#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