change buffer是一種特殊的數據結構，當要修改的輔助索引頁不在buffer pool中時，用來cache對輔助索引頁的修改。對輔助索引頁的操作可能是insert、update和delete操作。等到相關的索引頁被讀入buffer pool中后，才會使用change buffer中的內容對輔助索引頁進行修改(即merge操作)。
和聚集索引不同，輔助索引通常是不唯一的，插入輔助索引通常也是隨機的。同樣，對輔助索引的刪除、更新也通常是不連續的。
等到相關的索引頁被讀入buffer pool中后，才會使用change buffer中的內容對輔助索引頁進行修改(即merge操作)可以避免大量的磁盤隨機訪問I/O。
間歇性的，在系統空閑或關閉過程中，會執行purge操作，將新的索引頁寫入磁盤。purge操作一次寫多個索引值會比每次修改后就立即寫入磁盤的效率高。
對change buffer的merge可能需要好幾個小時，如果被更新的輔助索引行比較多。在merge過程中，磁盤的I/O會增加，可能會引起其他查詢的性能的降低。
merge操作也可能發生在事務提交后。事實上，即使在實例重啟后，還會可能發生merge操作。
在內存中，change buffer會占用buffer pool的空間；在物理磁盤上，change buffer是system tablespace的一部分，所以對索引的修改在數據庫重啟后仍然存在change buffer中。
change buffer包含的特性也叫作change buffering，包含insert buffering、delete buffering、purge buffering。
change buffer中數據類型和總量由參數innodb_change_buffering和innodb_chagne_buffer_max_size配置。查看change buffer中數據的信息，可以通過show engine innodb status查看。
change buffer在老版本中被稱作insert buffer。
1.監控change buffer
mysql> show engine innodb status\G ... ------------------------------------- INSERT BUFFER AND ADAPTIVE HASH INDEX ------------------------------------- Ibuf: size 1, free list len 0, seg size 2, 0 merges merged operations: insert 0, delete mark 0, delete 0 discarded operations: insert 0, delete mark 0, delete 0 Hash table size 4425293, used cells 32, node heap has 1 buffer(s) 13577.57 hash searches/s, 202.47 non-hash searches/s ...
2.查看information_schema
information_schema.innodb_metrics提供了change buffer的統計信息名稱和說明：
mysql> select name, comment from information_schema.innodb_metrics where name like '%ibuf%'; +-----------------------------------------+-------------------------------------------------------------+ | name | comment | +-----------------------------------------+-------------------------------------------------------------+ | buffer_page_read_index_ibuf_leaf | Number of Insert Buffer Index Leaf Pages read | | buffer_page_read_index_ibuf_non_leaf | Number of Insert Buffer Index Non-Leaf Pages read | | buffer_page_read_ibuf_free_list | Number of Insert Buffer Free List Pages read | | buffer_page_read_ibuf_bitmap | Number of Insert Buffer Bitmap Pages read | | buffer_page_written_index_ibuf_leaf | Number of Insert Buffer Index Leaf Pages written | | buffer_page_written_index_ibuf_non_leaf | Number of Insert Buffer Index Non-Leaf Pages written | | buffer_page_written_ibuf_free_list | Number of Insert Buffer Free List Pages written | | buffer_page_written_ibuf_bitmap | Number of Insert Buffer Bitmap Pages written | | ibuf_merges_insert | Number of inserted records merged by change buffering | | ibuf_merges_delete_mark | Number of deleted records merged by change buffering | | ibuf_merges_delete | Number of purge records merged by change buffering | | ibuf_merges_discard_insert | Number of insert merged operations discarded | | ibuf_merges_discard_delete_mark | Number of deleted merged operations discarded | | ibuf_merges_discard_delete | Number of purge merged operations discarded | | ibuf_merges | Number of change buffer merges | | ibuf_size | Change buffer size in pages | | innodb_ibuf_merge_usec | Time (in microseconds) spent to process change buffer merge | +-----------------------------------------+-------------------------------------------------------------+ 17 rows in set (0.00 sec) mysql>
information_schema.innodb_buffer_page提供了buffer pool中每個頁的元數據，包含change buffer 索引頁和change buffer位圖頁。
change buffer頁中page_type.ibuf_index表示change buffer索引頁；page_type.ibuf_bitmap表示change buffer位圖頁。
提醒：查看innodb_buffer_page會有很大的性能開銷。最好是在空閑時間或測試環境執行。
比如，可以查看change buffer的索引頁和位圖頁占據buffer pool的比例：
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 |
+---------------------+-------------+-------------------------------+
| 14 | 8192 | 0.1709 |
+---------------------+-------------+-------------------------------+
1 row in set (0.05 sec)
performance_schema還為高級性能監控提供了change buffer mutex的等待指令：
mysql> SELECT * FROM performance_schema.setup_instruments
-> WHERE NAME LIKE '%wait/synch/mutex/innodb/ibuf%';
+-------------------------------------------------------+---------+-------+
| NAME | ENABLED | TIMED |
+-------------------------------------------------------+---------+-------+
| wait/synch/mutex/innodb/ibuf_bitmap_mutex | NO | NO |
| wait/synch/mutex/innodb/ibuf_mutex | NO | NO |
| wait/synch/mutex/innodb/ibuf_pessimistic_insert_mutex | NO | NO |
+-------------------------------------------------------+---------+-------+
3 rows in set (0.00 sec)
3.配置change buffer
可以通過參數innodb_change_buffering來控制是否啟用change buffer。
--all： 默認值。開啟buffer inserts、delete-marking operations、purges --none： 不開啟change buffer --inserts： 只是開啟buffer insert操作 --deletes: 只是開delete-marking操作 --changes： 開啟buffer insert操作和delete-marking操作 --purges： 對只是在后台執行的物理刪除操作開啟buffer功能
從5.6.2開始，參數innodb_change_buffer_max_size設置了change buffer可以占用buffer pool的百分比，默認是25，最大可以設置為50。
mysql> show variables like 'innodb_change_buffer_max_size'; +-------------------------------+-------+ | Variable_name | Value | +-------------------------------+-------+ | innodb_change_buffer_max_size | 25 | +-------------------------------+-------+ 1 row in set (0.00 sec) mysql>