Is it possible to union-mount an existing disk image containing a root filesystem to the
upperdir of an
overlayfs mount for read-write access?
I'm trying to union-mount some old disk images to a
/tmp/ directory on an Ubuntu 16.04 machine using pre-existing scripts (originally written on Ubuntu 12.04 and ported to 14.04).
The old automation scripts used
overlayfs to create and union-mount writable
ext2/3 disk images on top of
squashfs images — something like this:
cd /tmp mkdir lower upper union losetup /dev/loop1 /tmp/image.sfs losetup /dev/loop2 /tmp/image.ext2 mount -t squashfs -o ro /dev/loop1 /tmp/lower mount -t ext2 -o rw /dev/loop2 /tmp/upper mount -t overlayfs -o rw,lowerdir=/tmp/lower,upperdir=/tmp/upper overlayfs /tmp/union
They were last used on an 3.x kernel (likely pre-3.18), and that method doesn't seem to work anymore. Overlay now requires a
mount -t overlay -o rw,lowerdir=/tmp/lower,upperdir=/tmp/upper,workdir=/tmp/work overlay /tmp/union
Q: Is it possible to tweak the automation scripts to mount and use the existing images in a writable state on a 4.x kernel? Can I avoid reformulating them to contain something like
work directories? In some cases that may break their use elsewhere.
Per the documentation, the
workdir has to be an empty directory on the same filesystem as the
upperdir to allow for atomic writes. That sounds to me like it is impossible to union-mount root filesystem images as read-write. The
workdir would have to exist within the images (separate from their data directory) which isn't possible in a root filesystem image.
I've considered loading up stock Ubuntu 14.04 in a VM for a couple one-offs, but it's not a long term solution.
Best Attempt So Far:
The only solution I've come up with that avoids double-writing the same data to disk is to mount both images as
lowerdir values, with
workdir directories sitting on a
tmpfs mount. Then I can use
rsync to copy the changes from the
tmpfs location to the mounted
ext2/3 image after the automated writes are complete and the overlay union has been unmounted—i.e.:
mount -t overlay -o rw,lowerdir=/tmp/upper:/tmp/lower,upperdir=/tmp/tmpfs/root,workdir=/tmp/tmpfs/work overlay /tmp/union ...perform automated reads/writes... umount /tmp/union ...rsync contents from /tmp/tmpfs/root to /tmp/upper...
It's strictly limited to available RAM, though, and it's an ugly hack to add to all of the automation scripts.