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I know that after switching to system images for Ubuntu on mobile devices that meant making the installation read-only.

Yet I don't quite understand whether that means that the whole OS is RO, as obviously system logs are updated, the home folder can be written to with e.g. user media and new click packages are installed in /opt.

So what does it exactly mean that the images are read-only, which areas are actually read-write, and how is that possible?

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up vote 3 down vote accepted

The Ubuntu touch images use multiple file systems. In the standard read only configuration, the root file system is mounted read only, while a number of locations under / are bind mounted to locations on the writeable /userdata file system.

These writeable locations include /var/log, /home, and /opt/, which is why those locations appeared writeable to you.

The full list of locations that are bind mounted to /userdata can be found in the /etc/fstab file. The list is fairly large, since a number of much of the traditional Linux software included in the images don't expect quite as much of the file system to be read only.

When you switch set the /userdata/.writable_image flag, these locations are still bind mounted to /userdata, but the underlying root file system is also mounted read/write allowing the entire system to be changed.

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