Recently I’ve got myself a little worried about how to monitor the threads executing data coming from the relay logs in a replication environment. I decided to go, raise some virtual machines, setup mysql on that and start investigating how to do that. All the things will start setting up the replication between some mysql servers using binary log. I haven’t tested the slave parallel workers with GTID replication and hearing folks around saying that parallel slave workers is a feature that is note working in GTID at this time (check the blog’s date, ok?)

Getting those stuff running, this is time to configure the slave_parallel_workers, which is the system variable that is in changer to control/handle the amount of threads will be dedicated to execute all those data being piled in relay logs. BTW, it’s very good that the mentioned variable as many others can be reconfigured with new value on the runtime and having said that…

mysql> show variables like ‘slave_parallel%’;
+——————————+———————–+
| Variable_name | Value |
+——————————+———————–+
| slave_parallel_workers | 0 |
+——————————+———————–+
1 rows in set (0.00 sec)

mysql> set global slave_parallel_workers=10;
Query OK, 0 rows affected (0.00 sec)

mysql> show variables like ‘slave_parallel%’;
+——————————+———————–+
| Variable_name | Value |
+——————————+———————–+
| slave_parallel_workers | 10 |
+——————————+———————–+
1 rows in set (0.00 sec)

As you can see, it’s easy to configure the number of threads the will execute data that is coming form the relay log/master server. It’s good to have in mind that is advisable that the number of threads keep up with the number of the available processor cores of the slave machine, as this is the slave one. This is the time so to configure a way to monitor the slave thread worker. Another variable that has a GLOBAL scope as well must be reconfigured:

# assure that the replication is stopped at this point as it’s just possible to run the below command if replication is stopped

mysql> set global relay_log_info_repository=’table’;
ERROR 1198 (HY000): This operation cannot be performed with a running slave; run STOP SLAVE first
mysql> stop slave;
Query OK, 0 rows affected (0.01 sec)

mysql> set global relay_log_info_repository=’table’;
Query OK, 0 rows affected (0.02 sec)

mysql> start slave;
Query OK, 0 rows affected, 1 warning (0.03 sec)

Now it’s a matter to start slave and query the mysql.slave_worker_info table…

mysql> select count(*) from mysql.slave_worker_info\G
*************************** 1. row ***************************
count(*): 10
1 row in set (0.00 sec)

Cheers, WB

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