Slave_IO_State

A copy of the State field of the output of SHOW PROCESSLIST for the slave I/O thread. This tells you what the thread is doing: trying to connect to the master, waiting for events from the master, reconnecting to the master, and so on. Possible states are listed in Section 6.4, “Replication Implementation Details”. It is necessary to check this field for older versions of MySQL which allowed the thread to continue running while unsuccessfully trying to connect to the master. If it is running, there is no problem; if it is not, you can find the error in the Last_Error field (described below).

Master_Host

The current master host.

Master_User

The current user used to connect to the master.

Master_Port

The current master port.

Connect_Retry

The current value of the --master-connect-retry option.

Master_Log_File

The name of the master binary log file from which the I/O thread is currently reading.

Read_Master_Log_Pos

The position up to which the I/O thread has read in the current master binary log.

Relay_Log_File

The name of the relay log file from which the SQL thread is currently reading and executing.

Relay_Log_Pos

The position up to which the SQL thread has read and executed in the current relay log.

Relay_Master_Log_File

The name of the master binary log file containing the most recent event executed by the SQL thread.

Slave_IO_Running

Whether the I/O thread is started and has connected successfully to the master. For older versions of MySQL (prior to 4.1.14 and 5.0.12) Slave_IO_Running is YES if the I/O thread is started, even if the slave hasn't connected to the master yet.

Slave_SQL_Running

Whether the SQL thread is started.

Replicate_Do_DB, Replicate_Ignore_DB

The lists of databases that were specified with the --replicate-do-db and --replicate-ignore-db options, if any.

Replicate_Do_Table, Replicate_Ignore_Table, Replicate_Wild_Do_Table, Replicate_Wild_Ignore_Table

The lists of tables that were specified with the --replicate-do-table, --replicate-ignore-table, --replicate-wild-do-table, and --replicate-wild-ignore_table options, if any.

Last_Errno, Last_Error

The error number and error message returned by the most recently executed query. An error number of 0 and message of the empty string mean “no error.” If the Last_Error value is not empty, it also appears as a message in the slave's error log. For example:

Last_Errno: 1051
Last_Error: error 'Unknown table 'z'' on query 'drop table z'

The message indicates that the table z existed on the master and was dropped there, but it did not exist on the slave, so DROP TABLE failed on the slave. (This might occur, for example, if you forget to copy the table to the slave when setting up replication.)

Skip_Counter

The most recently used value for SQL_SLAVE_SKIP_COUNTER.

Exec_Master_Log_Pos

The position of the last event executed by the SQL thread from the master's binary log (Relay_Master_Log_File). (Relay_Master_Log_File, Exec_Master_Log_Pos) in the master's binary log corresponds to (Relay_Log_File, Relay_Log_Pos) in the relay log.

Relay_Log_Space

The total combined size of all existing relay logs.

Until_Condition, Until_Log_File, Until_Log_Pos

The values specified in the UNTIL clause of the START SLAVE statement.

Until_Condition has these values:

Until_Log_File and Until_Log_Pos indicate the log filename and position values that define the point at which the SQL thread stops executing.

Master_SSL_Allowed, Master_SSL_CA_File, Master_SSL_CA_Path, Master_SSL_Cert, Master_SSL_Cipher, Master_SSL_Key

These fields show the SSL parameters used by the slave to connect to the master, if any.

Master_SSL_Allowed has these values:

The values of the other SSL-related fields correspond to the values of the --master-ca, --master-capath, --master-cert, --master-cipher, and --master-key options.

Seconds_Behind_Master

This field is an indication of how “late” the slave is:

In essence, this field measures the time difference in seconds between the slave SQL thread and the slave I/O thread.

If the network connection between master and slave is fast, the slave I/O thread is very close to the master, so this field is a good approximation of how late the slave SQL thread is compared to the master. If the network is slow, this is not a good approximation; the slave SQL thread may quite often be caught up with the slow-reading slave I/O thread, so Seconds_Behind_Master often shows a value of 0, even if the I/O thread is late compared to the master. In other words, this column is useful only for fast networks.

This time difference computation works even though the master and slave do not have identical clocks (the clock difference is computed when the slave I/O thread starts, and assumed to remain constant from then on). Seconds_Behind_Master is NULL (which means “unknown”) if the slave SQL thread is not running, or if the slave I/O thread is not running or not connected to master. For example if the slave I/O thread is sleeping for the number of seconds given by the --master-connect-retry option before reconnecting, NULL is shown, as the slave cannot know what the master is doing, and so cannot say reliably how late it is.

This field has one limitation. The timestamp is preserved through replication, which means that, if a master M1 is itself a slave of M0, any event from M1's binlog which originates in replicating an event from M0's binlog has the timestamp of that event. This enables MySQL to replicate TIMESTAMP successfully. However, the drawback for Seconds_Behind_Master is that if M1 also receives direct updates from clients, the value randomly deviates, because sometimes the last M1's event is from M0 and sometimes it is the most recent timestamp from a direct update.