pt-osc原理

Posted 2021-01-17 mysql-dba

pt-osc原理

1、检查设置环境

测试db是否可连通,并且验证database是否存在
SET SESSION innodb_lock_wait_timeout=1 //InnoDB事务等待行锁的超时时间
SET SESSION lock_wait_timeout=60  //设置获取元数据琐超时为60s
SET SESSION wait_timeout=10000   //交互超时时间
权限验证 show grants for current_user()

2、创建临时表_tablename_new并修改临时表结构

3、在源表创建三个触发器

--创建delete触发器
CREATE TRIGGER `pt_osc_darren_t_user_del` AFTER DELETE ON `darren`.`t_user` FOR EACH ROW DELETE IGNORE FROM `darren`.`_t_user_new`
WHERE `darren`.`_t_user_new`.`id` <=> OLD.`id`

--创建update触发器
CREATE TRIGGER `pt_osc_darren_t_user_upd` AFTER UPDATE ON `darren`.`t_user` FOR EACH ROW 
BEGIN 
    DELETE IGNORE FROM `darren`.`_t_user_new` WHERE !(OLD.`id` <=> NEW.`id`) AND `darren`.`_t_user_new`.`id` <=> OLD.`id`;
    REPLACE INTO `darren`.`_t_user_new` (`id`, `name`, `phone`, `gender`, `type`, `birth`, `is_delete`, `c1`, `c2`, `c3`) 
    VALUES (NEW.`id`, NEW.`name`, NEW.`phone`, NEW.`gender`, NEW.`type`, NEW.`birth`, NEW.`is_delete`, NEW.`c1`, NEW.`c2`, NEW.`c3`);
END;

--创建insert触发器
CREATE TRIGGER `pt_osc_darren_t_user_ins` AFTER INSERT ON `darren`.`t_user` 
  FOR EACH ROW 
     REPLACE INTO `darren`.`_t_user_new` (`id`, `name`, `phone`, `gender`, `type`, `birth`, `is_delete`, `c1`, `c2`, `c3`) 
     VALUES (NEW.`id`, NEW.`name`, NEW.`phone`, NEW.`gender`, NEW.`type`, NEW.`birth`, NEW.`is_delete`, NEW.`c1`, NEW.`c2`, NEW.`c3`)

4、开始迁移数据：

   1）估算表的大小，仅仅用于计算进度：EXPLAIN SELECT * FROM `darren`.`t_user` WHERE 1=1；
   2）最小值：SELECT `id` FROM `darren`.`t_user` FORCE INDEX(`PRIMARY`) ORDER BY `id` LIMIT 1;
   3) 根据chunk-size，计算本次chunk最大值及下一个chunk开始值：SELECT `id` FROM `darren`.`t_user` FORCE INDEX(`PRIMARY`) WHERE ((`id` >= ‘1‘)) 
                     ORDER BY `id` LIMIT 2, 2；
   4) 插入数据：
         INSERT LOW_PRIORITY IGNORE INTO `darren`.`_t_user_new` (`id`, `name`, `phone`, `gender`, `type`, `birth`, `is_delete`, `c1`, `c2`, `c3`) 
         SELECT `id`, `name`, `phone`, `gender`, `type`, `birth`, `is_delete`, `c1`, `c2`, `c3` FROM `darren`.`t_user` FORCE INDEX(`PRIMARY`) 
         WHERE ((`id` >= ‘1‘)) AND ((`id` <= ‘3‘)) LOCK IN SHARE MODE /*pt-online-schema-change 4670 copy nibble*/
   5）开始下一轮迭代......
   6) 何时copy结束？
       根据SELECT `id` FROM `darren`.`t_user` FORCE INDEX(`PRIMARY`) WHERE ((`id` >= ‘1‘)) ORDER BY `id` LIMIT 2, 2；
       如果获取不到下一个chunk开始值，视为表copy完成，相关源码片段如下：
                    sub _get_bounds {
                       my ($self) = @_;
                    
                       if ( $self->{one_nibble} ) {
                          if ( $self->{resume} ) {
                             $self->{no_more_boundaries} = 1;
                          }
                          return;
                       }
                    
                       my $dbh = $self->{Cxn}->dbh();
                    
                       $self->{first_lower} = $dbh->selectrow_arrayref($self->{first_lb_sql});
                       PTDEBUG && _d(‘First lower boundary:‘, Dumper($self->{first_lower}));
                    
                       if ( my $nibble = $self->{resume} ) {
                          if (    defined $nibble->{lower_boundary}
                               && defined $nibble->{upper_boundary} ) {
                             my $sth = $dbh->prepare($self->{resume_lb_sql});
                             my @ub  = split ‘,‘, $nibble->{upper_boundary};
                             PTDEBUG && _d($sth->{Statement}, ‘params:‘, @ub);
                             $sth->execute(@ub);
                             $self->{next_lower} = $sth->fetchrow_arrayref();
                             $sth->finish();
                          }
                       }
                       else {
                          $self->{next_lower}  = $self->{first_lower};
                       }
                       PTDEBUG && _d(‘Next lower boundary:‘, Dumper($self->{next_lower}));
                    
                       if ( !$self->{next_lower} ) {
                          PTDEBUG && _d(‘At end of table, or no more boundaries to resume‘);
                          $self->{no_more_boundaries} = 1;
                    
                          $self->{last_upper} = $dbh->selectrow_arrayref($self->{last_ub_sql});
                          PTDEBUG && _d(‘Last upper boundary:‘, Dumper($self->{last_upper}));
                       }
                    
                       return;
                    }

5、rename表

ANALYZE TABLE `darren`.`_t_user_new`;
RENAME TABLE `darren`.`t_user` TO `darren`.`_t_user_old`, `darren`.`_t_user_new` TO `darren`.`t_user`;

6、清理战场

DROP TABLE IF EXISTS `darren`.`_t_user_old`
DROP TRIGGER IF EXISTS `darren`.`pt_osc_darren_t_user_del`
DROP TRIGGER IF EXISTS `darren`.`pt_osc_darren_t_user_upd`
DROP TRIGGER IF EXISTS `darren`.`pt_osc_darren_t_user_ins`
SHOW TABLES FROM `darren` LIKE ‘_t_user_new‘

【copy rows和触发器实现数据迁移和增量写入原理：】

DDL功能	映射执行的SQL语句
insert触发器	replace into
update触发器	delete ignore + replace into
delete触发器	delete ignore
copy rows	insert ignore into