From https://www.kernel.org/doc/Documentation/filesystems/overlayfs.txt:
> The lower filesystem can be any filesystem supported by Linux and does
> not need to be writable. The lower filesystem can even be another
> overlayfs. The upper filesystem will normally be writable and if it
> is it must support the creation of trusted.* extended attributes, and
> must provide valid d_type in readdir responses, so NFS is not suitable.
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
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