原文地址：http://www.cnblogs.com/billyxp/p/3342969.html

对于越来越多的数据，数据库的容量越来越大，压缩也就越来越常见了。在我的实际工作中进行过多次压缩工作，也遇到多次问题，在此和大家分享一下。

首先，我们先说说怎么使用innodb的压缩.

第一，mysql的版本需要大于5.5第二，设置innodb_file_format=barracuda
第三，create table或者alter talble 增加 ROW_FORMAT=COMPRESSED KEY_BLOCK_SIZE=8;(默认的key_block_size=16)

其实很简单，根据经验，一般压缩比例可以达到30%-40%

然后，我们说说我在压缩过程中遇到的坑和发现的关联，当然有些比较二。

No1：

问题：使用脚本批量alter操作，只动态修改了实例的innodb_file_format=barracuda，然后alter所有数据库中的表。并没有修改配置文件中的设置。

结果：表中已有数据被压缩，但是在重启之后，由于innodb_file_format参数被重新修改成antelope，导致后续写入的数据没有被压缩（虽然表结构中有row_format=compressed，但是不会起作用），最终表体积仍然很大。

教训：实例和配置文件要同步修改。（这个错误最二，太低级 T_T，不解释了。）

No2:

问题：在innodb_file_format=antelope的情况下，建立压缩表（表结构中带有row_format=compressed），然后在设置innodb_file_format=barracuda。

结果：表结构中的row_format=compressed被忽略，后续写入表的数据并没有被压缩，最终导致表体积大。

教训：先修改innodb_file_format（session和global都需要修改），在create table或者alter table。

但是以上这点有个坑人的地方，在错误的顺序下，表是可以被成功建立了，只是会有warning，但是表结构中会有row_format=compressed，在后期排查的时候非常误导人！

+--------------------------+----------+| Variable_name            | Value    |+--------------------------+----------+| innodb_file_format       | Antelope || innodb_file_format_check | ON       || innodb_file_format_max   | Antelope |+--------------------------+----------+3 rows in set (0.00 sec)

test> create table test_1 (x int) ROW_FORMAT=COMPRESSED KEY_BLOCK_SIZE=8; 
Query OK, 0 rows affected, 4 warnings (0.07 sec)

test> show warnings; 
+---------+------+-----------------------------------------------------------------------+ 
| Level | Code | Message | 
+---------+------+-----------------------------------------------------------------------+ 
| Warning | 1478 | InnoDB: KEY_BLOCK_SIZE requires innodb_file_format > Antelope . | 
| Warning | 1478 | InnoDB: ignoring KEY_BLOCK_SIZE=8 . | 
| Warning | 1478 | InnoDB: ROW_FORMAT=COMPRESSED requires innodb_file_format > Antelope . | 
| Warning | 1478 | InnoDB: assuming ROW_FORMAT=COMPACT . | 
+---------+------+-----------------------------------------------------------------------+ 
4 rows in set (0.00 sec)

我们可以从warnings中看见，压缩设置被忽略了。但是最坑爹的一点是，如果我们show create table会有如下结果：

test_1 | CREATE TABLE `test_1` (  `x` int(11) DEFAULT NULL) ENGINE=InnoDB DEFAULT CHARSET=utf8 ROW_FORMAT=COMPRESSED KEY_BLOCK_SIZE=8

在这种情况下，我们吸取教训，不能使用show create table看压缩状态，而是应该用show table status;

show table status like 'test_1'\G;*************************** 1. row ***************************Name: test_1Engine: InnoDBVersion: 10Row_format: CompactRows: 0Avg_row_length: 0Data_length: 16384Max_data_length: 0Index_length: 0Data_free: 0Auto_increment: NULLCreate_time: 2013-09-27 15:59:13Update_time: NULLCheck_time: NULLCollation: utf8_general_ciChecksum: NULLCreate_options: row_format=COMPRESSED KEY_BLOCK_SIZE=8Comment:1 row in set (0.00 sec)

坑爹啊，不说了。正常应该这个样子

show table status like 'test_2'\G;*************************** 1. row ***************************Name: test_2Engine: InnoDBVersion: 10Row_format: CompressedRows: 0Avg_row_length: 0Data_length: 8192Max_data_length: 0Index_length: 0Data_free: 0Auto_increment: NULLCreate_time: 2013-09-27 16:09:51Update_time: NULLCheck_time: NULLCollation: utf8_general_ciChecksum: NULLCreate_options: row_format=COMPRESSED KEY_BLOCK_SIZE=8Comment:1 row in set (0.00 sec)

No3:

发现和innodb_file_format相关的2个参数:

+--------------------------+-----------+| Variable_name            | Value     |+--------------------------+-----------+| innodb_file_format       | Barracuda || innodb_file_format_check | ON        || innodb_file_format_max   | Barracuda |+--------------------------+-----------+3 rows in set (0.00 sec)

官方的解释可以参考如下的链接：http://dev.mysql.com/doc/refman/5.5/en/innodb-parameters.html#sysvar_innodb_file_format

测试过程中发现，如果是innodb_file_format=barracuda而innodb_file_format_max=antelop，那么在建立压缩表的时候，max会自动变成barracuda。

localhost.test>show global variables like 'innodb_file_format%';+--------------------------+-----------+| Variable_name            | Value     |+--------------------------+-----------+| innodb_file_format       | Barracuda || innodb_file_format_check | ON        || innodb_file_format_max   | Antelope  |+--------------------------+-----------+3 rows in set (0.00 sec)localhost.test>create table test_4(x int) ROW_FORMAT=COMPRESSED KEY_BLOCK_SIZE=8;Query OK, 0 rows affected (0.01 sec)localhost.test>show global variables like 'innodb_file_format%';+--------------------------+-----------+| Variable_name            | Value     |+--------------------------+-----------+| innodb_file_format       | Barracuda || innodb_file_format_check | ON        || innodb_file_format_max   | Barracuda |+--------------------------+-----------+3 rows in set (0.00 sec)

如果innodb_file_format_check这参数解释的，决定innodb是否会检查共享表空间中的表格式的tag，如果检查开启，那么当标记的表格式的tag高于innodb可以支撑的表格式，那么innodb会报错，并停止启动。如果支持，那么会将innodb_file_format_max的值改为这个tag的值。

以下是参考材料

• innodb_file_format

• The file format to use for new InnoDB tables. Currently, Antelope andBarracuda are supported. This applies only for tables that have their own tablespace, so for it to have an effect, innodb_file_per_table must be enabled. The Barracuda file format is required for certain InnoDB features such as table compression.

• innodb_file_format_check

• As of MySQL 5.5.5, this variable can be set to 1 or 0 at server startup to enable or disable whether InnoDB checks the file format tag in the shared tablespace (for example, Antelope or Barracuda ). If the tag is checked and is higher than that supported by the current version of InnoDB , an error occurs and InnoDBdoes not start. If the tag is not higher, InnoDB sets the value ofinnodb_file_format_max to the file format tag. 
  Before MySQL 5.5.5, this variable can be set to 1 or 0 at server startup to enable or disable whether InnoDB checks the file format tag in the shared tablespace. If the tag is checked and is higher than that supported by the current version ofInnoDB , an error occurs and InnoDB does not start. If the tag is not higher,InnoDB sets the value of innodb_file_format_check to the file format tag, which is the value seen at runtime. 
  Note 
  Despite the default value sometimes being displayed as ON or OFF , always use the numeric values 1 or 0 to turn this option on or off in your configuration file or command line.

• innodb_file_format_max

• At server startup, InnoDB sets the value of innodb_file_format_max to the file format tag in the shared tablespace (for example, Antelope or Barracuda ). If the server creates or opens a table with a “higher” file format, it sets the value ofinnodb_file_format_max to that format. 
  This variable was added in MySQL 5.5.5.