1、mysql5.7中分為2類：臨時表空間回滾段和普通回滾段。
2、mysql5.6中沒有區分。
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存儲在系統表空間，innodb_undo_logs值必須大於32
否則獨立存儲的話必須大於3。
