Correct unclear description in feature-online-ddl

en/feature-online-ddl.md

@@ -9,7 +9,7 @@ This document introduces the `online-ddl` feature of DM when DM is used to migra
 
 ## Overview
 
-DDL statements are always used in the database applications. MySQL 5.6 and later versions support `online-ddl` feature, but there are limitations for usage. For example, to acquire the MDL lock, some DDLs still need to be copied. In production scenarios, the table lock during DDL execution can block the reads or writes to and from the database to a certain extent.
+DDL statements are always used in database applications. MySQL 5.6 and later versions support the `online-ddl` feature, but there are limitations. For example, DDL execution sometimes acquires an MDL lock, resulting in a table lock that can block reads or writes to and from a database to a certain extent in production scenarios. In other cases, some DDLs copy the whole table and this affects overall database performance.
 
 Therefore, online DDL tools are often used to execute DDLs to reduce the impact on reads and writes. Common DDL tools are [gh-ost](https://github.com/github/gh-ost) and [pt-osc](https://www.percona.com/doc/percona-toolkit/3.0/pt-online-schema-change.html).
 
@@ -37,7 +37,7 @@ Generally, it is recommended to enbale the `online-ddl` configuration and you ca
 > **Note:**
 >
 > If you need to disable the `online-ddl` configuration, pay attention to the following effects:
-> 
+>
 > - The downstream TiDB replicates the behaviors of online DDL tools like gh-ost/pt-osc;
 > - You need to manually add various temporary tables and ghost tables generated by the online DDL tools to the task configuration white list;
 > - You cannot merge and migrate data from sharded tables.

zh/feature-online-ddl.md

@@ -8,7 +8,7 @@ title: 迁移使用 GH-ost/PT-osc 的源数据库
 
 ## 概述
 
-DDL 是数据库应用中必然会使用的一类 SQL。MySQL 虽然在 5.6 的版本以后支持了 online-ddl 功能，但是也有或多或少的限制。比如 MDL 锁的获取，某些 DDL 还是需要以 Copy 的方式来进行，在生产业务使用中，DDL 执行过程中的锁表会一定程度上阻塞数据库的读取或者写入。
+DDL 是数据库应用中必然会使用的一类 SQL。MySQL 虽然在 5.6 的版本以后支持了 online-ddl 功能，但是也有或多或少的限制。比如 MDL 锁的获取，某些 DDL 还是需要以 Copy 表的方式来进行，在生产业务使用中，DDL 执行过程中的锁表会一定程度上阻塞数据库的读取或者写入。
 
 因此，用户往往会选择 online DDL 工具执行 DDL，把对读写的影响降到最低。常见的 Online DDL 工具有 [gh-ost](https://github.com/github/gh-ost) 和 [pt-osc](https://www.percona.com/doc/percona-toolkit/3.0/pt-online-schema-change.html)。