MySQL monitor prompt does not return for slave

MySQLreplication

I'm trying to fix a replication error (or an ongoing failure to replicate) between our master and slave MySQL databases.

The procedure I have asks me to use MySQL Monitor to stop the slave.

stop slave;

When I try this, however, the prompt never returns. I've left it for several minutes before Ctrl+C'ing out.

Interestingly, the same thing occurs with:

show slave status;

…but

show master status;

Works fine. Is there some other way I can kill the slave process off, ideally without interrupting the master?

Best Answer

When this happens, I immediately go to the Slave and

If you have MySQL 5.1+

SELECT id FROM information_schema.processlist WHERE user='system user';

This will show you the Process ID of the IO Thread and SQL Thread of Replication.

If you have MySQL 5.0

SHOW PROCESSLIST;

You would have look for the two rows in the display that has 'system user' as the user.

Once you find the IDs...

You can run the KILL or KILL QUERY command against the Process IDs.

CAVEAT

This is a possibility that evn the KILL command can hang if the query must come to a completion (such as a transaction that must fully commit or fully rollback).

Master is oblivious to all this.

Why does `SHOW MASTER STATUS;` work just fine ???

Running SHOW MASTER STATUS; simply checks for binary logs on the DB Server. There are no DB threads to read, parse, examine, and coalesce.

Running SHOW SLAVE STATUS\G has to reconcile three(3) aspects:

Aspect #1 : the Contents of master.info
Aspect #2 : the current list of relay logs
Aspect #3 : information acquired from the IO and SQL threads (live)

This reconciliation helps produce the output of SHOW SLAVE STATUS\G. It's Aspect #3 that makes SHOW SLAVE STATUS\G freeze attempts to kill the SQL thread on a long running query.

Related Solutions

MySQL : if server_id is not explicitly set, then get error `ERROR 1200 (HY000) at line 1: The server is not configured as slave;`

Two servers are not allowed to have the same server_id. Also, If you omit --server-id from a master, the default ID is 0, in which case a master refuses connections from all slaves, and a slave refuses to connect to a master.

For more information, see Section 15.1.1.2, “Setting the Replication Slave Configuration”.

Even if both servers have different server_id values, please make sure the new master has binary logging enabled.

Mysql – Looking for an efficient way to fix “Could not parse relay log event entry…” error

There is more stable approach you can try

Here is something to remember

Whenever you run CHANGE MASTER TO, it will erase every relay log you have. You do not want to keep relay logs of commands you have not executed any SQL on as of yet

The following is an excerpt taken from a post I made back on Feb 03, 2012 : How to resolve the master server shut down/unavailability in mysql with master - slave replication :

Please notice that there are two sets of replication coordinates from the Master

(Master_Log_File,Read_Master_Log_Pos)

(Relay_Master_Log_File,Exec_Master_Log_Pos)

There is a major difference between them

(Master_Log_File,Read_Master_Log_Pos) tells you the last binlog statement from the Master's log file and log position that the Slave read from the Master and placed in its Relay Logs.

(Relay_Master_Log_File,Exec_Master_Log_Pos) tells you the last binlog statement from the Master's log file and log position that the Slave read from the Master and placed in its Relay Logs THAT IS NEXT TO BE EXECUTED ON THE SLAVE.

What you want are two things:

Erase Every Binary Log You Have
Start Collecting Binary Log Entries From the Last SQL You Successfully Executed.

In your case, you must use the second set of Replication Coordinates

Relay_Master_Log_File
Exec_Master_Log_Pos

It is easy to distrust a corrupt relay log as shown in the error message. The one that hurts the most is a corrupt Master Log. You will have to jump through hoops if that is the case. On the other hand, if one of the other situations was the reason for the corrupt relay log, the simplest and most concise approach is what I stated.

To make sure, whatever is reported for Relay_Master_Log_File, if that particular binary log still exists on the Master, perform a mysqlbinlog on it. If it dumps in its entirety without corrupt characters, go ahead and use the second set of replication coordinates.

From my same earlier post

mysql> show slave status\G
*************************** 1. row ***************************
             Slave_IO_State: Waiting for master to send event
                Master_Host: 10.48.20.253
                Master_User: replicant
                Master_Port: 3306
              Connect_Retry: 60
            Master_Log_File: mysql-bin.000254
        Read_Master_Log_Pos: 858190247
             Relay_Log_File: relay-bin.066069
              Relay_Log_Pos: 873918
      Relay_Master_Log_File: mysql-bin.000254
           Slave_IO_Running: Yes
          Slave_SQL_Running: Yes
            Replicate_Do_DB:
        Replicate_Ignore_DB:
         Replicate_Do_Table:
     Replicate_Ignore_Table:
    Replicate_Wild_Do_Table:
Replicate_Wild_Ignore_Table:
                 Last_Errno: 0
                 Last_Error:
               Skip_Counter: 0
        Exec_Master_Log_Pos: 858190247
            Relay_Log_Space: 873772
            Until_Condition: None
             Until_Log_File:
              Until_Log_Pos: 0
         Master_SSL_Allowed: No
         Master_SSL_CA_File:
         Master_SSL_CA_Path:
            Master_SSL_Cert:
          Master_SSL_Cipher:
             Master_SSL_Key:
      Seconds_Behind_Master: 0
1 row in set (0.00 sec)

notice that the Replication Coordinates from SHOW SLAVE STATUS\G for what was last executed are (mysql-bin.000254,858190247). The CHANGE MASTER TO command in this case would be:

CHANGE MASTER TO master_log_file='mysql-bin.000254',master_log_pos=858190247;

Give it a Try !!!

UPDATE 2012-09-14 16:38 EDT

If you worried about the stockpiling relay logs, just throttle the relay logs. In SHOW SLAVE STATUS\G, there is a field called Relay_Log_Space. That gives you the sum of all relay sizes in bytes. Did you know you could put a cap on that number ?

The option is called relay_log_space_limit.

For example, if you want to cap the total number of bytes to 10G, do the following

STEP 01) Add this to /etc/my.cnf on the Slave

[mysqld]
relay_log_space_limit = 10G

STEP 02) Run service mysql restart on the Slave

and that's it !!!

When the oldest relay has all its entries processed, it is deleted and a new relay log is created. That gets filled until all relay logs add up to 10G. That's the only way to control runaway relay log space issues.

UPDATE 2012-09-14 18:10 EDT

SUGGESTION : If you make mysqldump backups of the data on the Slave every midnight, you could set up the following to restrict having 1TB of binary logs:

STEP 01) Add this to /etc/my.cnf on the Master

[mysqld]
expire_logs_days = 14

STEP 02) Run this query on the Master

mysql> PURGE BINARY LOGS BEFORE DATE(NOW()) - INTERVAL 14 DAY;

STEP 03) service mysql restart on the Master

STEP 04) Add a mysqldump backup script to a crontab on the Slave

This will make the Slave more useful and would control having excess binary logs to worry about