This article documents thread states that are related to replication slave I/O threads. These correspond to the Slave_IO_State shown by SHOW SLAVE STATUS and the STATE values listed by the SHOW PROCESSLIST statement or in the Information Schema PROCESSLIST Table as well as the PROCESSLIST_STATE value listed in the Performance Schema threads Table.
| Value | Description |
|---|---|
| Checking master version | Checking the master's version, which only occurs very briefly after establishing a connection with the master. |
| Connecting to master | Attempting to connect to master. |
| Queueing master event to the relay log | Event is being copied to the relay log after being read, where it can be processed by the SQL thread. |
| Reconnecting after a failed binlog dump request | Attempting to reconnect to the master after a previously failed binary log dump request. |
| Reconnecting after a failed master event read | Attempting to reconnect to the master after a previously failed request. After successfully connecting, the state will change to Waiting for master to send event. |
| Registering slave on master | Registering the slave on the master, which only occurs very briefly after establishing a connection with the master. |
| Requesting binlog dump | Requesting the contents of the binary logs from the given log file name and position. Only occurs very briefly after establishing a connection with the master. |
| Waiting for master to send event | Waiting for binary log events to arrive after successfully connecting. If there are no new events on the master, this state can persist for as many seconds as specified by the slave_net_timeout system variable, after which the thread will reconnect. |
| Waiting for slave mutex on exit | Waiting for slave mutex while the thread is stopping. Only occurs very briefly. |
| Waiting for the slave SQL thread to free enough relay log space. | Relay log has reached its maximum size, determined by relay_log_space_limit (no limit by default), so waiting for the SQL thread to free up space by processing enough relay log events. |
| Waiting for master update | State before connecting to master. |
| Waiting to reconnect after a failed binlog dump request | Waiting to reconnect after a binary log dump request has failed due to disconnection. The length of time in this state is determined by the MASTER_CONNECT_RETRY clause of the CHANGE MASTER TO statement. |
| Waiting to reconnect after a failed master event read | Sleeping while waiting to reconnect after a disconnection error. The time in seconds is determined by the MASTER_CONNECT_RETRY clause of the CHANGE MASTER TO statement. |
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https://mariadb.com/kb/en/slave-io-thread-states/