change buffer官方说明： https://dev.mysql.com/doc/refman/5.7/en/innodb-change-buffer.html
change buffer官方解答： https://dev.mysql.com/doc/refman/5.7/en/faqs-innodb-change-buffer.html
设计背景：
与聚集索引不同，二级索引通常是非唯一的，像delete和update操作的二级索引树中通常都是顺序相对随机不相邻的索引页，
这样会照成大量随机I/O
工作机制：
DML(insert,update,delete)操作使用到的二级索引页如果不在buffer pool中，就把它缓存到buffer pool的change buffer中。后续再进行合并操作，批量写回disk( master thread在服务器空闲时和slow shutdown期间会合并缓冲更改)。可避免大量随机I/O
注：change buffer不支持descending index,Clustered indexes,full-text indexes,spatial indexes
控制参数:
innodb_change_buffering :change buffer缓存级别有
innodb_change_buffering values include:
-
all 默认开启
The default value: buffer inserts, delete-marking operations, and purges.
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none
Do not buffer any operations.
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inserts
Buffer insert operations.
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deletes
Buffer delete-marking operations.
-
changes (其实update操作就是做的 inserts and delete-marking)
Buffer both inserts and delete-marking operations.
-
purges
Buffer physical deletion operations that happen in the background.
innodb_change_buffer_max_size:change buffer是buffer pool中一块区域，5.6版本后默认大小是buffer pool的25%，参数 innodb_change_buffer_max_size 控制。
注：在内存中， change buffer占用buffer pool的一部分。在磁盘上， change buffer是system tablespace的一部分，当数据库服务器关闭时，索引更改将在其中缓冲。
使用监控
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mysql> SHOW ENGINE INNODB STATUSG-------------------------------------INSERT BUFFER AND ADAPTIVE HASH INDEX-------------------------------------Ibuf: size 1, free list len 0, seg size 2, 0 merges# size ：change buffer中使用的页数。change buffer大小等于seg size -（1+ free list len）#1+表示change buffer header page使用seg size ：更改缓冲区的大小，以页为单位。merged operations: insert 0, delete mark 0, delete 0discarded operations: insert 0, delete mark 0, delete 0Hash table size 4425293, used cells 32, node heap has 1 buffer(s)13577.57 hash searches/s, 202.47 non-hash searches/s |
change buffer 占用率
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mysql> SELECT ( SELECT COUNT (*) FROM INFORMATION_SCHEMA.INNODB_BUFFER_PAGE WHERE PAGE_TYPE LIKE 'IBUF%' ) AS change_buffer_pages, ( SELECT COUNT (*) FROM INFORMATION_SCHEMA.INNODB_BUFFER_PAGE) AS total_pages, ( SELECT ((change_buffer_pages/total_pages)*100)) AS change_buffer_page_percentage;+ ---------------------+-------------+-------------------------------+| change_buffer_pages | total_pages | change_buffer_page_percentage |+ ---------------------+-------------+-------------------------------+| 25 | 8192 | 0.3052 |+ ---------------------+-------------+-------------------------------+ |
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