Q: How do I configure a slave if the master is running and I do not want to stop it?
A: There are several
possibilities. If you have taken a snapshot backup of the master
at some point and recorded the binary log file name and offset
(from the output of
STATUS) corresponding to the snapshot, use the following
Make sure that the slave is assigned a unique server ID.
Execute the following statement on the slave, filling in appropriate values for each option:
CHANGE MASTER TO->
START SLAVE on the
If you do not have a backup of the master server, here is a quick procedure for creating one. All steps should be performed on the master host.
Issue this statement to acquire a global read lock:
FLUSH TABLES WITH READ LOCK;
With the lock still in place, execute this command (or a variation of it):
tar zcf /tmp/backup.tar.gz /var/lib/mysql
Issue this statement and record the output, which you will need later:
SHOW MASTER STATUS;
Release the lock:
An alternative to using the preceding procedure to make a binary copy is to make an SQL dump of the master. To do this, you can use mysqldump --master-data on your master and later load the SQL dump into your slave. However, this is slower than making a binary copy.
Regardless of which of the two methods you use, afterward follow the instructions for the case when you have a snapshot and have recorded the log file name and offset. You can use the same snapshot to set up several slaves. Once you have the snapshot of the master, you can wait to set up a slave as long as the binary logs of the master are left intact. The two practical limitations on the length of time you can wait are the amount of disk space available to retain binary logs on the master and the length of time it takes the slave to catch up.
Q: Does the slave need to be connected to the master all the time?
A: No, it does not. The slave can go down or stay disconnected for hours or even days, and then reconnect and catch up on updates. For example, you can set up a master/slave relationship over a dial-up link where the link is up only sporadically and for short periods of time. The implication of this is that, at any given time, the slave is not guaranteed to be in synchrony with the master unless you take some special measures.
Q: How do I know how late a slave is compared to the master? In other words, how do I know the date of the last statement replicated by the slave?
A: If the slave is 4.1.1 or
newer, read the
Seconds_Behind_Master column in
SHOW SLAVE STATUS, which shows the
number of seconds that the slave SQL thread is behind processing
the master binary log. A high number (or an increasing one) can
indicate that the slave is unable to cope with the large number of
queries from the master.
A value of 0 for
usually be interpreted as meaning that the slave has caught up
with the master, but there are some cases where this is not
strictly true. For example, this can occur if the network
connection between master and slave is broken but the slave I/O
thread has not yet noticed this—that is,
slave_net_timeout has not yet
It is also possible that transient values for
Seconds_Behind_Master may not reflect the
situation accurately. When the slave SQL thread has caught up on
Seconds_Behind_Master displays 0; but when
the slave I/O thread is still queuing up a new event,
Seconds_Behind_Master may show a large value
until the SQL thread finishes executing the new event. This is
especially likely when the events have old timestamps; in such
cases, if you execute
STATUS several times in a relatively short peiod, you
may see this value change back and forth repeatedly between 0 and
a relatively large value.
For versions of MySQL prior to 4.1.1, it is possible to determine
how far behind the slave is only if
SLAVE STATUS on the slave shows that the SQL thread is
running (or for MySQL 3.23, that the slave thread is running), and
that the thread has executed at least one event from the master.
See Section 14.3, “Replication Implementation Details”.
When the slave SQL thread executes an event read from the master,
it modifies its own time to the event timestamp. (This is why
TIMESTAMP is well replicated.) In
Time column in the output of
SHOW PROCESSLIST, the number of
seconds displayed for the slave SQL thread is the number of
seconds between the timestamp of the last replicated event and the
real time of the slave machine. You can use this to determine the
date of the last replicated event. Note that if your slave has
been disconnected from the master for one hour, and then
reconnects, you may immediately see
like 3600 for the slave SQL thread in
PROCESSLIST. This is because the slave is executing
statements that are one hour old.
Q: How do I force the master to block updates until the slave catches up?
A: Use the following procedure:
On the master, execute these statements:
FLUSH TABLES WITH READ LOCK;mysql>
SHOW MASTER STATUS;
Record the replication coordinates (the log file name and
offset) from the output of the
On the slave, issue the following statement, where the
arguments to the
MASTER_POS_WAIT() function are
the replication coordinate values obtained in the previous
SELECT statement blocks
until the slave reaches the specified log file and offset. At
that point, the slave is in synchrony with the master and the
On the master, issue the following statement to enable the master to begin processing updates again:
Q: What issues should I be aware of when setting up two-way replication?
A: MySQL replication currently does not support any locking protocol between master and slave to guarantee the atomicity of a distributed (cross-server) update. In other words, it is possible for client A to make an update to co-master 1, and in the meantime, before it propagates to co-master 2, client B could make an update to co-master 2 that makes the update of client A work differently than it did on co-master 1. Thus, when the update of client A makes it to co-master 2, it produces tables that are different from what you have on co-master 1, even after all the updates from co-master 2 have also propagated. This means that you should not chain two servers together in a two-way replication relationship unless you are sure that your updates can safely happen in any order, or unless you take care of mis-ordered updates somehow in the client code.
You should also realize that two-way replication actually does not improve performance very much (if at all) as far as updates are concerned. Each server must do the same number of updates, just as you would have a single server do. The only difference is that there is a little less lock contention, because the updates originating on another server are serialized in one slave thread. Even this benefit might be offset by network delays.
Q: How can I use replication to improve performance of my system?
A: You should set up one server
as the master and direct all writes to it. Then configure as many
slaves as you have the budget and rackspace for, and distribute
the reads among the master and the slaves. You can also start the
slaves with the
--delay-key-write=ALL options to
get speed improvements on the slave end. In this case, the slave
MyISAM tables instead of
BDB tables to get
more speed by eliminating transactional overhead.
Q: What should I do to prepare client code in my own applications to use performance-enhancing replication?
A: If the part of your code that is responsible for database access has been properly abstracted/modularized, converting it to run with a replicated setup should be very smooth and easy. Change the implementation of your database access to send all writes to the master, and to send reads to either the master or a slave. If your code does not have this level of abstraction, setting up a replicated system gives you the opportunity and motivation to it clean up. Start by creating a wrapper library or module that implements the following functions:
safe_ in each function name means that the
function takes care of handling all error conditions. You can use
different names for the functions. The important thing is to have
a unified interface for connecting for reads, connecting for
writes, doing a read, and doing a write.
Then convert your client code to use the wrapper library. This may be a painful and scary process at first, but it pays off in the long run. All applications that use the approach just described are able to take advantage of a master/slave configuration, even one involving multiple slaves. The code is much easier to maintain, and adding troubleshooting options is trivial. You need modify only one or two functions; for example, to log how long each statement took, or which statement among those issued gave you an error.
If you have written a lot of code, you may want to automate the conversion task by using the replace utility that comes with standard MySQL distributions, or just write your own conversion script. Ideally, your code uses consistent programming style conventions. If not, then you are probably better off rewriting it anyway, or at least going through and manually regularizing it to use a consistent style.
Q: When and how much can MySQL replication improve the performance of my system?
A: MySQL replication is most beneficial for a system that processes frequent reads and infrequent writes. In theory, by using a single-master/multiple-slave setup, you can scale the system by adding more slaves until you either run out of network bandwidth, or your update load grows to the point that the master cannot handle it.
To determine how many slaves you can use before the added benefits
begin to level out, and how much you can improve performance of
your site, you need to know your query patterns, and to determine
empirically by benchmarking the relationship between the
throughput for reads (reads per second, or
reads) and for writes
writes) on a typical master and a typical
slave. The example here shows a rather simplified calculation of
what you can get with replication for a hypothetical system.
Let's say that system load consists of 10% writes and 90% reads,
and we have determined by benchmarking that
reads is 1200 – 2 ×
writes. In other words, the system can do 1,200
reads per second with no writes, the average write is twice as
slow as the average read, and the relationship is linear. Suppose
that the master and each slave have the same capacity, and that we
have one master and
N slaves. Then we
have for each server (master or slave):
reads = 1200 – 2 ×
reads = 9 ×
N + 1) (reads are split, but writes
replicated to all slaves)
N + 1) + 2 ×
writes = 1200
writes = 1200 / (2 +
N + 1))
The last equation indicates the maximum number of writes for
N slaves, given a maximum possible read
rate of 1,200 per minute and a ratio of nine reads per write.
This analysis yields the following conclusions:
N = 0 (which means we have no
replication), our system can handle about 1200/11 = 109 writes
N = 1, we get up to 184 writes
N = 8, we get up to 400 writes
N = 17, we get up to 480 writes
infinity (and our budget negative infinity), we can get very
close to 600 writes per second, increasing system throughput
about 5.5 times. However, with only eight servers, we increase
it nearly four times.
Note that these computations assume infinite network bandwidth and
neglect several other factors that could be significant on your
system. In many cases, you may not be able to perform a
computation similar to the one just shown that accurately predicts
what will happen on your system if you add
N replication slaves. However,
answering the following questions should help you decide whether
and by how much replication will improve the performance of your
What is the read/write ratio on your system?
How much more write load can one server handle if you reduce the reads?
For how many slaves do you have bandwidth available on your network?
Q: How can I use replication to provide redundancy or high availability?
A: With the currently available features, you would have to set up a master and a slave (or several slaves), and to write a script that monitors the master to check whether it is up. Then instruct your applications and the slaves to change master in case of failure. Some suggestions:
To tell a slave to change its master, use the
CHANGE MASTER TO statement.
A good way to keep your applications informed as to the
location of the master is by having a dynamic DNS entry for
the master. With
bind you can use
nsupdate to dynamically update your DNS.
Run your slaves with the
--log-bin option and without
--log-slave-updates. In this
way, the slave is ready to become a master as soon as you
RESET MASTER, and
CHANGE MASTER TO statement on
the other slaves. For example, assume that you have the
WC \ v WC----> M / | \ / | \ v v v S1 S2 S3
In this diagram,
M means the master,
S the slaves,
clients issuing database writes and reads; clients that issue
only database reads are not represented, because they need not
S3 are slaves running with
--log-bin and without
updates received by a slave from the master are not logged in
the binary log unless
specified, the binary log on each slave is empty initially. If
for some reason
M becomes unavailable, you
can pick one of the slaves to become the new master. For
example, if you pick
WC should be redirected to
S1, which will log updates to its binary
then replicate from
The reason for running the slave without
--log-slave-updates is to
prevent slaves from receiving updates twice in case you cause
one of the slaves to become the new master. Suppose that
Then it will write updates that it receives from
M to its own binary log. When
S2 changes from
S1 as its master, it may receive updates
S1 that it has already received from
Make sure that all slaves have processed any statements in
their relay log. On each slave, issue
IO_THREAD, then check the output of
SHOW PROCESSLIST until you see
Has read all relay log. When this is true
for all slaves, they can be reconfigured to the new setup. On
S1 being promoted to become the
STOP SLAVE and
On the other slaves
CHANGE MASTER TO
represents the real host name of
CHANGE MASTER TO, add all
information about how to connect to
CHANGE MASTER TO, there is no
need to specify the name of the
log file or log position to read from: We know it is the first
binary log file and position 4, which are the defaults for
CHANGE MASTER TO. Finally, use
START SLAVE on
Then instruct all
WC to direct their
S1. From that point on, all
updates statements sent by
S1 are written to the binary log of
S1, which then contains every update
statement sent to
The result is this configuration:
WC / | WC | M(unavailable) \ | \ | v v S1<--S2 S3 ^ | +-------+
M is up again, you must issue on it
CHANGE MASTER TO as
that issued on
S3, so that
M becomes a
S1 and picks up all the
WC writes that it missed while it was down.
M a master again (because it is the
most powerful machine, for example), use the preceding
procedure as if
S1 was unavailable and
M was to be the new master. During this
procedure, do not forget to run
M before making
S3 slaves of
Otherwise, they may pick up old
from before the point at which
Note that there is no synchronization between the different slaves to a master. Some slaves might be ahead of others. This means that the concept outlined in the previous example might not work. In practice, however, the relay logs of different slaves will most likely not be far behind the master, so it would work, anyway (but there is no guarantee).
Q: How do I prevent GRANT and REVOKE statements from replicating to slave machines?
A: Start the server with the
Q: Does replication work on mixed operating systems (for example, the master runs on Linux while slaves run on Mac OS X and Windows)?
Q: Does replication work on mixed hardware architectures (for example, the master runs on a 64-bit machine while slaves run on 32-bit machines)?