理解innodb buffer pool

         今天组里有个同事说可以查看innodb buffer pool每个表和索引占的大小，为此我搜了下，还真有方法，记录下。

innodb buffer pool有几个目的：

• 缓存数据--众所周知，这个占了buffer pool的大半空间
• 缓存目录--数据字典
• insert buffer
• 排序的内部结构--比如自适应hash的结构或者一些行锁

1.buffer pool是怎样分配空间的?

SELECT engine,
  count(*) as TABLES,
  concat(round(sum(table_rows)/1000000,2),'M') rows,
  concat(round(sum(data_length)/(1024*1024*1024),2),'G') DATA,
  concat(round(sum(index_length)/(1024*1024*1024),2),'G') idx,
  concat(round(sum(data_length+index_length)/(1024*1024*1024),2),'G') total_size,
  round(sum(index_length)/sum(data_length),2) idxfrac
FROM information_schema.TABLES
WHERE table_schema not in ('mysql', 'performance_schema', 'information_schema')
GROUP BY engine
ORDER BY sum(data_length+index_length) DESC LIMIT 10;

得到的结果:

+--------+--------+----------+---------+--------+------------+---------+
| engine | TABLES | rows     | DATA    | idx    | total_size | idxfrac |
+--------+--------+----------+---------+--------+------------+---------+
| InnoDB |  71608 | 1644.51M | 130.79G | 82.76G | 213.55G    |    0.63 |
+--------+--------+----------+---------+--------+------------+---------+

2.获取buffer pool占的page个数：

select count(*) from information_schema.innodb_buffer_page;

结果：

+----------+
| count(*) |
+----------+
| 262142   |
+----------+

聪明的同学自己算下使用的buffer pool是多大吧。

3.获取page类型:

select
page_type as Page_Type,
sum(data_size)/1024/1024 as Size_in_MB
from information_schema.innodb_buffer_page
group by page_type
order by Size_in_MB desc;

结果：

+-------------------+--------------+
| Page_Type         | Size_in_MB   |
+-------------------+--------------+
| INDEX             | 158.66378689 |
| UNKNOWN           | 0.00000000   |
| TRX_SYSTEM        | 0.00000000   |
| SYSTEM            | 0.00000000   |
| FILE_SPACE_HEADER | 0.00000000   |
| IBUF_BITMAP       | 0.00000000   |
| EXTENT_DESCRIPTOR | 0.00000000   |
| ALLOCATED         | 0.00000000   |
| INODE             | 0.00000000   |
| BLOB              | 0.00000000   |
| UNDO_LOG          | 0.00000000   |
| IBUF_FREE_LIST    | 0.00000000   |
| IBUF_INDEX        | 0.00000000   |
+-------------------+--------------+

从这里可以看到数据和索引占了buffer pool的大部分空间。也可以看出来这里有几种重要的页类型：

• INDEX: B-Tree index
• IBUF_INDEX: Insert buffer index
• UNKNOWN: not allocated / unknown state
• TRX_SYSTEM: transaction system data

眼亮的同学可能会问，你上面不是说会缓存数据吗？怎么这里出来只有INDEX类型占多半buffer pool?数据哪里去了？数据在INDEX里！！！数据在聚簇索引的叶子节点上。

4.buffer pool里每个索引的使用

select
table_name as Table_Name, index_name as Index_Name,
count(*) as Page_Count, sum(data_size)/1024/1024 as Size_in_MB
from information_schema.innodb_buffer_page
group by table_name, index_name
order by Size_in_MB desc;

结果：

+--------------------------------------------+-----------------+------------+-------------+
| Table_Name                                 | Index_Name      | Page_Count | Size_in_MB  |
+--------------------------------------------+-----------------+------------+-------------+
| `magento`.`core_url_rewrite`               | PRIMARY         |       2829 | 40.64266014 |
| `magento`.`core_url_rewrite`               | FK_CORE_URL_... |        680 |  6.67517281 |
| `magento`.`catalog_product_entity_varchar` | PRIMARY         |        449 |  6.41064930 |
| `magento`.`catalog_product_index_price`    | PRIMARY         |        440 |  6.29357910 |
| `magento`.`catalog_product_entity`         | PRIMARY         |        435 |  6.23898315 |
+--------------------------------------------+-----------------+------------+-------------+

5.一个典型的buffer pool使用监控：

从这里图里我们可以看到buffer pool几乎是被填满的，另外预留了10%的空间用来做其他用途。

6.一般怎么设置buffer pool大小呢？

warm rows data size + warm indexes size (excl. clustered) + 20%

7.如何预热buffer pool？

在InnoDB上面执行select语句：

• 对于聚簇索引来说，大多数情况通过SELECT COUNT(*) 加载到buffer pool中了。
• 对于二级索引来说，要执行一些简单的语句来抓取全部数据，比如select * from tbname where 索引的第一列。或者select * from tbname force index(二级索引) where colname <>0.

另外，MySQL5.7支持动态修改buffer pool:

mysql> SET GLOBAL innodb_buffer_pool_size=size_in_bytes;

8.Dump & restore

在MySQL (5.6+), Percona Server (5.5.10+) or MariaDB (10.0+)可以通过以下配置把buffer pool里面的数据dump出来，并在启动的时候加载到内存中：

• innodb_buffer_pool_dump_at_shutdown=ON
• innodb_buffer_pool_load_at_startup=ON

参考资料:

https://michael.bouvy.net/blog/en/2015/01/18/understanding-mysql-innodb-buffer-pool-size/ 

http://www.speedemy.com/mysql/17-key-mysql-config-file-settings/innodb_buffer_pool_size/