mysql5.6和mysql5.7分配undo回滚段的区别
1、mysql5.7中分为2类：临时表空间回滚段和普通回滚段。
2、mysql5.6中没有区分。
As of
MySQL 5.7.2, 32 undo logs are reserved for use by temporary tables and are hosted in the temporary table tablespace (ibtmp1).
To allocate additional undo logs for data-modifying transactions that generate undo records,innodb_undo_logs must
be set to a value greater than 32 if undo logs are stored in the system tablespace only. If you have configured separate undo tablespaces, innodb_undo_logs must
be set to a value greater than 33 to allocate additional undo logs for data-modifying transactions. Each undo log can host up to a maximum of 1024 transactions.
如果undo log存储在系统表空间，innodb_undo_logs值必须大于32
否则独立存储的话必须大于3。
参考：
http://www.tuicool.com/articles/IzQNRrz
http://dev.mysql.com/doc/refman/5.7/en/innodb-parameters.html#sysvar_innodb_undo_logs
http://dev.mysql.com/doc/refman/5.6/en/innodb-parameters.html#sysvar_innodb_undo_logs
http://bbs.aliyun.com/read/244045.html?spm=5176.7189909.0.0.AB00as
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原文：http://blog.csdn.net/yanzongshuai/article/details/48141319