We don’t often see this option configured (default: unlimited) but it might be a good idea to set it. What it does is limit the amount of disk space the combined relay logs are allowed to take up.
A slave’s IO_Thread reads from the master and puts the events into the relay log; the slave’s SQL_Thread reads from the relay log and executes the query. If/when replication “breaks”, unless it’s connection related it tends to be during execution of a query. In that case the IO_Thread will keep running (receiving master events and storing in the relay log). Beyond some point, that doesn’t make sense.
The reason for having two separate replication threads …[Read more...]