关于log file sync事件

How can I minimize waits for 'log file sync' ?

Solution

For more information on troubleshooting 'log
file sync' waits see:

Document 1376916.1 Troubleshooting: "Log File Sync" Waits

Information to help diagnose log file sync can be obtained using
the script found in 
Document 1064487.1

A: Log file sync waits occur when sessions wait for redo data to be
written to disk. Typically this is caused by slow writes or
committing too frequently in the application. Checking the "user
commits" section in the AWR report can reveal if the issue is
related to frequent committing. 

The tips below will help you to reduce log file sync when writes
are slow:
Tune LGWR to get good throughput to disk . eg: Do not put redo
logs on RAID 5.

If there are lots of short duration transactions, see if it is
possible to BATCH transactions together so there are fewer distinct
COMMIT operations. Each commit must confirmed that the relevant
REDO is on disk before it can complete. Although commits can be
"piggybacked" by Oracle, reducing the overall number of commits by
batching transactions can have a very beneficial effect.

On 10g , See if any of the processing can use the COMMIT NOWAIT
option .

Note: From 11g The COMMIT_WRITE parameter is
deprecated. It is retained for backward compatibility only. It is
replaced by the COMMIT_LOGGING and COMMIT_WAIT parameters.

See:

Oracle® Database Reference

11g Release 2 (11.2)

Part Number E17110-08

for more details

In Oracle 10g Release 2 the COMMIT command has been enhanced with
the WRITE clause to give a degree of control over the way redo
information is written to the redo logs during the commit
operation. This can improve performance, but  it
should only be used for processes that meet the following
criteria:
They result in large numbers of transactions that require redo
log writes.

Data loss can be tolerated in the event of an instance crash
during the process.

Waiting for redo log writes is a significant part of the waits
associated with the process.The available options for the COMMIT
command and the WRITE clause are displayed below.

COMMIT;

COMMIT WRITE WAIT; --> The commit command is
synchronous. It doesn't return until the relevant redo information
is written to the online redo log.

COMMIT WRITE NOWAIT; --> The commit command is
asynchronous. It can return before the relevant redo information is
written to the online redo log.

COMMIT WRITE BATCH; --> The commit command is
synchronous. It doesn't return until the relevant redo information
is written to the online redo log.

COMMIT WRITE IMMEDIATE; --> The commit "prods" the
LGWR process by sending a message, so that the redo is written
immediately to the redo logs.

To avoid make modifications to your code, you can you can use the
COMMIT_WRITE parameter. You can set it on session level or on
system level.

SQL> ALTER [SYSTEM | SESSION] SET
COMMIT_WRITE='IMMEDIATE,NOWAIT';

You can use trigger to set the parameter for the user that run the
application:

SQL> CREATE OR REPLACE TRIGGER
sys.global_commit_session_settings AFTER LOGON ON
<your
application_username>.SCHEMA

BEGIN

execute immediate 'alter session set COMMIT_WRITE
=''IMMEDIATE,NOWAIT''';

END;

/

Please keep in mind that using this option, and in case of database
crash, the data that is not written to the redolog file will be
lost even though they are committed. This is part of the
requirements for commit nowait is that "Data loss can be tolerated
in the event of an instance crash during the process"

See if any activity can safely be done with NOLOGGING /
UNRECOVERABLE options