| Bug #74471 | most rollback segments may always be created in first undo tablespace | ||
|---|---|---|---|
| Submitted: | 21 Oct 2014 5:18 | Modified: | 23 Apr 2015 0:38 |
| Reporter: | zhai weixiang (OCA) | Email Updates: | |
| Status: | Closed | Impact on me: | |
| Category: | MySQL Server: Documentation | Severity: | S3 (Non-critical) |
| Version: | 5.7.5 | OS: | Any |
| Assigned to: | Daniel Price | CPU Architecture: | Any |
[4 Nov 2014 3:36]
zhai weixiang
Can anyone take a look of this bug ? I think this may lead to online undo truncate unusable
[10 Feb 2015 16:28]
MySQL Verification Team
Actually, this is not a bug in the code. InnoDB can use several undo log files, but it will move to the next one only if all previous ones have been filled. In the future, it will be possible to tie one or more log files to a specific tablespace, containing one or more tables. But, it is not yet possible. I find that manual is not too clear on this subject, so I can verify this as a documentation bug. I hope you agree with this decision.
[15 Feb 2015 5:48]
zhai weixiang
Agreed this behavior should be clarified in the document :)
[16 Feb 2015 13:41]
MySQL Verification Team
Documentation on InnoDB usage of undo tablespaces has to be improved and extended.
[23 Apr 2015 0:38]
Daniel Price
The innodb_undo_tablespaces option must be configured when the MySQL instance is created. Otherwise, there is only the undo space in the system tablespace, and that is where any additional rollback segments (i.e. undo logs) will be allocated (space 0). Also note that in MySQL 5.7.2, 32 rollback segments were allocated to the temporary table tablespace (ibtmp1). Consequently, you must set innodb_undo_logs=34 or more to allocate a rollback segment to an undo tablespace. Please refer to this documentation for information about undo tablespace setup and configuration. Revisions to this content should appear within 24-hours. http://dev.mysql.com/doc/refman/5.7/en/innodb-undo-tablespace.html Specific requirements regarding truncating undo logs that reside in undo tablespaces are documented here: http://dev.mysql.com/doc/refman/5.7/en/truncate-undo-tablespace.html
Description: While reading the new code of MySQL5.7.5 . I found that innodb always try to create new rollback segment starting from the first undo tablespace: quoted code from function: trx_sys_create_rsegs: 876 877 for (i = 0; i < new_rsegs; ++i) { 878 ulint space; 879 880 /* Tablespace 0 is the system tablespace. All UNDO 881 log tablespaces start from 1. */ 882 883 if (n_spaces > 0) { 884 space = (i % n_spaces) + 1; 885 } else { 886 space = 0; /* System tablespace */ 887 } 888 889 if (trx_rseg_create(space, 0) != NULL) { 890 ++n_used; Suppose we increase innodb_undo_logs one by one, then here new_rsegs= 1, and obviously all increased rollback segments will be inside the first undo tablespace. How to repeat: read the code or add some debug information in trx_sys_create_rsegs Suggested fix: I don't know