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Undo log

基本概念​

An undo log is a collection of undo log records associated with a single read-write transaction. An undo log record contains information about how to undo the latest change by a transaction to a clustered index record

记录了一条事务里对数据的操作，包含了如何撤销每个操作的信息，并且给mvcc提供了可追溯的历史版本的视图

配置​

#定义了分配给每个undo表空间和生成undo记录的事务的全局临时表空间的回滚段的数量。每个回滚段支持的事务数量取决于 InnoDB 页面大小和分配给每个事务的撤消日志数量
innodb_rollback_segments = 128
#创建undo tablespaces的路径 
innodb_undo_directory =