Wait Events and Potential Causes in Oracle

Wait Event	General Area	Possible Causes	Look For / Examine
buffer busy waits	Buffer cache, DBWR	Dependent on type of buffer: index block in a primary key that is based on an ascending sequence rollback segment header	Examine V$SESSION_WAIT while the problem is occurring to determine the type of block contended for.
free buffer waits	Buffer cache, DBWR, I/O	Slow DBWR (possibly due to I/O?) Cache too small	Examine write time using OS statistics. Check buffer cache statistics for evidence of too small cache.
db file scattered read	I/O, SQL statement tuning	Poorly tuned SQL Slow I/O system	Investigate V$SQLAREA to see whether there are SQL statements performing many disk reads. Cross-check I/O system and V$FILESTAT for poor read time.
db file sequential read	I/O, SQL statement tuning	Poorly tuned SQL Slow I/O system	Investigate V$SQLAREA to see whether there are SQL statements performing many disk reads. Cross-check I/O system and V$FILESTAT for poor read time.
enqueue	Locks	Depends on type of enqueue	Look at V$ENQUEUE_STAT.
latch free	Latch contention	Depends on latch	Check V$LATCH.
log buffer space	Log buffer, I/O	Log buffer small Slow I/O system	Check the statistic redo buffer allocation retries in V$SYSSTAT. Check configuring log buffer section Check the disks that house the online redo logs for resource contention.
log file sync	I/O, over- committing	Slow disks that store the online logs Un-batched commits	Check the disks that house the online redo logs for resource contention. Check the number of transactions (commits + rollbacks) per second, from V$SYSSTAT.