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等待事件的概念大概是从ORACLE 7.0.12中引入的，大致有100个等待事件。在ORACLE 8.0中这个数目增大到了大约150个，在ORACLE 8I中大约有220个事件，在ORACLE 9IR2中大约有400个等待事件，在ORACLE 10GR2中，大约有874个等待事件，在11GR2中约有1000多个等待事件。
虽然不同版本和组件安装可能会有不同数目的等待事件，但是这些等待事件都可以通过查V$EVENT_NAME视图获得：
10.2.0.5版本：
select count(1) from v$event_name;
11g：
select count(1) from v$event_name;
ORACLE的等待事件，主要可以分为两类，即空闲(IDLE)等待事件和非空闲(NON-IDLE)等待事件。
1). 空闲等待事件指ORACLE正等待某种工作，在诊断和优化数据库的时候，不用过多注意这部分事件。
2). 非空闲等待事件专门针对ORACLE的活动，指数据库任务或应用运行过程中发生的等待，这些等待事件是在调整数据库的时候需要关注与研究的。
下面来看一下ORACLE中等待事件的主要分类及各类等待事件的个数：
SELECT a.INST_ID, A.EVENT, COUNT(1)
FROM gv$session a
where a.username is not null
and a.STATUS = ‘ACTIVE‘
AND A.WAIT_CLASS<>‘Idle‘
GROUP BY a.INST_ID,A.EVENT;
SELECT wait_class#,
wait_class_id,
wait_class,
COUNT(*) AS "count"
FROM v$event_name
GROUP BY wait_class#,
wait_class_id,
wait_class
ORDER BY wait_class#;
11g：
常见的空闲事件有:
? dispatcher timer
? lock element cleanup
? Null event
? parallel query dequeue wait
? parallel query idle wait - Slaves
? pipe get
? PL/SQL lock timer
? pmon timer- pmon
? rdbms ipc message
? slave wait
? smon timer
? SQL*Net break/reset to client
? SQL*Net message from client
? SQL*Net message to client
? SQL*Net more data to client
? virtual circuit status
? client message
一些常见的非空闲等待事件有:
? db file scattered read
? db file sequential read
? buffer busy waits
? free buffer waits
? enqueue
? latch free
? log file parallel write
? log file sync
Every wait event belongs to a class of wait event. The following list describes each of
the wait classes.
Administrative
Waits resulting from DBA commands that cause users to wait (for example, an index
rebuild)
Application
Waits resulting from user application code (for example, lock waits caused by row
level locking or explicit lock commands)
Cluster
Waits related to Real Application Clusters resources (for example, global cache
resources such as ‘gc cr block busy‘)
Commit
This wait class only comprises one wait event - wait for redo log write confirmation
after a commit (that is, ‘log file sync‘)
Concurrency
Waits for internal database resources (for example, latches)
Configuration
Waits caused by inadequate configuration of database or instance resources (for
example, undersized log file sizes, shared pool size)
Idle
Waits that signify the session is inactive, waiting for work (for example, ‘SQL*Net
message from client‘)
Network
Waits related to network messaging (for example, ‘SQL*Net moredata to dblink‘)
Other
Waits which should not typically occur on a system (for example, ‘wait for EMON to
spawn‘)
Queue
Contains events that signify delays in obtaining additional data in a pipelined
environment. The time spent on these wait events indicates inefficiency or other
problems in the pipeline. It affects features such as Oracle Streams, parallel queries, or
DBMS_PIPEPL/SQL packages.
Scheduler
Resource Manager related waits (for example, ‘resmgr: become active‘)
System I/O
Waits for background process I/O (for example, DBWR wait for ‘db file parallel write‘)
User I/O
Waits for user I/O (for example ‘db file sequential read‘)
This section provides descriptions of some of the more common wait event parameters.
block#
This is the block number of the block for which Oracle needs to wait. The block
number is relative to the start of the file. Tofind the object to which this block belongs,
issue the following SQL statement:
select segment_name, segment_type, owner, tablespace_name
from dba_extents
where file_id = file#
and block#
between block_id and block_id + blocks - 1;
blocks
The number of blocks that is being either read from or written to the file. The block
size is dependent on the file type:
■ Database files have a block size of DB_BLOCK_SIZE
■ Logfiles and control files have a block size that is equivalent to the physical block
size of the platform
break?
If the value for this parameter equals 0, a reset was sent to the client. A nonzero value
indicates that a break was sent to the client.
class
The class of the block describes how the contents of the block are used. For example,
class 1 represents data block, and class 4 represents segment header.
dba
The initials "dba" represents the data block address, which consists of a file number
and a block number.
driver id
The address of the disconnect function of the driver that is currently being used.
file#
The following query returns the name of the database file:
select *
from v$datafile
where file# = file#;
id1
The first identifier (id1) of the enqueue or global lock takes its value from P2 or
P2RAW. The meaning of the identifier depends on the name (P1).
id2
The second identifier (id2) of the enqueue or global lock takes its value from P3 or
P3RAW. The meaning of the identifier depends on the name (P1).
le
The relative index number into V$GC_ELEMENT.
mode
Themodeis usually stored in the low order bytes of P1 or P1RAW and indicates the
mode of the enqueue or global lock request.This parameter has one of the following
values:
Use the following SQL statement to retrieve the name of the lock and the mode of the
lock request:
select chr(bitand(p1,-16777216)/16777215)||
chr(bitand(p1, 16711680)/65535) "Lock",
bitand(p1, 65535) "Mode"
from v$session_wait
where event = ‘DFS enqueue lock acquisition‘;
name and type
The name or "type" of the enqueue or globallock can be determined by looking at the
two high order bytes of P1 or P1RAW. The name is always two characters. Use the
following SQL statement to retrieve the lock name.
select chr(bitand(p1,-16777216)/16777215)||
chr(bitand(p1,16711680)/65535) "Lock"
from v$session_wait
where event = ‘DFS enqueue lock acquisition‘;
namespace
The name of the object namespace as it is displayed in V$DB_OBJECT_CACHEview.
requests
The number of I/Os that are "requested." This differs from the number of blocks in
that one request could potentially contain multiple blocks.
session#
The number of the inactive session. Use the following SQL statement to find more
information about the session:
select *
from v$session
where sid = session#;
waited
This is the total amount of time the sessionhas waited for this session to terminate.
Table C–1 Lock Mode Values
Mode Value Description
1 Null mode
2 Sub-Share
3 Sub-Exclusive
4Share 5Share/Sub-Exclusive 6Exclusive
数据文件I/O相关的等待事件：
? db file sequential read
? db file scattered read
? db file parallel read
? direct path read
? direct path write
? direct path read (lob)
? direct path write (lob)
控制文件I/O相关的等待事件：
? control file parallel write
? control file sequential read
? control file single write
重做日志文件I/O相关的等待事件：
? log file parallel write
? log file sync
? log file sequential read
? log file single write
? switch logfile command
? log file switch completion
? log file switch (clearing log file)
? log file switch (checkpoint incomplete)
? log switch/archive
? log file switch (archiving needed)
高速缓存区I/O相关的等待事件：
? db file parallel write
? db file single write
? write complete waits
? free buffer waits
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原文地址：http://www.cnblogs.com/lhrbest/p/5768663.html
