How to mount a qcow2 disk image

mount_qcow2.md

How to mount a qcow2 disk image

This is a quick guide to mounting a qcow2 disk images on your host server. This is useful to reset passwords, edit files, or recover something without the virtual machine running.

Step 1 - Enable NBD on the Host

modprobe nbd max_part=8

Step 2 - Connect the QCOW2 as network block device

qemu-nbd --connect=/dev/nbd0 /var/lib/vz/images/100/vm-100-disk-1.qcow2

Step 3 - Find The Virtual Machine Partitions

fdisk /dev/nbd0 -l

Step 4 - Mount the partition from the VM

mount /dev/nbd0p1 /mnt/somepoint/

Step 5 - After you done, unmount and disconnect

umount /mnt/somepoint/
qemu-nbd --disconnect /dev/nbd0
rmmod nbd

Is there any way to see path of the qcow2 file connected to nbd device (after successful "qemu-nbd -c")?
for example, want to see file name being mounted on /dev/nbd0 from a monitoring program.

works fine under OS

lsb_release -a
LSB Version:	core-11.1.0ubuntu4-noarch:printing-11.1.0ubuntu4-noarch:security-11.1.0ubuntu4-noarch
Distributor ID:	Ubuntu
Description:	Ubuntu 22.04.1 LTS
Release:	22.04
Codename:	jammy

i had .img file ending instead of .qcow2

after i checked with qemu-img info XXXX.img

i see that it is a qcow2 format

thanks a lot

gives me this error, since i made my qcow2 an LVM

“mount: unknown filesystem type LVM2_member”

Is there any other way for me to mount this as an LVM?

you have to remove missing disk from LVM member group, you can read my howto to fix it here:
(http://aysad.cloudns.cc/blog/2023/11/28/lvm/raid1/disk/mount/recovery/howto/)

Thanks. Ckear, concise. Worked on Fedora Server 38.

Very useful, but I think it worth to add nbds_max=1 (or 2) to the modprobe, so it only creates one or two devices, instead of 16.

Expanding a partitiion:

#!/bin/bash

# Script to expand the disk and Btrfs filesystem of an Arch Linux VM managed by virsh.
# This script performs *offline* filesystem resizing on the host.
# The VM is NOT booted during filesystem resize.
# *** IMPORTANT: THIS SCRIPT NOW INCLUDES BTRFS BALANCE FOR COMPLETE RESIZE ***

# --- Configuration ---
VM_NAME="archlinux"
QCOW2_IMAGE="/home/kreijstal/Downloads/vm/Arch-Linux-x86_64-basic-20240201.210938.qcow2"
SIZE_TO_ADD="40G"  # Amount to add to the existing virtual size
NBD_DEVICE="/dev/nbd0"
PARTITION_NUMBER=3  # Partition number to expand (usually root)
FILESYSTEM_TYPE="btrfs" # Filesystem type: btrfs, ext4, xfs (adjust if needed)
MOUNT_POINT="/mnt" # Mount point on the host for filesystem resize

# --- Step 1: Resize the .qcow2 image ---
echo "Step 1: Resizing qcow2 image..."
qemu-img resize "$QCOW2_IMAGE" "+$SIZE_TO_ADD"
if [ $? -ne 0 ]; then
  echo "Error resizing qcow2 image. Aborting."
  exit 1
fi
echo "qcow2 image resized."

# --- Step 2: Load the NBD kernel module ---
echo "Step 2: Loading NBD kernel module..."
sudo modprobe nbd max_part=8
if [ $? -ne 0 ]; then
  echo "Error loading NBD module. Aborting."
  exit 1
fi
echo "NBD module loaded."

# --- Step 3: Connect the qcow2 image to an NBD device ---
echo "Step 3: Connecting qcow2 image to NBD device..."
sudo qemu-nbd --connect="$NBD_DEVICE" "$QCOW2_IMAGE"
if [ $? -ne 0 ]; then
  echo "Error connecting qcow2 image to NBD device. Aborting."
  exit 1
fi
echo "qcow2 image connected to NBD device."

# --- Step 4: Fix the GPT backup header location ---
echo "Step 4: Fixing GPT backup header..."
sudo sgdisk -e "$NBD_DEVICE"
if [ $? -ne 0 ]; then
  echo "Error fixing GPT backup header. Proceeding anyway (may be non-critical)."
fi
echo "GPT backup header fixed (or attempted)."

# --- Step 5: Expand the partition using parted ---
echo "Step 5: Expanding partition $PARTITION_NUMBER using parted..."
sudo parted "$NBD_DEVICE" resizepart "$PARTITION_NUMBER" 100% -s
if [ $? -ne 0 ]; then
  echo "Error expanding partition. Aborting."
  sudo qemu-nbd --disconnect "$NBD_DEVICE"
  sudo rmmod nbd
  exit 1
fi
echo "Partition $PARTITION_NUMBER expanded."

# --- Step 6: Mount the resized partition on the host ---
echo "Step 6: Mounting partition $PARTITION_NUMBER on $MOUNT_POINT..."
sudo mount "${NBD_DEVICE}p${PARTITION_NUMBER}" "$MOUNT_POINT"
if [ $? -ne 0 ]; then
  echo "Error mounting partition. Aborting."
  sudo qemu-nbd --disconnect "$NBD_DEVICE"
  sudo rmmod nbd
  exit 1
fi
echo "Partition ${PARTITION_NUMBER} mounted on $MOUNT_POINT."

# --- Step 7: Resize the filesystem on the host ---
echo "Step 7: Resizing $FILESYSTEM_TYPE filesystem on the host..."
if [ "$FILESYSTEM_TYPE" = "btrfs" ]; then
  FS_RESIZE_COMMAND="sudo btrfs filesystem resize max $MOUNT_POINT"
elif [ "$FILESYSTEM_TYPE" = "ext4" ]; then
  FS_RESIZE_COMMAND="sudo resize2fs ${NBD_DEVICE}p${PARTITION_NUMBER}"
elif [ "$FILESYSTEM_TYPE" = "xfs" ]; then
  FS_RESIZE_COMMAND="sudo xfs_growfs $MOUNT_POINT" # xfs_growfs on mount point
else
  echo "Error: Unsupported filesystem type '$FILESYSTEM_TYPE' for offline resize in script."
  echo "Please resize the filesystem manually after booting the VM."
  FS_RESIZE_COMMAND="" # No command to run
fi

if [ -n "$FS_RESIZE_COMMAND" ]; then
  eval "$FS_RESIZE_COMMAND" # Execute the filesystem resize command
  if [ $? -ne 0 ]; then
    echo "Error resizing $FILESYSTEM_TYPE filesystem. Aborting."
    sudo umount "$MOUNT_POINT"
    sudo qemu-nbd --disconnect "$NBD_DEVICE"
    sudo rmmod nbd
    exit 1
  fi
  echo "$FILESYSTEM_TYPE filesystem resized."

  # --- Step 8: Balance Btrfs filesystem (REQUIRED for Btrfs to use new space) ---
  if [ "$FILESYSTEM_TYPE" = "btrfs" ]; then
    echo "Step 8: Balancing Btrfs filesystem (REQUIRED)..."
    sudo btrfs balance start --full-balance "$MOUNT_POINT"
    if [ $? -ne 0 ]; then
      echo "Warning: Error running btrfs balance (non-critical, but recommended to check manually)."
    else
      echo "Btrfs balance completed."
    fi
  fi
fi

# --- Step 9: Unmount the partition from the host ---
echo "Step 9: Unmounting partition from $MOUNT_POINT..."
sudo umount "$MOUNT_POINT"
if [ $? -ne 0 ]; then
  echo "Warning: Error unmounting $MOUNT_POINT (may need manual unmount)."
fi
echo "Partition unmounted from $MOUNT_POINT."


# --- Step 10: Disconnect the NBD device and unload the module (cleanup) ---
echo "Step 10: Disconnecting NBD device and unloading module..."
sudo qemu-nbd --disconnect="$NBD_DEVICE"
sudo rmmod nbd
echo "NBD device disconnected and module unloaded."

# --- Step 11: Start the VM ---
echo "Step 11: Starting VM '$VM_NAME'..."
virsh start "$VM_NAME"
if [ $? -ne 0 ]; then
  echo "Error starting VM '$VM_NAME'. Please check virsh logs."
  exit 1
fi
echo "VM '$VM_NAME' started."

echo "--- Disk and Filesystem Expansion Process Completed (Offline) ---"
echo "--- Log in to your VM to verify the expanded filesystem using 'df -h /' ---"

exit 0