RNBD_OP_BITS
#define RNBD_OP_MASK ((1 << RNBD_OP_BITS) - 1)
RNBD_F_SYNC = 1<<(RNBD_OP_BITS + 0),
RNBD_F_FUA = 1<<(RNBD_OP_BITS + 1),
RNBD_F_PREFLUSH = 1<<(RNBD_OP_BITS + 2),
RNBD_F_NOUNMAP = 1<<(RNBD_OP_BITS + 3)