MySQL version 5.6 and later allows you to execute replicated events using parallel threads. This feature is called Multi-Threaded Slave (MTS), and to enable it you need to modify theslave_parallel_workers variable to a value greater than 1.
Recently, a few customers asked about the meaning of some new statistics printed in their error log files when they enable MTS. These error messages look similar to the example stated below:
The MySQL reference manual doesn’t show information about these statistics. I’ve filled a bug report asking Oracle to add information about these statistics in the MySQL documentation. I reported this bug as #85747.
Before they update the documentation, we can use the MySQL code to get insight as to the statistics meaning. We can also determine how often these statistics are printed in the error log file. Looking into the rpl_slave.cc file, we find that when you enable MTS – and log-warnings variable is greater than 1 (log-error-verbosity greater than 2 for MySQL 5.7) – the time to print these statistics in MySQL error log is 120 seconds. It is determined by a hard-coded constant number. The code below shows this:
From the above code, you need MTS enabled and the modulo operation between the mts_events_assigned variable and 1024 equal to 1 in order to print the statistics. The mts_events_assigned variable stores the number of events assigned to the parallel queue. If you’re replicating a low level of events, or not replicating at all, MySQL won’t print the statistics in the error log. On the other hand, if you’re replicating a high number of events all the time, and themts_events_assigned variable increased its value until the remainder from the division between this variable and 1024 is 1, MySQL prints MTS statistics in the error log almost every 120 seconds.
You can find the explanation these statistics below (collected from information in the source code):
Multi-threaded slave is an exciting feature that allows you to replicate events faster, and keep in sync with master instances. By changing the log-warnings variable to a value greater than 1, you can get information from the slave error log file about how multi-threaded performance.
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