[摘录]Oracle Wait Interface之Buffer busy waits事件
1.获得cache buffers chains latch，遍历那条buffer chain直到找到需要的buffer header
2.根据需要进行的操作类型(读或写)，它需要在buffer header上获得一个共享或独占模式的buffer pin或者buffer lock
3.若进程获得buffer header pin，它会释放获得的cache buffers chains latch，然后执行对buffer block的操作
4.若进程无法获得buffer header pin，它就会在buffer busy waits事件上等待
进程之所以无法获得buffer header pin，是因为为了保证数据的一致性，同一时刻一个block只能被一个进程pin住进行存取，因此当一个进程需要存取buffer cache中一个被其他进程使用的block的时候，这个进程就会产生对该block的buffer busy waits事件。
截至Oracle 9i，buffer busy waits事件的p1,p2,p3三个参数分别是file#,block#和id，分别表示等待的buffer block所在的文件编号，块编号和具体的等待原因编号，到了Oracle 10g，前两个参数没变，第3个参数变成了块类型编号，这一点可以通过查询v$event_name视图来进行验证：
在诊断buffer busy waits事件的过程中，获取如下信息会很有用：PHP code:
Oracle 9i
SQL> select parameter1,parameter2,parameter3 from v$event_name where name='buffer busy waits';
PARAMETER1 PARAMETER2 PARAMETER3
------------------------ ------------------------ ------------------------
file# block# id
Oracle 10g
PARAMETER1 PARAMETER2 PARAMETER3
------------------------ ------------------------ ------------------------
file# block# class#
1.获取产生buffer busy waits事件的等待原因编号，这可以通过查询该事件的p3参数值获得
2.获取产生此事件的SQL语句，可以通过如下的查询获得：
select sql_text from v$sql t1,v$session t2,v$session_wait t3
where t1.address=t2.sql_address and t1.hash_value=t2.sql_hash_value
and t2.sid=t3.sid and t3.event='buffer busy waits';
3.获取等待的块的类型以及所在的segment，可以通过如下查询获得：
select 'Segment Header' class,a.segment_type,a.segment_name,a.partition_name from dba_segments a,v$session_wait b
where a.header_file=b.p1 and a.header_block=b.p2 and b.event='buffer busy waits'
union
select 'Freelist Groups' class,a.segment_type,a.segment_name,a.partition_name from dba_segments a,v$session_wait b
where a.header_file=b.p1 and b.p2 between a.header_block+1 and (a.header_block+a.freelist_groups) and a.freelist_groups>1 and b.event='buffer busy waits'
union
select a.segment_type||' block' class,a.segment_type,a.segment_name,a.partition_name from dba_extents a,v$session_wait b
where a.file_id=b.p1 and b.p2 between a.block_id and a.block_id+a.blocks-1 and b.event='buffer busy waits' and not exists(select 1 from dba_segments where
header_file=b.p1 and header_block= b.p2);
查询的第一部分：如果等待的块类型是segment header，那么可以直接拿buffer busy waits事件的p1和p2参数去dba_segments视图中匹配header_file和header_block字段即可找到等待的segment名称和segment类型，进行相应调整
查询的第二部分：如果等待的块类型是freelist groups，也可以在dba_segments视图中找出对应的segment名称和segment类型，注意这里的参数p2表示的freelist groups的位置是在segment的header_block+1到header_block+freelist groups组数之间，并且freelist groups组数大于1
查询的第三部分：如果等待的块类型是普通的数据块，那么可以用p1、p2参数和dba_extents进行联合查询得到block所在的segment名称和segment类型
对于不同的等待块类型，我们采取不同的处理办法：
1.data segment header：
进程经常性的访问data segment header通常有两个原因：获取或修改process freelists信息、扩展高水位标记，针对第一种情况，进程频繁访问process freelists信息导致freelist争用，我们可以增大相应的segment对象的存储参数freelist或者freelist groups；若由于数据块频繁进出freelist而导致进程经常要修改freelist，则可以将pctfree值和pctused值设置较大的差距，从而避免数据块频繁进出freelist；对于第二种情况，由于该segment空间消耗很快，而设置的next extent过小，导致频繁扩展高水位标记，解决的办法是增大segment对象的存储参数next extent或者直接在创建表空间的时候设置extent size uniform
2.data block：
某一或某些数据块被多个进程同时读写，成为热点块，可以通过如下这些办法来解决这个问题：
(1)降低程序的并发度，如果程序中使用了parallel查询，降低parallel degree，以免多个parallel slave同时访问同样的数据对象而形成等待降低性能
(2)调整应用程序使之能读取较少的数据块就能获取所需的数据，减少buffer gets和physical reads
(3)减少同一个block中的记录数，使记录分布于更多的数据块中，这可以通过若干途径实现：可以调整segment对象的pctfree值，可以将segment重建到block size较小的表空间中，还可以用alter table minimize records_per_block语句减少每块中的记录数
(4)若热点块对象是类似自增id字段的索引，则可以将索引转换为反转索引，打散数据分布，分散热点块
3.undo segment header：
undo segment header争用是因为系统中undo segment不够，需要增加足够的undo segment，根据undo segment的管理方法，若是手工管理模式，需要修改rollback_segments初始化参数来增加rollback segment，若是自动管理模式，可以减小transactions_per_rollback_segment初始化参数的值来使oracle自动增多rollback segment的数量
4.undo block：
undo block争用是由于应用程序中存在对数据的读和写同时进行，读进程需要到undo segment中去获得一致性数据，解决办法是错开应用程序修改数据和大量查询数据的时间
小结：buffer busy waits事件是oracle等待事件中比较复杂的一个，其形成原因很多，需要根据p3参数对照Oracle提供的原因代码表进行相应的诊断，10g以后则需要根据等待的block类型结合引起等待时间的具体SQL进行分析，采取相应的调整措施
附录：
P1 = file# (Absolute File# in Oracle8 onwards)
P2 = block#
P3 = id (Reason Code) ，在10g中P3表示块类型编号 class#
原因代码P3 ：
A block is being read
=====
100 We want to NEW the block but the block is currently being read by another session (most likely for undo).
200 We want to NEW the block but someone else has is using the current copy so we have to wait for them to finish.
230 Trying to get a buffer in CR/CRX mode , but a modification has started on the buffer that has not yet been completed.
- A modification is happening on a SCUR or XCUR buffer, but has not yet completed
(dup.) 231 CR/CRX scan found the CURRENT block, but a modification has started on the buffer that has not yet been completed.
130 Block is being read by another session and no other suitable block image was found, so we wait until the read is completed. This may also occur after a buffer cache assumed deadlock. The kernel can't get a buffer in a certain amount of time and assumes a deadlock. Therefor it will read the CR version of the block.
110 We want the CURRENT block either shared or exclusive but the Block is being read into cache by another session, so we have to wait until their read() is completed.
(duplicate) 120 We want to get the block in current mode but someone else is currently reading it into the cache. Wait for them to complete the read. This occurs during buffer lookup.
210 The session wants the block in SCUR or XCUR mode. If this is a buffer exchange or the session is in discrete TX mode, the session waits for the first time and the second time escalates the block as a deadlock and so does not show up as waiting very long. In this case the statistic: "exchange deadlocks" is incremented and we yield the CPU for the "buffer deadlock" wait event.
(duplicate) 220 During buffer lookup for a CURRENT copy of a buffer we have found the buffer but someone holds it in an incompatible mode so we have to wait.
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