InnoDB的undo log是其实现多版本的关键组件，在物理上以数据页的形式进行组织。在早期版本中(<5.6)，undo tablespace是在ibdata中，因此一个常见的问题是由于大事务不提交导致ibdata膨胀，这时候通常只有重建数据库一途来缩小空间。到了MySQL5.6版本， InnoDB开始支持独立的undo tablespace，也就是说，undo log可以存储于ibdata之外。但这个特性依然鸡肋：
到2. 了MySQL5.7版本中，终于引入了一个期待已久的功能：即在线truncate undo tablespace。DBA终于摆脱了undo空间膨胀的苦恼。
WL#10583: Stop using rollback segments in the system tablespace * Change the minimum value of innodb_undo_tablespaces to 2 * Fix code that allows and checks for innodb_undo_tablespaces=0 * Fix all testcases affected
WL#9507: Make innodb_undo_tablespaces variable dynamic WL#10498: InnoDB: Change Default for innodb_undo_tablespaces from 0 to 2 WL#10499: InnoDB: Change Default for innodb_undo_log_truncate from OFF to ON * Introduce innodb_undo_tablespace_update() and srv_undo_tablespaces_update() for online updates. * Introduce innodb_rollback_segments_update() for online updates. * Introduce srv_undo_tablespaces_uprade() to convert 5.7 undo tablespaces to 8.0.1. * Introduce srv_undo_tablespaces_downgrade() in case the upgrade from 5.7 fails. * Introduce trx_rseg_adjust_rollback_segments() and trx_rseg_add_rollback_segments() to consolidate the creation and use of rollback segments in any tablespace. * Introduce a new file format for undo tablespaces including a reserved range for undo space IDs and an RSEG_ARRAY page. * Rename auto-generated undo tablespace names from undonnn to undo-nnn * Expand the undo namespace to support new undo file format. * Various changes to allow online creation of undo spaces and rollback segments. * Change round robin routine for supplying rsegs to transactions to support rseg arrays in each tablespace. * Handle conversions between undo space_id and undo space number. * Introduce undo_settings.test * Adjust and improve a lot of testcases.
WL#10322: Deprecate innodb_undo_logs in 5.7 * Add (deprecated) to innodb-undo-logs description and mention that it is actually setting the number of rollback segments. * Delete “(deprecated)” from innodb-rollback-segments message. * Add a deprecation warning message when innodb_undo_logs is used at runtime and also at startup in a config or the command line. * Return a warning when innodb_undo_logs is used at runtime. * Rename srv_undo_logs to srv_rollback_segments in code * Rename innodb_undo_logs to innodb_rollback_segments in all collections and testcases except sysvars.innodb_undo_logs_basic. * Fix sysvars.innodb_undo_logs_basic to suppress the deprecation warning. Add a restart to exercise the deprecation code for using it at startup.
Bug #25572279 WARNING ALLOCATED TABLESPACE ID N FOR INNODB_UNDO00N OLD MAXIMUM WAS N This extra bootstrap warning was introduced in rb#13164: Bug#24462978: Free up 32 slots in TRX_SYS page for non-temp rsegs. It occurs when the database is initialized with undo tablespaces > rollback segments. In this case, only the first undo tablespaces associated with the limited rollback segments will be used. The remaining undo tablespaces cannot be used because the rollback segment slots are not available in the TRX_SYS page. But when starting the server, we have to open all unused undo tablespaces. Along with opening the unused undo tablespaces, the max_assigned_id needs to be incremented to avoid the warning. The rb#13164 patch was not doing that. The solution is simply to call to fil_set_max_space_id_if_bigger(space_id) in srv_undo_tablespaces_open() instead of the other three place it does now. This makes sure that the condition "id > fil_system->max_assigned_id" in fil_space_create() is false which will prevent the reported warning message.
Bug #25551311 BACKPORT BUG #23517560 REMOVE SPACE_ID RESTRICTION FOR UNDO TABLESPACES Description: ============ The restriction that required the first undo tablespace to use space_id 1 is removed. The first undo tablespace can now use a space_id other than 1. space_id values for undo tablespaces are still assigned in a consecutive sequence.
WL#9289 InnoDB: Support Transparent Data Encryption for Undo Tablespaces WL#9290 InnoDB: Support Transparent Data Encryption for Redo Log Based on wl#8548, we provide encryption support for redo log and undo tablespaces. For encrypting redo/undo log, as same as we did in wl#8548, we will en/decrypt the redo log blocks/undo log pages in the I/O layer. Which means, the en/decryption only happens when the redo/undo log read or write from/to disk. For redo log, encryption metadata will be stored in the header of first log file. Same as wl#8548, there're 2 key levels here, master key and tablespace key. Master key is stored in keyring plugin, and it's used to en/decrypt tablespace key and iv. Tablespace key is for en/decrypt redo log blocks, and it will be stored into the 3rd block of first redo log file(ib_logfile0). For undo log, Same as regular tablespace, the encryption metadata will be stored in the first page of data file. We also added 2 new global variables innodb_redo_log_encrypt=ON/OFF, innodb_undo_log_encrypt=ON/OFF for en/disable redo/undo log encryption.