💊 Quick Pill: QEMU/VirtualBox Disk Resize
Info
Use case: Expand virtual machine disk when running out of space. Works with QEMU raw images, VirtualBox VDI/VHD, and similar formats.
🚀 Complete Resize Process
Warning
⚠️ Critical: Always backup your VM disk before resizing!
Step-by-Step Guide
# 1. Shutdown your VM first!
# 2. Resize the disk image file
qemu-img resize -f raw Ubuntu.vhd 200G
# 3. Load NBD kernel module
sudo modprobe nbd
# 4. Connect disk as network block device
sudo qemu-nbd --connect=/dev/nbd0 -f raw Ubuntu.vhd
# 5. Expand the partition
sudo parted /dev/nbd0
Inside parted:
(parted) print # Show current partitions
(parted) resizepart 2 100% # Resize partition 2 to use all space
(parted) quit # Exit parted
# 6. Resize the filesystem
sudo resize2fs /dev/nbd0p2
# 7. Verify the changes
sudo parted /dev/nbd0 print
# 8. Disconnect the NBD device
sudo qemu-nbd --disconnect /dev/nbd0
# 9. Start your VM - done!
📋 Understanding Each Step
1. Resize Disk Image
qemu-img resize -f raw Ubuntu.vhd 200G
What it does:
-f raw: Specifies format (raw, qcow2, vdi, vhd, vmdk)Ubuntu.vhd: Your disk image file200G: New total size (not added space!)
Size formats:
200G: Set to 200GB total+50G: Add 50GB to current size+10240M: Add 10GB (in megabytes)
2. Connect as Block Device
sudo modprobe nbd
sudo qemu-nbd --connect=/dev/nbd0 -f raw Ubuntu.vhd
What it does:
nbd: Network Block Device module/dev/nbd0: First NBD device (use nbd1, nbd2 if busy)- Mounts disk image as if it were a physical disk
3. Resize Partition
sudo parted /dev/nbd0
(parted) resizepart 2 100%
What it does:
resizepart 2: Resize partition number 2100%: Use all available space- Note: Adjust partition number if different!
4. Resize Filesystem
sudo resize2fs /dev/nbd0p2
What it does:
- Expands ext4/ext3 filesystem to fill partition
p2= partition 2 (match with resizepart!)- Automatic: detects new size from partition
💡 Pro Tips
Work with different disk formats
QCOW2 (QEMU native format)
# Resize
qemu-img resize -f qcow2 disk.qcow2 +50G
# Connect
sudo qemu-nbd --connect=/dev/nbd0 -f qcow2 disk.qcow2
VirtualBox VDI
# Resize using VBoxManage
VBoxManage modifymedium disk Ubuntu.vdi --resize 204800 # Size in MB
# Or convert to raw first, then resize
qemu-img convert -f vdi -O raw Ubuntu.vdi Ubuntu.raw
qemu-img resize Ubuntu.raw 200G
VMDK (VMware)
# Resize VMDK
qemu-img resize -f vmdk disk.vmdk 200G
# Connect
sudo qemu-nbd --connect=/dev/nbd0 -f vmdk disk.vmdk
Check partition layout first
# Connect disk
sudo qemu-nbd --connect=/dev/nbd0 -f raw Ubuntu.vhd
# List partitions
sudo parted /dev/nbd0 print
# Example output:
# Number Start End Size Type File system Flags
# 1 1049kB 538MB 537MB primary fat32 boot, esp
# 2 538MB 100GB 99.5GB primary ext4
# Note the partition number (usually 2 for Linux root)
# Disconnect when done
sudo qemu-nbd --disconnect /dev/nbd0
Resize XFS filesystem instead of ext4
# After resizing partition
# XFS requires mounting first
sudo mkdir -p /mnt/temp
sudo mount /dev/nbd0p2 /mnt/temp
# Resize XFS
sudo xfs_growfs /mnt/temp
# Unmount
sudo umount /mnt/temp
Resize LVM volumes
# After resizing partition with LVM
# 1. Resize physical volume
sudo pvresize /dev/nbd0p2
# 2. Check volume group
sudo vgdisplay
# 3. Resize logical volume
sudo lvextend -l +100%FREE /dev/ubuntu-vg/root
# 4. Resize filesystem
sudo resize2fs /dev/ubuntu-vg/root
# Or for XFS:
# sudo xfs_growfs /dev/ubuntu-vg/root
Add space incrementally
# Add 50GB to current size (safer than setting absolute size)
qemu-img resize -f raw Ubuntu.vhd +50G
# Add in smaller chunks
qemu-img resize -f raw Ubuntu.vhd +10G
# Test VM, then add more if needed
qemu-img resize -f raw Ubuntu.vhd +10G
Multiple NBD devices in use
# Check which NBD devices are available
ls -la /dev/nbd*
# Try different device if nbd0 is busy
sudo qemu-nbd --connect=/dev/nbd1 -f raw Ubuntu.vhd
sudo qemu-nbd --connect=/dev/nbd2 -f raw Ubuntu.vhd
# List active NBD connections
ps aux | grep qemu-nbd
Create complete resize script
Save as resize_vm_disk.sh:
#!/bin/bash
# Configuration
DISK_IMAGE="${1}"
NEW_SIZE="${2}"
PARTITION_NUM="${3:-2}" # Default partition 2
NBD_DEVICE="/dev/nbd0"
if [ -z "$DISK_IMAGE" ] || [ -z "$NEW_SIZE" ]; then
echo "Usage: $0 <disk_image> <new_size> [partition_number]"
echo "Example: $0 Ubuntu.vhd 200G 2"
exit 1
fi
if [ ! -f "$DISK_IMAGE" ]; then
echo "Error: Disk image not found: $DISK_IMAGE"
exit 1
fi
echo "================================================"
echo " VM Disk Resize Script"
echo "================================================"
echo "Disk image: $DISK_IMAGE"
echo "New size: $NEW_SIZE"
echo "Partition: $PARTITION_NUM"
echo ""
read -p "Continue? (y/N) " -n 1 -r
echo
if [[ ! $REPLY =~ ^[Yy]$ ]]; then
echo "Aborted."
exit 1
fi
# Detect format
FORMAT=$(qemu-img info "$DISK_IMAGE" | grep "file format:" | awk '{print $3}')
echo "Detected format: $FORMAT"
echo ""
# Step 1: Resize image
echo "[1/6] Resizing disk image..."
qemu-img resize -f "$FORMAT" "$DISK_IMAGE" "$NEW_SIZE"
# Step 2: Load NBD module
echo "[2/6] Loading NBD module..."
sudo modprobe nbd max_part=8
# Step 3: Connect NBD
echo "[3/6] Connecting disk as NBD device..."
sudo qemu-nbd --connect=$NBD_DEVICE -f "$FORMAT" "$DISK_IMAGE"
# Wait for device
sleep 2
# Step 4: Show current layout
echo "[4/6] Current partition layout:"
sudo parted $NBD_DEVICE print
# Step 5: Resize partition
echo "[5/6] Resizing partition $PARTITION_NUM..."
sudo parted $NBD_DEVICE resizepart $PARTITION_NUM 100%
# Step 6: Resize filesystem
echo "[6/6] Resizing filesystem..."
PART_DEVICE="${NBD_DEVICE}p${PARTITION_NUM}"
# Detect filesystem type
FS_TYPE=$(sudo blkid -o value -s TYPE $PART_DEVICE)
echo "Filesystem type: $FS_TYPE"
case $FS_TYPE in
ext4|ext3|ext2)
sudo resize2fs $PART_DEVICE
;;
xfs)
TEMP_MOUNT="/mnt/temp_resize_$$"
sudo mkdir -p "$TEMP_MOUNT"
sudo mount $PART_DEVICE "$TEMP_MOUNT"
sudo xfs_growfs "$TEMP_MOUNT"
sudo umount "$TEMP_MOUNT"
sudo rmdir "$TEMP_MOUNT"
;;
*)
echo "Warning: Unknown filesystem type: $FS_TYPE"
echo "You may need to resize manually."
;;
esac
# Verify
echo ""
echo "New partition layout:"
sudo parted $NBD_DEVICE print
# Disconnect
echo ""
echo "Disconnecting NBD device..."
sudo qemu-nbd --disconnect $NBD_DEVICE
echo ""
echo "================================================"
echo " Resize Complete!"
echo "================================================"
echo "You can now start your VM."
echo ""
Usage:
chmod +x resize_vm_disk.sh
sudo ./resize_vm_disk.sh Ubuntu.vhd 200G 2
Shrink disk (advanced, risky)
Danger
Warning: Shrinking is dangerous and can cause data loss!
# 1. Connect disk
sudo qemu-nbd --connect=/dev/nbd0 -f raw Ubuntu.vhd
# 2. Check filesystem usage
sudo e2fsck -f /dev/nbd0p2
sudo resize2fs /dev/nbd0p2 150G # Shrink filesystem first!
# 3. Shrink partition
sudo parted /dev/nbd0
(parted) resizepart 2 150GB
(parted) quit
# 4. Disconnect
sudo qemu-nbd --disconnect /dev/nbd0
# 5. Shrink image (only for raw/qcow2)
qemu-img resize --shrink -f raw Ubuntu.vhd 160G
# Note: Always leave some extra space!
🔍 Troubleshooting
NBD module not found
Problem: modprobe: FATAL: Module nbd not found
Solution:
# Ubuntu/Debian
sudo apt install qemu-utils qemu-block-extra
# Fedora/RHEL
sudo dnf install qemu-img
# Try loading again
sudo modprobe nbd max_part=8
Device or resource busy
Problem: Cannot connect to /dev/nbd0
Solution:
# Check if nbd0 is in use
sudo qemu-nbd --disconnect /dev/nbd0
# Or use different device
sudo qemu-nbd --connect=/dev/nbd1 -f raw Ubuntu.vhd
# List all NBD devices
cat /proc/partitions | grep nbd
Partition not found after resize
Problem: /dev/nbd0p2 doesn’t exist
Solution:
# Reload partition table
sudo partprobe /dev/nbd0
# Or reconnect with max_part
sudo qemu-nbd --disconnect /dev/nbd0
sudo modprobe -r nbd
sudo modprobe nbd max_part=8
sudo qemu-nbd --connect=/dev/nbd0 -f raw Ubuntu.vhd
Filesystem still shows old size in VM
Problem: VM shows old disk size after resize
Solution:
# Inside the VM (after resizing from host)
# 1. Check current size
df -h
# 2. Rescan disk
sudo partprobe
# 3. Grow filesystem (if not done from host)
sudo resize2fs /dev/sda2 # or your root partition
# 4. Verify
df -h
Cannot resize: filesystem has errors
Problem: resize2fs: filesystem has errors
Solution:
# Check and repair filesystem first
sudo e2fsck -f -y /dev/nbd0p2
# Then resize
sudo resize2fs /dev/nbd0p2
VirtualBox disk resize fails
Problem: VBoxManage resize not working
Solution:
# Convert to raw format first
VBoxManage clonehd Ubuntu.vdi Ubuntu.vhd --format VHD
# Resize using qemu-img
qemu-img resize Ubuntu.vhd 200G
# Follow NBD process above
# Or use VBoxManage with correct UUID
VBoxManage showhdinfo Ubuntu.vdi # Get UUID
VBoxManage modifymedium disk <UUID> --resize 204800
📊 Supported Formats
| Format | Command | Notes |
|---|---|---|
| RAW | qemu-img resize -f raw disk.raw 200G | Fastest, largest file |
| QCOW2 | qemu-img resize -f qcow2 disk.qcow2 +50G | QEMU native, dynamic |
| VDI | VBoxManage modifymedium disk.vdi --resize 204800 | VirtualBox (size in MB) |
| VHD | qemu-img resize -f vpc disk.vhd 200G | Hyper-V/VirtualBox |
| VMDK | qemu-img resize -f vmdk disk.vmdk 200G | VMware |
⚠️ Important Warnings
Danger
Before Resizing:
- Shutdown the VM completely - never resize while running
- Backup the disk image - copy to safe location
- Test in snapshot/copy first if possible
- Verify format with
qemu-img info disk.img - Check free space on host (need room for larger file)
- Remember: Shrinking is dangerous, expanding is safe
🎯 Common Scenarios
Expand Ubuntu VM
# Typical Ubuntu setup: partition 2 is root
qemu-img resize -f raw Ubuntu.vhd 200G
sudo modprobe nbd
sudo qemu-nbd --connect=/dev/nbd0 -f raw Ubuntu.vhd
sudo parted /dev/nbd0 resizepart 2 100%
sudo resize2fs /dev/nbd0p2
sudo qemu-nbd --disconnect /dev/nbd0
Expand Windows VM (NTFS)
# Windows needs ntfsresize instead of resize2fs
qemu-img resize -f raw Windows.vhd 300G
sudo modprobe nbd
sudo qemu-nbd --connect=/dev/nbd0 -f raw Windows.vhd
sudo parted /dev/nbd0 resizepart 2 100%
sudo ntfsresize -f /dev/nbd0p2 # For NTFS
sudo qemu-nbd --disconnect /dev/nbd0
Quick Check Without Resizing
# Just inspect disk
sudo modprobe nbd
sudo qemu-nbd --connect=/dev/nbd0 -f raw Ubuntu.vhd
sudo parted /dev/nbd0 print
sudo fdisk -l /dev/nbd0
df -h # Won't show until mounted
sudo qemu-nbd --disconnect /dev/nbd0
📈 Size Planning Guide
| VM Purpose | Recommended Size | Resize When |
|---|---|---|
| Testing/Dev | 20-50GB | Below 5GB free |
| Desktop VM | 50-100GB | Below 10GB free |
| Server | 100-500GB | Below 20GB free |
| Production | 500GB+ | Below 10% free |
🚀 Quick Reference Commands
# 1. RESIZE IMAGE
qemu-img resize -f raw disk.vhd +50G
# 2. CONNECT
sudo modprobe nbd max_part=8
sudo qemu-nbd --connect=/dev/nbd0 -f raw disk.vhd
# 3. RESIZE PARTITION (interactive)
sudo parted /dev/nbd0
> resizepart 2 100%
> quit
# 4. RESIZE FILESYSTEM
sudo resize2fs /dev/nbd0p2 # ext4
# OR
sudo xfs_growfs /mnt/mountpoint # XFS
# OR
sudo ntfsresize -f /dev/nbd0p2 # NTFS
# 5. VERIFY
sudo parted /dev/nbd0 print
# 6. DISCONNECT
sudo qemu-nbd --disconnect /dev/nbd0
🔄 Alternative: Resize from Inside VM
If NBD doesn’t work, resize from within the VM:
# 1. Resize image from host (VM off)
qemu-img resize disk.vhd 200G
# 2. Boot VM and run inside it
sudo growpart /dev/sda 2 # Grow partition
sudo resize2fs /dev/sda2 # Grow filesystem
# Or use parted
sudo parted /dev/sda resizepart 2 100%
sudo resize2fs /dev/sda2
Expand your virtual machines without the hassle!