bytes "$ROOT_DISK")"
+  CONT_B="$(dev_bytes "$ROOT_DEV")"
+  FS_B="$(fs_bytes "$ROOT_DEV")"
+  VG_FREE_B="$(vg_free_bytes)"
+
+  # A filesystem is always a little smaller than the thing holding it —
+  # metadata, journal, reserved blocks. Only a real gap is worth reporting.
+  SLACK=$((1024 * 1024 * 1024))
+
+  echo ""
+  info "Disk space — whether the root filesystem is actually using the whole drive"
+  echo "     Ubuntu's installer, left on its defaults, gives the root volume a"
+  echo "     fixed size and leaves the rest of the drive unallocated. On a 2TB"
+  echo "     disk that is a 100G root and no sign anything is wrong: lsblk shows"
+  echo "     the whole drive, df shows 100G, and the two are never seen together"
+  echo "     until the day it fills up. Growing a virtual disk at the provider"
+  echo "     leaves the same shape."
+  echo ""
+  echo "     drive:        $(human_bytes "$DISK_B")   ${ROOT_DISK}"
+  echo "     volume:       $(human_bytes "$CONT_B")   ${ROOT_DEV}"
+  echo "     filesystem:   $(human_bytes "$FS_B")   $(root_fstype), mounted at /"
+  ((VG_FREE_B > SLACK)) && echo "     unused in LVM: $(human_bytes "$VG_FREE_B")"
+
+  # growpart is the authority on whether a partition can move, but installing a
+  # package just to ask is too eager — the arithmetic decides whether it is even
+  # worth looking.
+  MIGHT_GROW=false
+  ((DISK_B - CONT_B > SLACK)) && MIGHT_GROW=true
+  [[ -n "$ROOT_PART" ]] && $MIGHT_GROW && ensure_growpart
+
+  if ((VG_FREE_B > SLACK)); then
+    echo ""
+    echo "     $(human_bytes "$VG_FREE_B") is sitting unallocated in the volume group"
+    echo "     the fix is lvextend, then growing the filesystem into it — both online"
+    if confirm "Use it?"; then
+      grow_lv && grow_fs
+      ok "root filesystem is now $(human_bytes "$(fs_bytes "$ROOT_DEV")")"
+      SUMMARY+=("Disk: reclaimed $(human_bytes "$VG_FREE_B") from the volume group")
+    else
+      warn "skipped by request"
+      SUMMARY+=("Disk: SKIPPED — $(human_bytes "$VG_FREE_B") left unallocated")
+    fi
+
+  elif [[ -n "$ROOT_PART" ]] && partition_can_grow "$ROOT_PART"; then
+    echo ""
+    echo "     the partition stops short of the end of the drive"
+    echo "     this is the one step that edits the partition table — it only ever"
+    echo "     moves the end of ${ROOT_PART} outwards, and never touches another one"
+    if confirm "Extend it?"; then
+      grow_partition "$ROOT_PART"
+      if root_is_lvm; then
+        grow_pv "$ROOT_PART"
+        grow_lv
+      fi
+      grow_fs
+      ok "root filesystem is now $(human_bytes "$(fs_bytes "$ROOT_DEV")")"
+      SUMMARY+=("Disk: partition extended, filesystem now $(human_bytes "$(fs_bytes "$ROOT_DEV")")")
+    else
+      warn "skipped by request"
+      SUMMARY+=("Disk: SKIPPED — partition left short of the drive")
+    fi
+
+  elif ((FS_B > 0 && CONT_B - FS_B > SLACK)); then
+    echo ""
+    echo "     the filesystem is smaller than the volume holding it"
+    if confirm "Grow it?"; then
+      grow_fs
+      ok "root filesystem is now $(human_bytes "$(fs_bytes "$ROOT_DEV")")"
+      SUMMARY+=("Disk: filesystem grown to $(human_bytes "$(fs_bytes "$ROOT_DEV")")")
+    else
+      warn "skipped by request"
+      SUMMARY+=("Disk: SKIPPED — filesystem left short of its volume")
+    fi
+
+  else
+    echo ""
+    echo "     the whole drive is in use, nothing to reclaim"
+    SUMMARY+=("Disk: already using the whole drive")
+  fi
+  step_ok
+fi
+
+# =============================================================================
+# 3. System update
 # =============================================================================
 #
 # Its own section because it is the only thing in the script that moves versions
@@ -125,7 +222,7 @@ if ! skip; then
 fi
 
 # =============================================================================
-# 3. Core utils
+# 4. Core utils
 # =============================================================================
 #
 # What the distribution provides: the six this script would break without, and
@@ -140,7 +237,7 @@ if ! skip; then
 fi
 
 # =============================================================================
-# 4. Command-line tools
+# 5. Command-line tools
 # =============================================================================
 #
 # A different thing from core utils, and kept apart from them: upstream binaries
@@ -158,7 +255,7 @@ fi
 
 
 # =============================================================================
-# 5. Locale
+# 6. Locale
 # =============================================================================
 #
 # LOCALE in the environment overrides the default.
@@ -196,7 +293,7 @@ if ! skip; then
 fi
 
 # =============================================================================
-# 6. Timezone
+# 7. Timezone
 # =============================================================================
 #
 # TIMEZONE in the environment answers the prompt ahead of time.
@@ -262,7 +359,7 @@ if ! skip; then
 fi
 
 # =============================================================================
-# 7. Swap
+# 8. Swap
 # =============================================================================
 #
 # Disk the kernel can park cold pages on when RAM fills, so a spike costs
@@ -316,7 +413,7 @@ if ! skip; then
 fi
 
 # =============================================================================
-# 8. Emergency disk ballast
+# 9. Emergency disk ballast
 # =============================================================================
 #
 # Always offered, whatever the role — the role only decides which way the
@@ -359,7 +456,7 @@ if ! skip; then
 fi
 
 # =============================================================================
-# 9. earlyoom
+# 10. earlyoom
 # =============================================================================
 
 step "earlyoom"
@@ -394,7 +491,7 @@ if ! skip; then
 fi
 
 # =============================================================================
-# 10. inotify watch limit
+# 11. inotify watch limit
 # =============================================================================
 
 step "inotify watch limit"
@@ -447,7 +544,7 @@ fi
 
 
 # =============================================================================
-# 23. Summary
+# 12. Summary
 # =============================================================================
 
 echo ""