Hello Marko, InnoDB uses 5 bits to store the combination of mbminlen and mbmaxlen, and its calculated as: mbmaxlen * 5 + mbminlen Note, combinations with mbminlen=0 and mbmaxlen=0 are not possible. Also, combinations with mbminlen>mbmaxlen are not possible. This is a summary: mbminlen mbmaxlen 0 1 2 3 4 0 (0) (1) (2) (3) (4) 1 (5) 6 (7) (8) (9) 2 (10) 11 12 (13) (14) 3 (15) 16 17 18 (19) 4 (20) 21 22 23 24 I marked impossible numbers with parentheses. In the range 0..24: - 10 pairs are valid - 15 pairs are not valid It should be safe to use any impossible number to mean mbmaxlen=5 & mbminlen=1 It seems there is no really a need to extend the header to use separate 3 bits for both mbminlen and mbmaxlen.