OverlayFS has a
workdir option, beside two other directories
upperdir, which needs to be an empty directory.
Unfortunately the kernel documentation of overlayfs does not talk much about the purpose of this option.
The "workdir" needs to be an empty directory on the same filesystem as upperdir.
For readonly overlays the
workdir might be ommittet among the
upperdir. This give me the clue that it has to do with writing the merged files.
Please explain what's happening in the
workdir when files are written or changed in the merged directory. Why is the writable
upperdir not enough?