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zenDev120
Asked: 2020-03-29 10:08:03 +0800 CST

Overlayfs to an existing directory

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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?

mount overlayfs

2 Answers

  1. 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.

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  2. 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)
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