InnoDB change buffer
InnoDB change buffer 一种重要的数据变更日志
Change buffer的主要目的是将对二级索引的数据操作缓存下来，以此减少二级索引的随机IO，并达到操作合并的效果。
在MySQL5.5之前的版本中，由于只支持缓存insert操作，所以最初叫做insert buffer，只是后来的版本中支持了更多的操作类型缓存，才改叫change buffer。
当更新/插入的非聚集索引的数据所对应的页不在内存中时（对非聚集索引的更新操作通常会带来随机IO），会将其放到一个insert buffer中，当随后页面被读到内存中时，会将这些变化的记录merge到页中。当服务器比较空闲时，后台线程也会做merge操作
但change buffer会占用buffer pool，并且在非聚集索引很少时，并不总是必要的，反而会降低buffer pool做data cache的能力。
The INFORMATION_SCHEMA.INNODB_BUFFER_PAGE table provides metadata about each page in the buffer pool, including change buffer index and change buffer bitmap pages. Change buffer pages are identified by PAGE_TYPE. IBUF_INDEX is the page type for change buffer index pages, and IBUF_BITMAP is the page type for change buffer bitmap pages.
For example, you can query the INNODB_BUFFER_PAGE table to determine the approximate number of IBUF_INDEX and IBUF_BITMAP pages as a percentage of total buffer pool pages.
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 |
+---------------------+-------------+-------------------------------+
没有对比，不知道这个结果的性能好坏。
