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I want Linux's OverlayFS to behave like AUFS when writing to a lower file. I want it to write through to the lower directory. For example, I have two files named L/lower and U/upper.

mount -t overlay -o lowerdir=L,upperdir=U,workdir=W overlay X

This merges L and U into a single OverlayFS directory named X. So now the two files are accessible as X/lower and X/upper.

Then I modify the lower file through the OverlayFS directory. This is where it misbehaves on me:

echo 'This is a modification' >> X/lower

It does not actually modify the lower file L/lower. Instead it creates a new upper file called U/lower and writes my modification there. This is not what I want. I want X to serve as a convenient, single access point for editing purposes.

How can I make the modification to X/lower write through to L/lower?

  • lower is readonly. thats how it works. you can recursively mount overlay over overlays, though. – mikeserv Oct 29 '15 at 22:31
  • I've posted a couple of answers to my own question (unix.stackexchange.com/q/393930) which covers this topic. – ejm Sep 23 '17 at 2:50
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You can write to the underlying directory by not using the created overlay to access the file (the directory "X" in this case). Do it with:

echo 'This is a modification' >> L/lower
  • That wouldn’t be done through the overlay, however, which is what I ask about. I’d like X/lower to be a single point of access for editing purposes regardless of whether the file actually sits in L/ or in U/; so apparently merging the two directories while actually keeping them apart. – Michael Allan Dec 5 '19 at 0:08

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