The basic problem stems from the Slave's IO Thread. Sometimes, it has the nasty habit of playing dumb and not properly checking or pinging the master.
MySQL 5.5's semisynchronous replication allows you to have the master time itself out and degrade back to asynchronous replication. Slaves running semisynchronous replication should have a more sensitive IO Thread now.
Concerning your particular situation, you are using MySQL 5.0.24 ??? That's a very old version. There are two bug reports (Bug1 and Bug2) discussing this.
It just dawned on me that you asked this question earlier and DTest answered it. Giving credit where credit is due, the bug reports came from his answer.
It would be nice to to be informed when the Public IP is being disabled from Slave access. That way, you could run STOP SLAVE; on the slave in advance.
You also asked is there a way to come to know the master status from the slave, when this type of behavior occurs.
Here is a typical SHOW SLAVE STATUS\G
mysql> show slave status\G
*************************** 1. row ***************************
Slave_IO_State: Waiting for master to send event
Master_Host: 10.64.80.136
Master_User: replicant
Master_Port: 3306
Connect_Retry: 60
Master_Log_File: mysql-bin.000019 <<------
Read_Master_Log_Pos: 277892198 <<------
Relay_Log_File: relay-bin.000058
Relay_Log_Pos: 37535484
Relay_Master_Log_File: mysql-bin.000019 <<------
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: 277892198 <<------
Relay_Log_Space: 277892637 <<------
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
Master_SSL_Verify_Server_Cert: No
Last_IO_Errno: 0
Last_IO_Error:
Last_SQL_Errno: 0
Last_SQL_Error:
1 row in set (0.00 sec)
Please note the following:
- Master_Log_File : Log file from Master whose latest SQL was last copied to relay logs
- Read_Master_Log_Pos : Log position within
Master_Log_File whose latest SQL was last copied to relay logs
- Relay_Master_Log_File : Log file from Master whose SQL was last executed from the relay logs
- Exec_Master_Log_Pos : Log position within
Relay_Master_Log_File whose SQL was last executed from the relay logs
If none of these value are moving, that could mean the all SQL is processed or that the slave is currently processing an SQL statement in the SQL thread that came from position Exec_Master_Log_Pos of the Master Log Relay_Master_Log_File.
Now, look at Relay_Log_Space. This number represents the sum total of all filesizes for all relay logs. If Slave_IO_Running is Yes and Relay_Log_Space is not changing, then go check the master by running SHOW MASTER STATUS;. It should like something like this:
mysql> show master status;
+------------------+-----------+--------------+------------------+
| File | Position | Binlog_Do_DB | Binlog_Ignore_DB |
+------------------+-----------+--------------+------------------+
| mysql-bin.000019 | 682563021 | | |
+------------------+-----------+--------------+------------------+
1 row in set (0.00 sec)
If the Master's (File,Position) of SHOW MASTER STATUS; is beyond the Slave's (Master_Log_File,Read_Master_Log_Pos) of SHOW SLAVE STATUS\G, then the IO Thread on the Slave is dead for intents and purposes, even if Slave_IO_Running is Yes.
Best Answer
If the Master was already acting as a Slave, then you really had a Master-Master setup, and the replication from the old Slave (current Master) will flow to the old Master that had been down.
Otherwise, you are in deep weeds. You may as well promote the currently-acting-as-Master machine to be the Master. And then rebuild the other machine as a Slave. Or rather a Master in Master-Master setup so you don't hit this problem again.