Technology sharing | MySQL parallel DDL

author ： Peng Bo Li

Love can be born DBA Team members , Will change , Mainly responsible for MySQL Troubleshooting and SQL Audit optimization . A commitment to technology , Be responsible for the customer .

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With MySQL The version is constantly updated , Yes DDL The operation support is also constantly improved and updated ： For instance from MySQL 5.6 introduce Online DDL , stay MySQL 5.7 Yes Online DDL Further refinement , To the present 8.0 edition , On the other hand DDL The implementation of is redesigned , such as DDL The operation supports atomic properties , stay MySQL 8.0.27 Introduce parallelism DDL . This article will explore MySQL 8.0.27 Parallel of DDL about DDL Increase of operating speed .

MySQL 8.0.14 Introduced innodb_parallel_read_threads Variable to control the parallel thread scanning the cluster index .MySQL 8.0.27 Introduced innodb_ddl_threads Variable to control the number of parallel threads used to create secondary indexes , This parameter is generally introduced together with innodb_ddl_buffer_size Use it together ,innodb_ddl_buffer_size Used to specify parallel processing DDL Can be used during operation buffer size ,buffer In all DDL The average number of threads allocated in parallel , So generally, if you turn it up innodb_ddl_threads variable , It also needs to be turned up innodb_ddl_buffer_size Size .

innodb_ddl_threads 、innodb_ddl_buffer_size and innodb_parallel_read_threads The default sizes of are ：

mysql> select @@global.innodb_ddl_threads;
+-----------------------------+
| @@global.innodb_ddl_threads |
+-----------------------------+
|                           4 |
+-----------------------------+
1 row in set (0.00 sec)

mysql> select @@global.innodb_ddl_buffer_size;
+---------------------------------+
| @@global.innodb_ddl_buffer_size |
+---------------------------------+
|                         1048576 |
+---------------------------------+
1 row in set (0.00 sec)

mysql> select @@global.innodb_parallel_read_threads;
+---------------------------------------+
| @@global.innodb_parallel_read_threads |
+---------------------------------------+
|                                     4 |
+---------------------------------------+
1 row in set (0.00 sec)

Next, test a big innodb_ddl_threads 、innodb_ddl_buffer_size and innodb_parallel_read_threads The parameter value pairs DDL Performance improvement of operation .

First create a 5000 Ten thousand watches ：

--  The database version is 8.0.28
mysql> select @@version;
+----------+
| @@version|
+----------+
| 8.0.28   |
+----------+
1 row in set (0.00 sec)

-- buffer pool The size is 24G
mysql> select @@global.innodb_buffer_pool_size;
+----------------------------------+
| @@global.innodb_buffer_pool_size |
+----------------------------------+
|                      25769803776 |
+----------------------------------+
1 row in set (0.001 sec)

mysql> create database action;
Query OK, 1 row affected (0.01 sec)

# sysbench /usr/share/sysbench/oltp_read_write.lua --mysql-socket=/data/mysql/data/3306/mysqld.sock  --mysql-user=root --mysql-password='123' --mysql-db=action --tables=1 --table-size=50000000 --report-interval=1 --threads=8 prepare

mysql> select count(*) from action.sbtest1;
+----------+
| count(*) |
+----------+
| 50000000 |
+----------+
1 row in set (21.64 sec)

--  The table space size is 12G
# ll -h
total 12G
-rw-r-----. 1 mysql mysql 12G Jan 20 17:38 sbtest1.ibd

Test the performance of different number of threads and buffer size DDL Operating time , for example ：

--  Set up concurrency DDL Thread is 1
mysql> set innodb_ddl_threads = 1;
Query OK, 0 rows affected (0.01 sec)

--  Set up buffer The size is 512M
mysql> set innodb_ddl_buffer_size = 536870912;
Query OK, 0 rows affected (0.00 sec)

--  Set the parallel index scanning thread to 1
mysql> set innodb_parallel_read_threads = 1;
Query OK, 0 rows affected (0.01 sec)

--  perform DDL operation 
mysql> alter table action.sbtest1 add index idx_c(c);
Query OK, 0 rows affected (6 min 54.21 sec)
Records: 0  Duplicates: 0  Warnings: 0

--  see DDL The maximum memory usage of 
mysql> select event_name,CURRENT_NUMBER_OF_BYTES_USED/1024/1024 from performance_schema.memory_summary_global_by_event_name where event_name='memory/innodb/ddl';
+-------------------+----------------------------------------+
| event_name        | CURRENT_NUMBER_OF_BYTES_USED/1024/1024 |
+-------------------+----------------------------------------+
| memory/innodb/ddl |                           513.08750916 |
+-------------------+----------------------------------------+
1 row in set (0.00 sec)

By continuously adjusting relevant parameters, the following results are obtained ：

innodb_ddl_threads	innodb_ddl_buffer_size	innodb_parallel_read_threads	DDL Maximum memory consumption	DDL Time
1	512M	1	513M	6 min 54.21 sec
2	1G	2	1230M	4 min 12.08 sec
4	2G	4	2735M	3 min 43.01 sec
8	4G	8	5791M	3 min 19.63 sec
16	8G	16	5975M	3 min 12.33 sec
32	16G	32	6084M	3 min 11.11 sec

You can see , With the increase of concurrent threads and buffer An increase in ,DDL The more resources the operation takes up , and DDL The less time it takes to operate . But by comparing the consumption of resources with DDL The rate of increase in speed , The most reasonable number of parallel threads is 4-8 individual , and buffer The size can be adjusted according to the situation .

Reference link ：https://dev.mysql.com/doc/ref...