MySQL Replication Slave I/O Thread States
Waiting for master update
The initial state before Connecting to master.
Connecting to master
The thread is attempting to connect to the master.
Checking master version
A state that occurs very briefly, after the connection to the master is established.
Registering slave on master
A state that occurs very briefly after the connection to the master is established.
Requesting binlog dump
A state that occurs very briefly, after the connection to the master is established. The thread sends to the master a request for the contents of its binary logs, starting from the requested binary log file name and position.
Waiting to reconnect after a failed binlog dump request
If the binary log dump request failed (due to disconnection), the thread goes into this state while it sleeps, then tries to reconnect periodically. The interval between retries can be specified using the CHANGE MASTER TO statement or the --master-connect-retry option.
Reconnecting after a failed binlog dump request
The thread is trying to reconnect to the master.
Waiting for master to send event
The thread has connected to the master and is waiting for binary log events to arrive. This can last for a long time if the master is idle. If the wait lasts for slave_net_timeout seconds, a timeout occurs. At that point, the thread considers the connection to be broken and makes an attempt to reconnect.
Queueing master event to the relay log
The thread has read an event and is copying it to the relay log so that the SQL thread can process it.
Waiting to reconnect after a failed master event read
An error occurred while reading (due to disconnection). The thread is sleeping for the number of seconds set by the CHANGE MASTER TO statement or --master-connect-retry option (default 60) before attempting to reconnect.
Reconnecting after a failed master event read
The thread is trying to reconnect to the master. When connection is established again, the state becomes Waiting for master to send event.
Waiting for the slave SQL thread to free enough relay log space
You are using a nonzero relay_log_space_limit value, and the relay logs have grown large enough that their combined size exceeds this value. The I/O thread is waiting until the SQL thread frees enough space by processing relay log contents so that it can delete some relay log files.
Waiting for slave mutex on exit
A state that occurs briefly as the thread is stopping.
