If you have followed the instructions, and your replication setup is not working, the first thing to do is check the error log for messages. Many users have lost time by not doing this soon enough after encountering problems.
If you cannot tell from the error log what the problem was, try the following techniques:
Verify that the master has binary logging enabled by issuing a
SHOW MASTER STATUS statement. If logging is
enabled, Position is non-zero. If binary
logging is not enabled, verify that you are running the master
with the --log-bin and
--server-id options.
Verify that the slave is running. Use SHOW SLAVE
STATUS to check whether the
Slave_IO_Running and
Slave_SQL_Running values are both
Yes. If not, verify the options that were
used when starting the slave server. For example,
--skip-slave-start prevents the slave threads
from starting until you issue a START SLAVE
statement.
If the slave is running, check whether it established a
connection to the master. Use SHOW
PROCESSLIST, find the I/O and SQL threads and check
their State column to see what they
display. See
Section 6.3, “Replication Implementation Details”. If the
I/O thread state says Connecting to master,
verify the privileges for the replication user on the master,
the master hostname, your DNS setup, whether the master is
actually running, and whether it is reachable from the slave.
If the slave was running previously but has stopped, the reason usually is that some statement that succeeded on the master failed on the slave. This should never happen if you have taken a proper snapshot of the master, and never modified the data on the slave outside of the slave thread. If the slave stops unexpectedly, it is a bug or you have encountered one of the known replication limitations described in Section 6.7, “Replication Features and Known Problems”. If it is a bug, see Section 6.12, “How to Report Replication Bugs or Problems”, for instructions on how to report it.
If a statement that succeeded on the master refuses to run on the slave, try the following procedure if it is not feasible to do a full database resynchronization by deleting the slave's databases and copying a new snapshot from the master:
Determine whether the affected table on the slave is
different from the master table. Try to understand how
this happened. Then make the slave's table identical to
the master's and run START SLAVE.
If the preceding step does not work or does not apply, try to understand whether it would be safe to make the update manually (if needed) and then ignore the next statement from the master.
If you decide that you can skip the next statement from the master, issue the following statements:
mysql>SET GLOBAL SQL_SLAVE_SKIP_COUNTER =mysql>N;START SLAVE;
The value of N should be 1 if
the next statement from the master does not use
AUTO_INCREMENT or
LAST_INSERT_ID(). Otherwise, the value
should be 2. The reason for using a value of 2 for
statements that use AUTO_INCREMENT or
LAST_INSERT_ID() is that they take two
events in the binary log of the master.
If you are sure that the slave started out perfectly synchronized with the master, and that no one has updated the tables involved outside of the slave thread, then presumably the discrepancy is the result of a bug. If you are running the most recent version of MySQL, please report the problem. If you are running an older version, try upgrading to the latest production release to determine whether the problem persists.