NAME				     TYPE	 VALUE
------------------------------------ ----------- ------------------------------
disk_asynch_io			     boolean	 TRUE
SQL> COL NAME FORMAT A50
SQL> SELECT NAME,ASYNCH_IO FROM V$DATAFILE F,V$IOSTAT_FILE I
WHERE F.FILE#=I.FILE_NO
AND FILETYPE_NAME='Data File';  2    3  

NAME						   ASYNCH_IO
-------------------------------------------------- ---------
/u01/app/oracle/oradata/yaoy11G/system01.dbf	   ASYNC_OFF
/u01/app/oracle/oradata/yaoy11G/sysaux01.dbf	   ASYNC_OFF
/u01/app/oracle/oradata/yaoy11G/undotbs01.dbf	   ASYNC_OFF
/u01/app/oracle/oradata/yaoy11G/users01.dbf	   ASYNC_OFF
/u01/app/oracle/oradata/yaoy11G/example01.dbf	   ASYNC_OFF

SQL> show parameter systemio

NAME				     TYPE	 VALUE
------------------------------------ ----------- ------------------------------
filesystemio_options		     string	 none
SQL> alter system set filesystemio_options=asynch;
alter system set filesystemio_options=asynch
                 *
ERROR at line 1:
ORA-02095: specified initialization parameter cannot be modified




SQL>  alter system set filesystemio_options=setall  scope=spfile;

System altered.OPTIONS:

  "asynch"    - This allows asynchronous IO to be used where supported by the OS. 
  
   "directIO"  - This allows directIO to be used where supported by the OS. Direct IO bypasses any Unix buffer cache. As of 10.2 most platforms will try to use "directio" option for NFS mounted disks (and will also check NFS attributes are sensible).

"setall"    - Enables both ASYNC and DIRECT IO.

"none"      - This disables ASYNC IO and DIRECT IO so that  Oracle uses normal synchronous writes, without  any direct io options.


SQL> startup force;
ORACLE instance started.

Total System Global Area  835104768 bytes
Fixed Size		    2217952 bytes
Variable Size		  511707168 bytes
Database Buffers	  318767104 bytes
Redo Buffers		    2412544 bytes
Database mounted.
Database opened.
SQL> COL NAME FORMAT A50
SQL>  SELECT NAME,ASYNCH_IO FROM V$DATAFILE F,V$IOSTAT_FILE I
WHERE F.FILE#=I.FILE_NO
AND FILETYPE_NAME='Data File';  2    3  

NAME						   ASYNCH_IO
-------------------------------------------------- ---------
/u01/app/oracle/oradata/yaoy11G/system01.dbf	   ASYNC_ON
/u01/app/oracle/oradata/yaoy11G/sysaux01.dbf	   ASYNC_ON
/u01/app/oracle/oradata/yaoy11G/undotbs01.dbf	   ASYNC_ON
/u01/app/oracle/oradata/yaoy11G/users01.dbf	   ASYNC_ON
/u01/app/oracle/oradata/yaoy11G/example01.dbf	   ASYNC_ON

As for ASM, ASM Inherently Performs Asynchronous I/O Regardless of filesystemio_options Parameter (Doc ID 751463.1)
FILESYSTEMIO_OPTIONS enables asynchronous I/O (AIO) when the value is set to SETALL/ASYNC. However, ASM bypasses the filesystem layer in this case because ASM I/O is entirely controlled by DISK_ASYNCH_IO parameter. AIO needs to be enabled/disabled by setting disk_asynch_io to parameter values TRUE/FALSE.