Technology sharing | what if Undo is too big

author ： Wang Yuchen

Aikesheng Database Engineer , be responsible for MySQL Routine maintenance and DMP Product support .

In this paper, the source ： Original contribution

* Produced by aikesheng open source community , Original content is not allowed to be used without authorization , For reprint, please contact the editor and indicate the source .

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The problem background

There are users using MySQL5.7 The database of , encounter undo Soaring situation , After investigation, there is a slow SQL It has been executed for tens of thousands of seconds, but it is still not over , Resulting in subsequent transactions undo Can't clean up , More and more

On-line truncate undo log Enabled , Will slow down SQL kill After falling ,undo Larger than innodb_max_undo_log_size Set size , but undo The file did not shrink immediately

Test verification

The test parameters are as follows , Turn on innodb_undo_log_truncate

mysql> show variables like '%undo%';
+--------------------------+-----------+
| Variable_name            | Value     |
+--------------------------+-----------+
| innodb_max_undo_log_size | 104857600 |
| innodb_undo_directory    | ./        |
| innodb_undo_log_truncate | ON        |
| innodb_undo_logs         | 128       |
| innodb_undo_tablespaces  | 3         |
+--------------------------+-----------+
5 rows in set (0.00 sec)

simulation undo growth , exceed innodb_max_undo_log_size Set size

# du -sh ./undo*
152M    ./undo001
296M    ./undo002
15M     ./undo003

Check out the official documents undo Clean up strategy , It can be summarized as follows ：

1、 Enable innodb_undo_log_truncate after , exceed innodb_max_undo_log_size Set the size of undo Table spaces are marked as truncated

2、 Marked undo The rollback segment of the table space is set to inactive , Cannot assign to a new transaction

3、purge Threads release unnecessary rollback segments

4、 After releasing the rollback segment ,undo The table space is truncated to its initial size 10M

You can see it shrinking undo Before size , need purge The thread releases the rollback segment first , Here is another parameter innodb_purge_rseg_truncate_frequency, The default value is 128, Express purge Every time a thread calls 128 Time , Release the rollback segment once

In the background of this problem , This parameter sets the default value

mysql> show variables like 'innodb_purge_rseg_truncate_frequency';
+--------------------------------------+-------+
| Variable_name                        | Value |
+--------------------------------------+-------+
| innodb_purge_rseg_truncate_frequency | 128   |
+--------------------------------------+-------+
1 row in set (0.01 sec)

So in order to shrink as soon as possible undo file , We can innodb_purge_rseg_truncate_frequency Turn it down , Improve purge How often does the thread release rollback segments

// Turn down the value 
mysql> show variables like 'innodb_purge_rseg_truncate_frequency';
+--------------------------------------+-------+
| Variable_name                        | Value |
+--------------------------------------+-------+
| innodb_purge_rseg_truncate_frequency | 16    |
+--------------------------------------+-------+
1 row in set (0.01 sec)

// achieve purge Number of thread calls , Release the rollback segment ,undo The table space is truncated 
# du -sh ./undo*
10M     ./undo001
10M     ./undo002
15M     ./undo003

MySQL8.0 newly added Manual Truncation

MySQL8.0 New support for SQL Statement to manage undo Table space

1、 Need at least three active undo Table space , Because there are two active undo Table space to support Automated Truncation

Create a... By hand undo Table space , Must be .ibu ending

mysql> create undo tablespace undo_003 add datafile '/data/mysql/data/3307/undo_003.ibu';
Query OK, 0 rows affected (0.27 sec)
// Three are in  active  State of  undo  Table space 
mysql> SELECT NAME, STATE FROM INFORMATION_SCHEMA.INNODB_TABLESPACES WHERE NAME LIKE '%undo%';
+-----------------+--------+
| NAME            | STATE  |
+-----------------+--------+
| innodb_undo_001 | active |
| innodb_undo_002 | active |
| undo_003        | active |
+-----------------+--------+
3 rows in set (0.00 sec)

2、 Manually cut undo Table space , You need to undo The tablespace is set to inactive

// simulation  undo  growth 
# du -sh ./undo*
81M     ./undo_001
157M    ./undo_002
26M     ./undo_003.ibu

mysql> ALTER UNDO TABLESPACE innodb_undo_002 SET INACTIVE;
Query OK, 0 rows affected (0.01 sec)

3、 Manual settings inactive after ,undo Table spaces are marked as truncated ,purge Threads will increase the return frequency , Quickly empty and finally cut undo Table space , The status changes to empty

mysql> SELECT NAME, STATE FROM INFORMATION_SCHEMA.INNODB_TABLESPACES WHERE NAME LIKE '%undo%';
+-----------------+--------+
| NAME            | STATE  |
+-----------------+--------+
| innodb_undo_001 | active |
| innodb_undo_002 | empty  |
| undo_003        | active |
+-----------------+--------+
//undo  File shrink 
# du -sh ./undo*
81M     ./undo_001
2.1M    ./undo_002
26M     ./undo_003.ibu

4、empty State of undo Tablespaces can be reactivated

mysql> ALTER UNDO TABLESPACE innodb_undo_002 SET ACTIVE;
Query OK, 0 rows affected (0.00 sec)

mysql> SELECT NAME, STATE FROM INFORMATION_SCHEMA.INNODB_TABLESPACES WHERE NAME LIKE '%undo%';
+-----------------+--------+
| NAME            | STATE  |
+-----------------+--------+
| innodb_undo_001 | active |
| innodb_undo_002 | active |
| undo_003        | active |
+-----------------+--------+
3 rows in set (0.01 sec)

5、MySQL8.0 Support deleting tablespaces , But the premise is that the table space is empty state

mysql> ALTER UNDO TABLESPACE undo_003 SET INACTIVE;
Query OK, 0 rows affected (0.01 sec)

mysql> SELECT NAME, STATE FROM INFORMATION_SCHEMA.INNODB_TABLESPACES WHERE NAME LIKE '%undo%';
+-----------------+--------+
| NAME            | STATE  |
+-----------------+--------+
| innodb_undo_001 | active |
| innodb_undo_002 | active |
| undo_003        | empty  |
+-----------------+--------+
3 rows in set (0.01 sec)

mysql> DROP UNDO TABLESPACE undo_003;
Query OK, 0 rows affected (0.02 sec)

mysql> SELECT TABLESPACE_NAME, FILE_NAME FROM INFORMATION_SCHEMA.FILES WHERE FILE_TYPE LIKE 'UNDO LOG';
+-----------------+------------+
| TABLESPACE_NAME | FILE_NAME  |
+-----------------+------------+
| innodb_undo_001 | ./undo_001 |
| innodb_undo_002 | ./undo_002 |
+-----------------+------------+
2 rows in set (0.01 sec)