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 ofMySQL 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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