随着MySQL版本的不断更新，对DDL操作的支持也在不断的完善和更新：比如从MySQL 5.6引入Online DDL，在MySQL 5.7对Online DDL进一步完善，到现在的8.0版本，则对DDL的实现重新进行了设计，比如DDL操作支持原子特性，在MySQL 8.0.27引入并行DDL。本篇就来探究一下MySQL 8.0.27的并行DDL对于DDL操作速度的提升。
MySQL 8.0.14引入了innodb_parallel_read_threads变量来控制扫描聚簇索引的并行线程。MySQL 8.0.27引入了innodb_ddl_threads变量来控制用于创建二级索引时的并行线程数量，此参数一般和一并引入的innodb_ddl_buffer_size一起使用，innodb_ddl_buffer_size用于指定进行并行DDL操作时能够使用的buffer大小，buffer是在所有的DDL并行线程中平均分配的，所以一般如果调大innodb_ddl_threads变量时，也需要调大innodb_ddl_buffer_size的大小。
innodb_ddl_threads、innodb_ddl_buffer_size和innodb_parallel_read_threads的默认大小分别为：
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)
接下来测试一下调大innodb_ddl_threads、innodb_ddl_buffer_size和innodb_parallel_read_threads参数值对DDL操作的性能提升。
首先创建一张5000万的表：
-- 数据库版本为8.0.28 mysql> select @@version; +----------+ | @@version| +----------+ | 8.0.28 | +----------+ 1 row in set (0.00 sec) -- buffer pool大小为24G mysql> select @@global.innodb_buffer_pool_size; +----------------------------------+ | @@global.innodb_buffer_pool_size | +----------------------------------+ | 1073741824 | +----------------------------------+ 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) -- 表空间大小为12G # ll -h total 12G -rw-r-----. 1 mysql mysql 12G Jan 20 17:38 sbtest1.ibd
分别测试不同的线程数量和缓冲区大小的DDL操作时间，例如：
-- 设置并发DDL线程为1 mysql> set innodb_ddl_threads = 1; Query OK, 0 rows affected (0.01 sec) -- 设置buffer大小为512M mysql> set innodb_ddl_buffer_size = 536870912; Query OK, 0 rows affected (0.00 sec) -- 设置并行索引扫描线程为1 mysql> set innodb_parallel_read_threads = 1; Query OK, 0 rows affected (0.01 sec) -- 执行DDL操作 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 -- 查看DDL的内存最大占用 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)
通过不断调整相关参数得到以下结果
|
innodb_ddl_threads
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innodb_ddl_buffer_size
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innodb_parallel_read_threads
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DDL占用最大内存
|
DDL时间
|
|
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
|
可以看到，随着并发线程的增多和buffer的增加，DDL操作所占用的资源也越多，而DDL操作所花费的时间则越少。不过通过对比资源的消耗和DDL速度的提升比例，最合理的并行线程数量为4-8个，而buffer大小可以根据情况进行调整。