Mount a QCOW2 File as a Filesystem

Typically, your VM disks will exist on the host as qcow2 files. However, if you ever need to do something on the VM disk without booting the VM, you can mount the qcow2 file directly on the host. Note that you can only do this either 1) while the guest is shut down or 2) on a clone of the guest’s disk, not on a running guest.

This can be useful for automating tasks in the guest from the host, or pulling a file from an old snapshot.

This technique can be particularly useful in combination with ZFS snapshots on the host, where you use the read-only snapshot of the VM disk and mount it to fetch a file from within the guest, at the time of the snapshot, reducing the need for in-guest backup agents.

Which One?

If you want to change files within the guest’s disk, you’ll need to mount read-write. However, if you’re simply trying to read a file from the guest, especially from something like a ZFS snapshot, you’ll need to take the additional steps of mounting read-only, which doesn’t work by default. Pick one of the two below.

Mount Read Write

First, enable qemu-nbd on the host, which is the daemon that allows the qcow2 file to be mounted as a filesystem.

sudo modprobe nbd max_part=8

Next, create the mount with qemu-nbd.

sudo qemu-nbd --connect=/dev/nbd0 /var/lib/libvirt/images/app01.qcow2

Then, use fdisk to view the partitions that can be mounted.

root@host:~# fdisk /dev/nbd0 -l

Disk /dev/nbd0: 50 GiB, 53687091200 bytes, 104857600 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: gpt
Disk identifier: 0A8E7183-678B-4618-84F5-8A5F44F7FEC3

Device      Start       End   Sectors Size Type
/dev/nbd0p1  2048      4095      2048   1M BIOS boot
/dev/nbd0p2  4096 104855167 104851072  50G Linux filesystem
root@host:~#

From fdisk, it appears that /dev/nbd0p2 is the the VM’s root volume.

Now, make a spot to mount it, and run the mount.

sudo mkdir /mnt/app01/
sudo mount /dev/nbd0p2 /mnt/app01

Now, check your mount point - you should have your VM root mounted to your mountpoint.

$ df -h
Filesystem     Size  Used Avail Use% Mounted on
/dev/nbd0p2    50G   43G  4.6G  91% /mnt/app01

You may now copy out the files you need.

When done, you need to tear down the mount, and disable qemu-nbd.

sudo umount /mnt/app01
sudo qemu-nbd --disconnect /dev/nbd0
sudo rmmod nbd

Mount Read Only