一些常见的等待事件及原因

参考文档：

https://docs.oracle.com/cd/E11882_01/server.112/e41573/instance_tune.htm#PFGRF94465

10.2.3 Table of Wait Events and Potential Causes

Table 10-1 links wait events to possible causes and gives an overview of the Oracle data that could be most useful to review next.

Table 10-1 Wait Events and Potential Causes

Wait Event	General Area	Possible Causes	Look for / Examine
buffer busy waits	Buffer cache, DBWR	Depends on buffer type. For example, waits for an index block may be caused by a primary key that is based on an ascending sequence.	Examine V$SESSION while the problem is occurring to determine the type of block in contention.
free buffer waits	Buffer cache, DBWR, I/O	Slow DBWR (possibly due to I/O?) Cache too small	Examine write time using operating system statistics. Check buffer cache statistics for evidence of too small cache.
db file scattered read	I/O, SQL statement tuning	Poorly tuned SQL Slow I/O system	Investigate V$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$FILESTAT for poor read time.
enqueue waits (waits starting with enq:)	Locks	Depends on type of enqueue	Look at V$ENQUEUE_STAT.
library cache latch waits: library cache, librarycache pin, and librarycache lock	Latch contention	SQL parsing or sharing	Check V$SQLAREA to see whether there are SQL statements with a relatively high number of parse calls or a high number of child cursors (column VERSION_COUNT). Check parse statistics in V$SYSSTAT and their corresponding rate for each second.
log buffer space	Log buffer, I/O	Log buffer small Slow I/O system	Check the statistic redobuffer allocation retriesin V$SYSSTAT. Check configuring log buffer section in configuring memory chapter. Check the disks that house the online redo logs for resource contention.
log file sync	I/O, over- committing	Slow disks that store the online logs Un-batched commits	Check the disks that house the online redo logs for resource contention. Check the number of transactions (commits + rollbacks) each second, from V$SYSSTAT.

You may also want to review the My Oracle Support notices on buffer busy waits (34405.1) and free buffer waits (62172.1). You can also access these notices and related notices by searching for "busy buffer waits" and "free buffer waits" at: