#value_indexed
IOCB_CMD_PREAD 0
IOCB_CMD_PWRITE 1
IOCB_CMD_FSYNC 2
IOCB_CMD_FDSYNC 3
IOCB_CMD_PREADX 4
IOCB_CMD_POLL 5
IOCB_CMD_NOOP 6
IOCB_CMD_PREADV 7
IOCB_CMD_PWRITEV 8
