Migrate existing ext4 partition to LVM physical volume in-place. Inspired by https://chromium.googlesource.com/chromiumos/platform2/+/main/thinpool_migrator/

lvm_migrator.sh

#!/bin/sh

# Copyright (c) 2025 TheKit <thekit@disroot.org>
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# Constants
STARTING_PHYSICAL_EXTENT_ADDRESS=1048576  # 1MB
PARTITION_HEADER_SIZE=1048576  # 1MB
REGULAR_LVM_PHYSICAL_EXTENT_SIZE=1048576  # 1MB

# Default values
RESERVED_SIZE_MB=0

# Function to print usage
print_usage() {
    echo "Usage: $0 --device=<device> --vg-name=<name> --lv-name=<name> [--reserve=<mb>]"
    echo "  --device=<device>    Path to the device to migrate"
    echo "  --vg-name=<name>     Volume group name (required)"
    echo "  --lv-name=<name>     Logical volume name (required)"
    echo "  --reserve=<mb>       Reserve space in MB for additional logical volumes (optional)"
    echo "  --help              Show this help message"
}

# Function to generate random ID (POSIX compatible)
generate_id() {
    # Use /dev/urandom if available, otherwise use date + process ID
    if [ -r /dev/urandom ]; then
        hexdump -n 16 -e '4/4 "%08X" "\n"' /dev/urandom | sed 's/\([0-9A-F]\{8\}\)\([0-9A-F]\{4\}\)\([0-9A-F]\{4\}\)\([0-9A-F]\{4\}\)\([0-9A-F]\{12\}\)/\1-\2-\3-\4-\5/'
    else
        # Fallback: use date and process ID
        date +%s | od -tx4 -N4 | awk '{printf "%08X", $2}' | sed 's/\([0-9A-F]\{8\}\)\([0-9A-F]\{4\}\)\([0-9A-F]\{4\}\)\([0-9A-F]\{4\}\)\([0-9A-F]\{12\}\)/\1-\2-\3-\4-\5/'
    fi
}

# Function to get device size
get_device_size() {
    device="$1"
    blockdev --getsize64 "$device" 2>/dev/null || {
        echo "Failed to get device size for $device" >&2
        exit 1
    }
}

# Function to resize filesystem
resize_filesystem() {
    device="$1"
    target_size_bytes="$2"
    target_size_blocks=$((target_size_bytes / 4096))

    resize2fs "$device" "$target_size_blocks"
}

# Function to copy header
copy_header() {
    device="$1"
    src_offset="$2"
    dst_offset="$3"

    dd if="$device" of="$device" bs=512 count=2048 \
       seek=$((dst_offset / 512)) skip=$((src_offset / 512)) \
       conv=notrunc
}

# Function to create LVM metadata
create_lvm_metadata() {
    device="$1"
    device_size="$2"
    vg_name="$3"
    lv_name="$4"
    reserved_size_mb="$5"

    # Calculate extents
    total_extents=$(((device_size - STARTING_PHYSICAL_EXTENT_ADDRESS) / REGULAR_LVM_PHYSICAL_EXTENT_SIZE))
    header_extents=1  # 1MB header
    reserved_extents="$reserved_size_mb"  # Reserved space in MB = extents (since 1 extent = 1MB)
    main_data_extents=$((total_extents - header_extents - reserved_extents))

    # Generate random IDs
    vg_id=$(generate_id)
    pv_id=$(generate_id)
    lv_id=$(generate_id)

    # Get current time
    creation_time=$(date +%s)

    # Generate metadata
    cat > /tmp/vgcfgrestore.txt << EOF
contents = "Text Format Volume Group"
version = 1
description = "Generated by lvm_migrator"
creation_host = "localhost"
creation_time = $creation_time
$vg_name {
  id = "$vg_id"
  seqno = 0
  format = "lvm2"
  status = ["READ", "WRITE", "RESIZEABLE"]
  flags = []
  extent_size = 2048
  max_lv = 0
  max_pv = 1
  metadata_copies = 0
  physical_volumes {

  pv0 {
    id = "$pv_id"
    device = $device
    status = ["ALLOCATABLE"]
    flags = []
    dev_size = $((total_extents * 2048))
    pe_start = 2048
    pe_count = $total_extents
  }
  }
  logical_volumes {

    $lv_name {
    id = "$lv_id"
    status = ["READ", "WRITE", "VISIBLE"]
    flags = []
    creation_time = $creation_time
    creation_host = "localhost"
    segment_count = 2

      segment1 {
        start_extent = 0
        extent_count = 1
        type = "striped"
stripe_count = 1
stripes = [
"pv0", $((total_extents - 1))
]

      }
      segment2 {
        start_extent = 1
        extent_count = $main_data_extents
        type = "striped"
stripe_count = 1
stripes = [
"pv0", 0
]

      }
    }
  }
}
EOF

    # Create physical volume
    pvcreate --uuid "$pv_id" --restorefile /tmp/vgcfgrestore.txt "$device"

    # Restore volume group
    vgcfgrestore -f /tmp/vgcfgrestore.txt "$vg_name"

    echo "Created volume group: $vg_name"
    echo "Created logical volume: $vg_name-$lv_name"
}

# Parse command line arguments
while [ $# -gt 0 ]; do
    case "$1" in
        --help)
            print_usage
            exit 0
            ;;
        --device=*)
            DEVICE_PATH="${1#*=}"
            shift
            ;;
        --vg-name=*)
            VG_NAME="${1#*=}"
            shift
            ;;
        --lv-name=*)
            LV_NAME="${1#*=}"
            shift
            ;;
        --reserve=*)
            RESERVED_SIZE_MB="${1#*=}"
            shift
            ;;
        *)
            echo "Unknown argument: $1" >&2
            print_usage
            exit 1
            ;;
    esac
done

# Validate required arguments
if [ -z "${DEVICE_PATH:-}" ]; then
    echo "Error: --device argument is required" >&2
    print_usage
    exit 1
fi

if [ -z "${VG_NAME:-}" ]; then
    echo "Error: --vg-name argument is required" >&2
    print_usage
    exit 1
fi

if [ -z "${LV_NAME:-}" ]; then
    echo "Error: --lv-name argument is required" >&2
    print_usage
    exit 1
fi

# Validate reserve size (POSIX compatible number check)
case "$RESERVED_SIZE_MB" in
    ''|*[!0-9]*)
        echo "Error: Invalid reserve size: $RESERVED_SIZE_MB" >&2
        exit 1
        ;;
esac

# Main migration logic
echo "Starting LVM migration for device: $DEVICE_PATH"

# Get device size
DEVICE_SIZE=$(get_device_size "$DEVICE_PATH")
echo "Device size: $DEVICE_SIZE bytes ($((DEVICE_SIZE / 1048576)) MB)"

# Calculate filesystem size (reserve 1MB for LVM header + optional reserved space)
RESERVED_SIZE_BYTES=$((RESERVED_SIZE_MB * 1048576))
FILESYSTEM_SIZE=$((DEVICE_SIZE - STARTING_PHYSICAL_EXTENT_ADDRESS - RESERVED_SIZE_BYTES))
echo "Resizing filesystem to: $FILESYSTEM_SIZE bytes ($((FILESYSTEM_SIZE / 1048576)) MB)"
if [ "$RESERVED_SIZE_MB" -gt 0 ]; then
    echo "Reserving $RESERVED_SIZE_MB MB for additional logical volumes"
fi

# Resize filesystem
echo "Resizing filesystem..."
resize_filesystem "$DEVICE_PATH" "$FILESYSTEM_SIZE"

# Copy header to end
HEADER_OFFSET="$FILESYSTEM_SIZE"
echo "Copying 1MB header to offset: $HEADER_OFFSET"
copy_header "$DEVICE_PATH" 0 "$HEADER_OFFSET"

# Create LVM metadata
echo "Creating LVM metadata..."
create_lvm_metadata "$DEVICE_PATH" "$DEVICE_SIZE" "$VG_NAME" "$LV_NAME" "$RESERVED_SIZE_MB"

echo "Migration completed successfully!"