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This diagram shows a ring of extents in an undo segment: E1, E2, E3, E4, and E5. Extents E1-E4 are active extents without space. In the diagram, transaction T1 and T2 both write to E4 and then start writing to E5. E5 is a new extent. E1 through E4 are active extents without space. E5 is a nonactive extent with space.