Oracle buffer busy waits

 buffer busy waits

This wait indicates that there are some buffers in the buffer cache that multiple processes are attempting to access concurrently. Query 

V$WAITSTAT

 for the wait statistics for each class of buffer. Common buffer classes that have buffer busy waits include 

data

 

block

, 

segment

 

header

, 

undo

 

header

, and 

undo

 

block

.多个进程同时访问Buffer cache中的相同数据导致。常见的遭遇buffer busy waits 的缓冲类包括： 

data

 

block

, 

segment

 

header

, 

undo

 

header

, 和

undo

 

block

Check the following 

V$SESSION_WAIT

 parameter columns:

P1

: File ID

P2

: Block ID

P3

: Class ID

 Causes

To determine the possible causes, first query 

V$SESSION

 to identify the value of 

ROW_WAIT_OBJ#

 when the session waits for 

buffer

 

busy

 

waits

. For example:确定问题的原因，需要结合

V$SESSION

 和DBA_OBJECTS，详细参见如下示例：SELECT row_wait_obj#
FROM V$SESSION
WHERE EVENT = 'buffer busy waits';

To identify the object and object type contended for, query 

DBA_OBJECTS

 using the value for 

ROW_WAIT_OBJ#

 that is returned from 

V$SESSION

.
For example:

SELECT owner, object_name, subobject_name, object_type
FROM DBA_OBJECTS
WHERE data_object_id = &row_wait_obj;

 Actions

The action required depends on the class of block contended for and the actual segment.
  解决方法依赖于class of block contended 和actual
segment。


segment header
If the contention is on the segment header, then this is most likely free list contention.
Automatic segment-space management in locally managed tablespaces eliminates the need to specify the 

PCTUSED

, 

FREELISTS

,
and 

FREELIST

 

GROUPS

 parameters. If possible, switch from manual space management to automatic segment-space management (ASSM).
尽可能的将MSSM转化为ASSM.

The following information is relevant if you are unable to use ASSM (for example, because the tablespace uses dictionary space management).
A free list is a list of free data blocks that usually includes blocks existing in several different extents within the segment. Free lists are composed of blocks in which
free space has not yet reached PCTFREE or used space has shrunk below PCTUSED. Specify the number of process free lists with the 

FREELISTS

parameter. The default value of 

FREELISTS

 is one.
The maximum value depends on the data block size.
To find the current setting for free lists for that segment, run the following:

SELECT SEGMENT_NAME, FREELISTS
FROM DBA_SEGMENTS
WHERE SEGMENT_NAME = segment name
AND SEGMENT_TYPE = segment type;

Set free lists, or increase the number of free lists. If adding more free lists does not alleviate the problem, then use free list groups (even in single instance this can make a difference). If using Oracle RAC,
then ensure that each instance has its own free list group(s)


data block
If the contention is on tables or indexes (not the segment header):
若争用是在表或者索引上（非segment header）

Check for right-hand indexes. These are indexes that are inserted into at the same point by many processes. For example, those that use sequence number generators for the key values.
检查右侧 indexe

Consider using ASSM, global hash partitioned indexes, or increasing free lists to avoid multiple processes attempting to insert into the same block.
 使用ASSM,全局hash分区索引或者是增加free list 来避免多个进程插入相同的块。


 undo header
For contention on rollback segment header:

If you are not using automatic undo management, then add more rollback segments.
若未使用auto undo management就增加更多的rollback
segments。


undo block
For contention on rollback segment block:

If you are not using automatic undo management, then consider making rollback segment sizes larger.
若未使用auto undo management可考虑增大rollback
segment 大小。