Slave SQL Thread States
This article documents thread states that are related to replication slave SQL threads. These correspond to the Slave_SQL_State shown by SHOW SLAVE STATUS as well as the STATE values listed by the SHOW PROCESSLIST statement and the Information Schema PROCESSLIST as well as the PROCESSLIST_STATE value listed in the Performance Schema threads Table.
| Value | Description |
|---|---|
| Apply log event | Log event is being applied. |
| Making temp file | Creating a temporary file containing the row data as part of a LOAD DATA INFILE statement. |
| Reading event from the relay log | Reading an event from the relay log in order to process the event. |
| Slave has read all relay log; waiting for the slave I/O thread to update it | All relay log events have been processed, now waiting for the I/O thread to write new events to the relay log. |
| Waiting for work from SQL thread | In parallel replication the worker thread is waiting for more things from the SQL thread. |
| Waiting for prior transaction to start commit before starting next transaction | In parallel replication the worker thread is waiting for conflicting things to end before starting executing. |
| Waiting for worker threads to be idle | Happens in parallel replication when moving to a new binary log after a master restart. All slave temporary files are deleted and worker threads are restarted. |
| Waiting due to global read lock | In parallel replication when worker threads are waiting for a global read lock to be released. |
| Waiting for worker threads to pause for global read lock | FLUSH TABLES WITH READ LOCK is waiting for worker threads to finish what they are doing. |
| Waiting while replication worker thread pool is busy | Happens in parallel replication during a FLUSH TABLES WITH READ LOCK or when changing number of parallel workers. |
| Waiting for other master connection to process GTID received on multiple master connections | A worker thread noticed that there is already another thread executing the same GTID from another connection and it's waiting for the other to complete. |
| Waiting for slave mutex on exit | Thread is stopping. Only occurs very briefly. |
| Waiting for the next event in relay log | State before reading next event from the relay log. |
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https://mariadb.com/kb/en/slave-sql-thread-states/