Recover Data from a VM Disk Image

This guide shows how to retrieve files from a Cubic virtual machine that can no longer boot or is otherwise inaccessible via cubic ssh. The VM disk is a standard qcow2 image, so qemu-img and qemu-nbd can read it directly from the host.

Stop the Virtual Machine

The disk image must not be in use by a running VM before it is mounted on the host:

$ cubic stop <instance>

Locate the Disk Image

Cubic stores each instance under the user data directory:

Snap: $SNAP_USER_COMMON/cubic/machines/<instance>/machine.img (typically ~/snap/cubic/common/cubic/machines/<instance>/machine.img)
Linux: $XDG_DATA_HOME/cubic/machines/<instance>/machine.img (typically ~/.local/share/cubic/machines/<instance>/machine.img)
macOS: ~/Library/cubic/machines/<instance>/machine.img
Windows: %LOCALAPPDATA%\cubic\machines\<instance>\machine.img

Copy Files with `virt-cat` (read-only, recommended)

If the host has libguestfs-tools installed, files can be extracted without mounting the image:

$ virt-filesystems -a ~/.local/share/cubic/machines/<instance>/machine.img --all --long -h
$ virt-cat -a ~/.local/share/cubic/machines/<instance>/machine.img /etc/hostname
$ virt-copy-out -a ~/.local/share/cubic/machines/<instance>/machine.img /home/<user>/work .

virt-copy-out writes the listed guest paths into the current host directory. The image is opened read-only, so the running guest state cannot be corrupted.

Mount the Image with `qemu-nbd`

When libguestfs-tools is not available, expose the disk as a block device on Linux with the Network Block Device kernel module:

$ sudo modprobe nbd max_part=8
$ sudo qemu-nbd --read-only --connect=/dev/nbd0 \
    ~/.local/share/cubic/machines/<instance>/machine.img
$ lsblk /dev/nbd0

Mount the guest root partition (typically /dev/nbd0p1) somewhere on the host and copy files out:

$ sudo mkdir -p /mnt/cubic
$ sudo mount -o ro /dev/nbd0p1 /mnt/cubic
$ cp -a /mnt/cubic/home/<user>/work ~/recovered/

Always mount with -o ro to avoid writing to the guest filesystem.

Detach the Image

When the recovery is done, unmount and disconnect the device:

$ sudo umount /mnt/cubic
$ sudo qemu-nbd --disconnect /dev/nbd0