MySQL可用的在线备份工具有三个，一个是官方的clone plugin， 一个是percona的xtrabackup，这两个是物理备份， 官方的clone plugin 是MySQL 8 版本才提供的，第三个也是官方的，就是逻辑备份工具mysqldump。
这三个工具虽然可以进行在线备份，但这种在线备份是有限制的，严格的说不能称为在线备份，因为它们都需要阻塞某些DDL操作。
clone plugin在进行数据库克隆时要获得源和目标实例的backup lock，这个锁阻塞某些DDL操作，下面是MySQL参考手册：
LOCK INSTANCE FOR BACKUP prevents files from being created, renamed, or removed.
REPAIRTABLE TRUNCATE TABLE, OPTIMIZE TABLE, and account management statements are blocked. Operations that modify InnoDB files that are notrecorded in the InnoDB redo log are also blocked.
xtrabackup 在进行数据库备份时，也需要获取备份表的DDL锁以保持数据的一致性，这个锁在8.0.24版本采用的是MySQL官方的backup锁，在5.7 版本及8.0.24之间的版本有两个选项：一个是lock-ddl，这个选项阻塞所有的DDL，另一个是-lock-ddl-per-table，这个在每一个要备份的的文件加ddl锁至备份结束。使用lock_ddl_per_table，备份期间可以对表进行创建、删除（未加锁前）等操做，这些操作的一致性要靠redo日志来保证。
mysqldump如要在线备份需要使用--single-transaction（innodb存储引擎）选项，这个选项官方说明如下：
This option sets the transaction isolation mode to REPEATABLE READ and sends a STARTTRANSACTION SQL statement to the server before dumping data. It is useful only with transactionaltables such as InnoDB, because then it dumps the consistent state of the database at the time whenSTART TRANSACTION was issued without blocking any applications.
While a --single-transaction dump is in process, to ensure a valid dump file (correct tablecontents and binary log coordinates), no other connection should use the following statements: ALTERTABLE, CREATE TABLE, DROP TABLE, RENAME TABLE, TRUNCATE TABLE. A consistent read is notisolated from those statements, so use of them on a table to be dumped can cause the SELECT that isperformed by mysqldump to retrieve the table contents to obtain incorrect contents or fail.
