Direct I/O (DIO) and Concurrent I/O (CIO) on AIX 5L [ID 257338.1]

출처  :   https://support.oracle.com



Direct I/O (DIO) and Concurrent I/O (CIO) on AIX 5L [ID 257338.1]
--------------------------------------------------------------------------------

수정 날짜 13-SEP-2005     유형 BULLETIN     상태 PUBLISHED  



PURPOSE
-----------
To discuss the use of Direct I/O (DIO) and Concurrent I/O (CIO) on AIX 5L


 
SCOPE & APPLICATION
-------------------------
System Administrators managing AIX 5L that has Oracle RDBMS on top.



DIRECT I/O (DIO) AND CONCURRENT I/O (CIO) on AIX
-------------------------------------------------------
If you use a Journaled File System, it is easier to manage and maintain database files
than it is if you use raw devices. In prior versions of AIX, file systems used to support only
buffered read and write and added extra contention due to undesirable inode locking.

These two issues are solved by JFS2's Concurrent I/O and GPFS' Direct I/O, allowing
file systems to be used in place of raw devices even when optimal performance is required.

AIX 5L includes Direct I/O support on JFS, JFS2 and GPFS. In addition, AIX 5L version 5.2
and later includes Concurrent I/O support. Direct I/O and Concurrent I/O support allows
database files to exist on file systems while bypassing the operating system buffer cache
and removing inode locking operations that are redundant with functionality provided by
Oracle Database 9i. Where possible, Oracle recommends isolating Oracle data files on their
own dedicated file system(s) on which the Concurrent I/O or Direct I/O option is enabled.


The following table lists file systems available on AIX and the recommended setting:

File System        Option        Description
-----------        ------        ------------------------------------------------------
JFS                    dio           Concurrent I/O is not available on JFS. 
                                         Direct I/O (dio) is available, but performance
                                         is degraded compared to JFS2 with Concurrent I/O.

JFS large file       none         Oracle does not recommend using JFS big file for 
                                         Oracle Database because it's 128 KB alignment
                                         constraint prevents you from using Direct I/O.

JFS2                   cio           Concurrent I/O (cio) is a better setting than
                                         Direct I/O (dio) on JFS2 because it has support
                                         for multiple concurrent readers/multiple
                                         concurrent writers on the same file.

GPFS                   -            Oracle Database 9i release 2 and later silently enables
                                         Direct I/O on GPFS for optimum performance.
                                         GPFS already supports multiple reader/multiple writers/multiple nodes.
                                         Therefore, Direct I/O and Concurrent I/O are the same thing on GPFS.


If GPFS is used, the same file system can be used for all purposes including
$ORACLE_HOME, datafiles, logs. A large GPFS block size (typically at least 512KB)
should be used for optimal performance. GPFS is designed for scalability and there
is no requirement to create multiple GPFS file systems as long as the amount of
data fits in a single GPFS file system.

If placing the Oracle Database's logs on a JFS2 filesystem, the optimal configuration
is to create the file system with option "agblksize=512" and to mount it with option "cio".
This delivers logging performance similar, within a few percents, to the performance of a raw device.

In Oracle Database 9i, Direct I/O and/ or Concurrent I/O can only be enabled at
filesystem level on JFS/JFS2 and therefore $ORACLE_HOME and datafiles have to be
placed in separate filesystems for optimal performance:
$ORACLE_HOME would reside on a file system mounted with default options,
whereas datafiles and logs would reside on file systems mounted with the "dio" or "cio" option.

Having on ORACLE_HOME on a filesystem mounted with "cio" option is not supported.
Such a configuration will cause, installation, relinking and other unexpected problems.



RELATED DOCUMENTS
--------------------------
Note 232935.1 How to Enable DIRECT IO Support within Oracle and AIX with JFS2 Filesystem
Note 302806.1 IBM General Parallel File System (GPFS) and Oracle RAC on AIX 5L and IBM eServer pSeries
Note 282036.1 Minimum Software Versions and Patches Required to Support Oracle Products on IBM pSeries