When a slave starts after being stopped for some time, or a new slave starts that was created from a backup from some time back, a lot of old binlog events may need to be downloaded from the master. If this happens from many slaves simultaneously, it can put a lot of load on the master.
The option read_binlog_speed_limit can be used to reduce such load, by limiting the speed at which events are downloaded. The limit is given as maximum kilobytes per second to download on one slave connection.
With this option set, the replication I/O thread will limit the rate of download. Since the I/O thread is often much faster to download events than the SQL thread is at applying them, an appropriate value for read_binlog_speed_limit may reduce load spikes on the master without much limit in the speed of the replication slave.
The option read_binlog_speed_limit is available starting from MariaDB 10.2.3.
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