MySQLump 参数--Lock-Tables浅析

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mysqldump有一个参数--lock-tables，以前对这个参数也没有深入、详细了解过，直到上次有个网友问“参数lock-tables 是一次性锁定当前库的所有表，还是锁定当前导出表?“ ，之前一直以为只是锁定当前导出表，后面看了参数说明后，

-l, --lock-tables   Lock all tables for read. 
 
                      (Defaults to on; use --skip-lock-tables to disable.) 

自己也不怎么确认了，当时就测试了一下。准备一个稍微大一点的库，如果数据库太小，那么可能mysqldum命令一下子就导出了所有库，很难清晰的看到实验结果。

执行下面命令做逻辑备份

[root@DB-Server ~]# mysqldump -u root -p --default-character-set=utf8  --opt --extended-insert=false --lock-tables MyDB > db_backup_MyDB.sql 
 
Enter password: 

同时立即执行下面命令

mysql> show open tables where in_use >0; 
+----------+--------------------------------+--------+-------------+ 
| Database | Table                          | In_use | Name_locked | 
+----------+--------------------------------+--------+-------------+ 
| MyDB     | AO_60DB71_VERSION              |      1 |           0 | 
| MyDB     | AO_AEFED0_TEAM_TO_MEMBER       |      1 |           0 | 
| MyDB     | AO_4B00E6_STASH_SETTINGS       |      1 |           0 | 
| MyDB     | AO_2D3BEA_FOLIOCF              |      1 |           0 | 
| MyDB     | AO_AEFED0_TEAM_ROLE            |      1 |           0 | 
| MyDB     | AO_60DB71_DETAILVIEWFIELD      |      1 |           0 | 
| MyDB     | AO_60DB71_LEXORANK             |      1 |           0 | 
| MyDB     | AO_6714C7_REPORT_SCHEDULE      |      1 |           0 | 
| MyDB     | AO_E8B6CC_SYNC_AUDIT_LOG       |      1 |           0 | 
| MyDB     | cwd_application                |      1 |           0 | 
| MyDB     | clusternode                    |      1 |           0 | 
| MyDB     | AO_86ED1B_GRACE_PERIOD         |      1 |           0 | 
| MyDB     | AO_60DB71_WORKINGDAYS          |      1 |           0 | 
| MyDB     | cwd_directory                  |      1 |           0 | 
| MyDB     | AO_2D3BEA_BASELINE             |      1 |           0 | 
| MyDB     | fieldlayoutitem                |      1 |           0 | 
| MyDB     | JQUARTZ_BLOB_TRIGGERS          |      1 |           0 | 
| MyDB     | AO_013613_HD_SCHEME_MEMBER     |      1 |           0 | 
| MyDB     | AO_2D3BEA_ALLOCATION           |      1 |           0 | 
| MyDB     | AO_013613_WL_SCHEME            |      1 |           0 | 
| MyDB     | AO_7DEABF_EXEC_CLUSTER_MESSAGE |      1 |           0 | 
.................................................................... 

如上截图，执行mysqldump命令的时候，使用show open tables where in_use >0命令，你会看到MyDB里面的所有表的In_use的值都为1，意味着是当执行mysqldump命令时，是一次性锁定当前库的所有表。而不是锁定当前导出表。

In_use说明

• The number of table locks or lock requests there are for the table. For example, if one client acquires a lock for a table using LOCK TABLE t1 WRITE, In_use will be 1. If another client issues LOCK TABLE t1 WRITE while the table remains locked, the client will block waiting for the lock, but the lock request causes In_use to be 2. If the count is zero, the table is open but not currently being used. In_use is also increased by the HANDLER ... OPEN statement and decreased by HANDLER ... CLOSE.

参考资料：

https://dev.mysql.com/doc/refman/5.7/en/show-open-tables.html

https://oracle-base.com/articles/mysql/mysql-identify-locked-tables