理解innodb buffer pool

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理解innodb buffer pool

漫天花雨 2018-10-09 11:32:51 浏览1205
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前言

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.查看表的数据和索引使用情况?

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','test') 
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 |
+--------+--------+----------+---------+--------+------------+---------+

idxfrac这个值越低越好,举个例子,表里只有一个唯一索引的数据如下:

+--------+--------+----------+---------+--------+------------+---------+
| engine | TABLES | rows     | DATA    | idx    | total_size | idxfrac |
+--------+--------+----------+---------+--------+------------+---------+
| InnoDB |     16 | 3120.61M | 386.59G | 58.09G | 444.68G    |    0.15 |
+--------+--------+----------+---------+--------+------------+---------+

可见idxfrac可见这个值越低越好。

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:

148847_20161201164947334_1077294502

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/

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