Problem:
========

You are experiencing performance problems.  Your applications have lots of
COMMITs and ROLLBACKs.

V$SYSTEM_EVENT.TOTAL_WAITS / TOTAL_TIMEOUTS / TIME_WAITED / AVERAGE_WAIT
has high numbers for "log file sync".

You are not having disk problems which is often the problem cause of
"log file sync".


Solution:
=========

1. Ensure that your redo log buffer in the SGA is large enough.

2. Over time (since database startup), if you see increasing values for:

     select * from v$sysstat where name like 'redo%space%';

   your redo log buffer is too small.

   "Redo log space requests" event is the number of times that the
   background was requested to allocate space in the redo file.

   This gives an indication of how many times a user process waited for
   space in the redo logs.

3. Increase the redo log buffer by increasing the init.ora "log_buffer"
   parameter.

4. Then bounce your instance.


Explanation:
============

The "log file sync" event is often thought to be a disk-only issue which is
not always true.  This event can be more than disk I/O problems.  If the redo
log buffer cannot be flushed/allocated fast enough, you experience memory
contention rather than disk contention.
 
引自：http://blog.chinaunix.net/u/12960/showart_127878.html