Change Buffer【Mysql InnoDB系列】

什麼是Change Buffer？

Change Buffer是一種用於緩存二級索引頁變化的特殊數據結構，是緩衝池中一塊獨立的區域。當需要修改的二級索引頁不在緩衝池中而在磁盤中時，會將這些索引頁的變化緩存在change buffer中。

爲什麼二級索引需要Change Buffer？

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• 減少隨機讀取IO

  對於聚集索引頁，DML操作帶來的數據磁盤讀取和緩存修改往往是順序的、集中的，這就很好地利用了聚集索引的優點，但隨着聚集索引頁的順序IO，各個二級索引頁的讀取和修改通常是隨機IO的。而Change buffer將分散的、未緩存的二級索引頁的變化進行緩存而非直接從磁盤中讀取索引頁，待到其他操作需要訪問二級索引時，只需將change buffer中的變化緩存合併然後加入緩衝池就行了，這樣就大大地減少了磁盤隨機讀取的IO。

• 定期批量寫入索引變化

  爲了避免change buffer使用過多，二級索引頁的變化也必須定期地寫入磁盤，這個操作叫做purge，相較於立即將索引頁變化寫入磁盤而言，這種批量的寫入自然是更加高效。InnoDB主線程會在服務器空閒時進行purge操作

  但需要注意的是，當需要更新的二級索引頁過多時，change buffer的purge操作可能需要花費幾個小時，期間，磁盤IO會增加，可能會嚴重影響磁盤IO密集的查詢。合理配置change buffer可避免這種情況的發生。

如何配置change buffer？

可以用innodb_change_buffering系統變量控制change buffer相關的行爲。可以控制change buffer只對部分類操作生效。該變量的值和含義如下：

• all

  默認值，表示緩存所有DML語句和物理purge操作的索引頁變化。

• none

  不對任何操作緩存，相當於禁用

• inserts

  只緩存insert

• deletes

  只緩存標記性delete操作

• changes

  只緩存寫入和標記性delete

• purges

  只緩存後臺物理刪除操作

注意：
innodb_change_buffering參數可動態修改，只對修改後發生的操作生效，修改前已緩存的索引頁不受影響

可以用innodb_change_buffer_max_size變量控制change buffer區域的最大大小，表示佔緩衝池總大小的百分點數。默認爲25。最大可以設置爲50。

當DML語句量較多時，可以適當擴大innodb_change_buffer_max_size.反則反之。

如何監控Change buffer的使用？

• show engine innodb status命令可用於查看change buffer的使用情況，包含於INSERT BUFFER AND ADAPTIVE HASH INDEX部分：

-------------------------------------
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

Ibuf: size 1, free list len 0, seg size 2, 0 merges
size爲當前使用的change buffers頁數，seg size表示change buffer分配的總頁數。

• 也可以用下面的SQL查看change buffer更詳細的使用統計

select * from information_schema.innodb_metrics 
where subsystem like 'change_buffer'\G

*************************** 1. row ***************************
           NAME: ibuf_merges_insert
      SUBSYSTEM: change_buffer
          COUNT: 198469
      MAX_COUNT: 198469
      MIN_COUNT: NULL
      AVG_COUNT: 0.13473886448652062
    COUNT_RESET: 198469
MAX_COUNT_RESET: 198469
MIN_COUNT_RESET: NULL
AVG_COUNT_RESET: NULL
   TIME_ENABLED: 2019-07-30 16:04:56
  TIME_DISABLED: NULL
   TIME_ELAPSED: 1472990
     TIME_RESET: NULL
         STATUS: enabled
           TYPE: status_counter
        COMMENT: Number of inserted records merged by change buffering
*************************** 2. row ***************************
           NAME: ibuf_merges_delete_mark
      SUBSYSTEM: change_buffer
          COUNT: 35361
      MAX_COUNT: 35361
      MIN_COUNT: NULL
      AVG_COUNT: 0.02400627295500988
    COUNT_RESET: 35361
MAX_COUNT_RESET: 35361
MIN_COUNT_RESET: NULL
AVG_COUNT_RESET: NULL
   TIME_ENABLED: 2019-07-30 16:04:56
  TIME_DISABLED: NULL
   TIME_ELAPSED: 1472990
     TIME_RESET: NULL
         STATUS: enabled
           TYPE: status_counter
        COMMENT: Number of deleted records merged by change buffering
*************************** 3. row ***************************
           NAME: ibuf_merges_delete
      SUBSYSTEM: change_buffer
          COUNT: 875
      MAX_COUNT: 875
      MIN_COUNT: NULL
      AVG_COUNT: 0.0005940298304808587
    COUNT_RESET: 875
MAX_COUNT_RESET: 875
MIN_COUNT_RESET: NULL
AVG_COUNT_RESET: NULL
   TIME_ENABLED: 2019-07-30 16:04:56
  TIME_DISABLED: NULL
   TIME_ELAPSED: 1472990
     TIME_RESET: NULL
         STATUS: enabled
           TYPE: status_counter
        COMMENT: Number of purge records merged by change buffering
*************************** 4. row ***************************
           NAME: ibuf_merges_discard_insert
      SUBSYSTEM: change_buffer
          COUNT: 0
      MAX_COUNT: 0
      MIN_COUNT: NULL
      AVG_COUNT: 0
    COUNT_RESET: 0
MAX_COUNT_RESET: 0
MIN_COUNT_RESET: NULL
AVG_COUNT_RESET: NULL
   TIME_ENABLED: 2019-07-30 16:04:56
  TIME_DISABLED: NULL
   TIME_ELAPSED: 1472990
     TIME_RESET: NULL
         STATUS: enabled
           TYPE: status_counter
        COMMENT: Number of insert merged operations discarded
*************************** 5. row ***************************
           NAME: ibuf_merges_discard_delete_mark
      SUBSYSTEM: change_buffer
          COUNT: 0
      MAX_COUNT: 0
      MIN_COUNT: NULL
      AVG_COUNT: 0
    COUNT_RESET: 0
MAX_COUNT_RESET: 0
MIN_COUNT_RESET: NULL
AVG_COUNT_RESET: NULL
   TIME_ENABLED: 2019-07-30 16:04:56
  TIME_DISABLED: NULL
   TIME_ELAPSED: 1472990
     TIME_RESET: NULL
         STATUS: enabled
           TYPE: status_counter
        COMMENT: Number of deleted merged operations discarded
*************************** 6. row ***************************
           NAME: ibuf_merges_discard_delete
      SUBSYSTEM: change_buffer
          COUNT: 0
      MAX_COUNT: 0
      MIN_COUNT: NULL
      AVG_COUNT: 0
    COUNT_RESET: 0
MAX_COUNT_RESET: 0
MIN_COUNT_RESET: NULL
AVG_COUNT_RESET: NULL
   TIME_ENABLED: 2019-07-30 16:04:56
  TIME_DISABLED: NULL
   TIME_ELAPSED: 1472990
     TIME_RESET: NULL
         STATUS: enabled
           TYPE: status_counter
        COMMENT: Number of purge merged  operations discarded
*************************** 7. row ***************************
           NAME: ibuf_merges
      SUBSYSTEM: change_buffer
          COUNT: 178009
      MAX_COUNT: 178009
      MIN_COUNT: NULL
      AVG_COUNT: 0.12084874982179106
    COUNT_RESET: 178009
MAX_COUNT_RESET: 178009
MIN_COUNT_RESET: NULL
AVG_COUNT_RESET: NULL
   TIME_ENABLED: 2019-07-30 16:04:56
  TIME_DISABLED: NULL
   TIME_ELAPSED: 1472990
     TIME_RESET: NULL
         STATUS: enabled
           TYPE: status_counter
        COMMENT: Number of change buffer merges
*************************** 8. row ***************************
           NAME: ibuf_size
      SUBSYSTEM: change_buffer
          COUNT: 1
      MAX_COUNT: 1
      MIN_COUNT: NULL
      AVG_COUNT: 0.0000006788912348352671
    COUNT_RESET: 1
MAX_COUNT_RESET: 1
MIN_COUNT_RESET: NULL
AVG_COUNT_RESET: NULL
   TIME_ENABLED: 2019-07-30 16:04:56
  TIME_DISABLED: NULL
   TIME_ELAPSED: 1472990
     TIME_RESET: NULL
         STATUS: enabled
           TYPE: status_counter
        COMMENT: Change buffer size in pages
8 rows in set (0.01 sec)

• information_schema.innodb_buffer_page系統表中存放的是每個緩衝池中的頁的元數據，也可以用於統計change buffer的使用情況：

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 |
+---------------------+-------------+-------------------------------+