MySQL replication - slave is continuously lagging behind master
The Seconds_Behind_Master is really like viewing the past via time travel.

Think of it this way:


The Sun is 93,000,000 miles away from the Earth
The speed of light is 186,000 miles/sec
Simple division shows that it takes roughly 500 sec (8 min 20 sec) for the Sun's light to reach Earth
When you look at the Sun, you actually don't see the Sun. You see where it was 8 min 20 sec ago.


In like manner, it seems that the Master is processing a lot of queries at the same time. 

You look back at the Slave, run SHOW SLAVE STATUS\G and it says 200 for Seconds_Behind_Master. How is that number calculated? Slave's Clock Time (UNIX_TIMESTAMP(NOW()) - TIMESTAMP of the Query when it was completed and recorded in the Master's Binary Log.

There is another metric to look at besides Seconds_Behind_Master. That metric is called Relay_Log_Space. That represents the sum of all bytes for all relay files on the Slave. By default, the largest single relay log is limited to 1GB. If Relay_Log_Space is less than 1GB, this indicates that many long running queries executed on the Master in parallel. Unfortunately, due to the single-threaded nature Replication's SQL thread, queries are executed one behind the other.

For example, suppose you have the following scenario on the Master:


Slow Query log is enabled
20 queries executed in parallel on the Master
Each query took 3 seconds
Each query get recorded in the Master Binary Log with the same timestamp


When the Slave reads those queries from its relay log and processes them one by one


the Slave's Clock will be moving
the TIMESTAMP for each of the 20 queries will be identical
the difference will increasing 3 seconds be completed query
this results in 60 seconds for Seconds_Behind_Master


Concerning the Slow Log, the default for long_query_time is 10 seconds. If all your queries in the relay logs are less than 10 seconds, you will never catch anything in the Slow Query Log.

I have the following recommendations for both Master and Slave servers


RECOMMENDATION #1 : Upgrade to MySQL 5.5. Under MySQL 5.5 and Percona Server 5.1.38+, you can tune InnoDB to access multiple CPUs. I have written past posts about this


Apr 26, 2012 : Is the CPU performance relevant for a database server?
Sep 20, 2011 : Multi cores and MySQL Performance
Sep 12, 2011 : Possible to make MySQL use more than one core?
May 26, 2011 : About single threaded versus multithreaded databases performance

RECOMMENDATION #2 : Use InnoDB for all tables. InnoDB caches data and indexes in RAM, MyISAM only caches indexes.
RECOMMENDATION #3 : Increase RAM. You must cache more data and indexes on the Slave and Master alike
RECOMMENDATION #4 : Tune all queries. Reducing milliseconds from queries that run hundreds of times goes a long way to reduce Seconds_Behind_Master.


FURTHER TROUBLESHOOTING

If you want to see the queries causing the replciation lag, do the following:


SHOW SLAVE STATUS\G
Get name of relay log from Relay_Log_File
STOP SLAVE;
START SLAVE;
In the OS, cd /var/lib/mysql or wherever the relay logs are written
Dump the relay log to a text file


For example, Let's do SHOW SLAVE STATUS\G

               Slave_IO_State: Waiting for master to send event
                  Master_Host: 10.64.51.149
                  Master_User: replicant
                  Master_Port: 3306
                Connect_Retry: 60
              Master_Log_File: mysql-bin.000009
          Read_Master_Log_Pos: 1024035856
               Relay_Log_File: relay-bin.000030
                Relay_Log_Pos: 794732078
        Relay_Master_Log_File: mysql-bin.000009
             Slave_IO_Running: Yes
            Slave_SQL_Running: Yes
              Replicate_Do_DB:
          Replicate_Ignore_DB: search_cache
           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: 1024035856
              Relay_Log_Space: 794732271
              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:
  Replicate_Ignore_Server_Ids:
             Master_Server_Id: 106451149


If I run STOP SLAVE; START SLAVE;, the relay log closes and a new one is open. Yet, you want relay-bin.000030.

Dump the contents as follows:

cd /var/lib/mysql
mysqlbinlog relay-bin.000030 &gt; /root/RelayLogQueries.txt
less /root/RelayLogQueries.txt


You can now see the queries the Slave is currently trying to process. You can use those queries as the starting point for tuning.