Q:

cp: cannot create directory 'ABC.DEF/G/H': Structure needs cleaning

What does it mean "Structure needs cleaning" ?
I've never seen such error code before - and man cp is not that helpful.
It has happened to me on ext4.
I was trying to copy directory using:
cp -arv dirname dirname.bak

A:

Some googling told me that there was a patch to ext4 last year that mentions returning EUCLEAN upon an out of space error.
E.g. https://patchwork.ozlabs.org/patch/452275/

The fix is either to run e2fsck -E bmap2extent, or to chattr +e the file.

