I have looked at How do I use OverlayFS? but it doesn't answer my question.
I need to install some third party applications and they require /opt to be writable but the device I am running on has /opt on a readable rootfs. Essentially, I have a directory /opt on a read-only filesystem (lower) and let's say I have a read-write directory /mnt/optw (writable). I would like to merge /opt & /mnt/optw and mount it to /opt.
Is this possible at all?
This can also be achieved through kernel in-built overlayfs module:
# mount -t overlay -o lowerdir=/opt,upperdir=/mnt/optw,workdir=/tmp/opt-overlay overlay /opt
That can be checked to be properly mounted as:
$ mount | tail -1
overlay on /opt type overlay (rw,relatime,lowerdir=/opt,upperdir=/mnt/optw,workdir=/tmp/opt-overlay)
You could bind /mnt/optw to /opt like so:
sudo mount --bind /mnt/optw/ /opt/
This way /opt will be a reflection of /mnt/optw contents and permissions.
To access the contents of the two directories under /opt, mount with aufs like so:
sudo mount -t aufs -o br=/opt/:/mnt/optw/ none /opt/
This way, the two directories are merged and you'll be able to access their contents under /opt/.
Notice:
If you do not have aufs support available on your system, you can add support by installing aufs-tools like so:
sudo apt install aufs-tools
For further reading man aufs.
Edit:
Regarding the OP's comment, The same can be achieved through kernel in-built overlayfs module:
sudo mount -t overlay -o lowerdir=/opt,upperdir=/mnt/optw,workdir=/tmp/optoverlay overlay /opt
