linux FS tools {mount, overlay, chroot}

loop device

mount the qemu raw system to host linux.

# you may need sudo on these

losetup /dev/loop0 disk.raw
kpartx -a /dev/loop0
mount /dev/mapper/lop0p1 /guest_fs

to close the devices:

$ umount # whatever                    # unmount guest fs
$ kpartx -d /dev/loop<whatever>        # remove the mapper
$ losetup -d /dev/loop0                # remove the loop device

# if the above doesn't clean it up correctly, also
$ kpartx -d /dev/mapper/<whatever>     # remove the mapper
$ dmsetup remove /dev/mapper/<whatever>

filesystem overlay

chroot
I find arch-chroot works out-of-the-box even for other disk images from other distributions. I have a debian disk image and the chroot works as a charm, it feels like I’m running another distribution inside of arch.

overlay filesystem

$ mount -t overlay overlay -o lowerdir=/lower1:/lower2:/lower3,upperdir=/upper,workdir=/work /merged

Situation: I have a raw disk image mounted to the host system via a loop device/mapper, however I want to put something else (data dir on host) into the guest fs for a latter chroot environment. This can not be done via a symlink… because the target path is not available in the chroot.

Here I simply mount the target data dir into the “guest os root mounted” – nested. Notably, if the datadir is on a remote machine and you need to sshfs mount it to the guest fs, you will need the -o allow_other option for sshfs otherwise it will prevent you from accessing the files in a chroot.

the -o allow_other needs to be toggled in the /etc/fuse.conf

-----------------------------------------------------
                                    Host Machine env

+-------------+
|raw disk img | (guest rootfs)
+-----+-------+
      |
      | loop dev
      | + kpartx                                       
      |                                                        
      v                                      +-----------+
+-------------+                              | DATA DIR  |
|guest fs dev | (/dev/mapper/guest-rootfs)   | ON HOST   |
+-------------+                              +-----+-----+
     |                                             |
     | mount                                       |
     v                                             |
+------------------------+                         |
|guest os root mounted   |                         |
|   |- /home             |                         |
|   |- /lib              |                         |
|   |- /etc              |                         |
|   |- suplementary dir<-------- ?????-------------+
|   ......               |
+----------+-------------+
           |
           |
-----------|-----------------------------------------
           |                      (virtual) guest env
           | chroot
           | 
     "guest" shell
     -----------------
     $ ~
     -----------------