The manual for aufs makes several references to something called a "whiteout", but it's not clear to me what this is.

What is a whiteout and why would one exist?


It is how the union file system supports deleting from a union where the underlying file to be deleted resides on a read-only file system.

Say you have

  • /bin (read-only)
  • /usr/bin (read-only)
  • /home/USERNAME/bin (read-write)

all union mounted together in /bin (read-write).

And you do rm /bin/ls.

It would place a whiteout for ls in /home/USERNAME/bin, which makes it look like /bin/ls has been deleted.

See State of the unionfs for a good read and a full explanation.

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