回收InnoDB表空间_MySQL

回收InnoDB表空间

以下论述均假定innodb_file_per_table开启

先用常规optimize回收：

[plain]

mysql> select count(*) from t; 
+----------+ 
| count(*) | 
+----------+ 
| 1200096 | 
+----------+ 
[mysql@even employees]$ ls -alh t.ibd 
-rw-rw---- 1 mysql dba 72M 10-08 17:39 t.ibd 
mysql> delete from t limit 800000; 
mysql> select count(*) from t; 
+----------+ 
| count(*) | 
+----------+ 
| 400096 | 
+----------+ 
[mysql@even employees]$ ls -alh t.ibd 
-rw-rw---- 1 mysql dba 72M 10-08 17:41 t.ibd 
mysql> optimize table t; 
+-------------+----------+----------+-------------------------------------------------------------------+ 
| Table | Op | Msg_type | Msg_text | 
+-------------+----------+----------+-------------------------------------------------------------------+ 
| employees.t | optimize | note | Table does not support optimize, doing recreate + analyze instead | 
| employees.t | optimize | status | OK | 
+-------------+----------+----------+-------------------------------------------------------------------+ 
[mysql@even employees]$ ls -alh t.ibd 
-rw-rw---- 1 mysql dba 29M 10-08 17:43 t.ibd

来看下这种方法的缺陷

[plain]

mysql> show processlist;

+----+------+-----------+-----------+---------+------+-------------------+------------------+

| Id | User | Host | db | Command | Time | State | Info |

+----+------+-----------+-----------+---------+------+-------------------+------------------+

| 5 | root | localhost | NULL | Query | 0 | NULL | show processlist |

| 6 | root | localhost | employees | Query | 64 | copy to tmp table | optimize table t |

+----+------+-----------+-----------+---------+------+-------------------+------------------+

mysqladmin debug结果如下

[plain]

Thread database.table_name Locked/Waiting Lock_type

6 employees.t Locked - read Read lock without concurrent inserts

用optimize缺点显而易见：读锁，特别当你有大表时性能恐怕会深受影响

这里推荐使用percona公司：pt-online-schema-change，避免加锁

[plain]

$ pt-online-schema-change -uroot -poracle --alter "ENGINE=InnoDB" D=employees,t=t --execute

执行pt工具时有可能发生的类似错误：

[plain]

Cannot chunk the original table There is no good index and the table is oversized

这是因为被作用的表需要含有主键或者唯一索引，这或许也能成为这款工具的小bug吧

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