How to re-sync the Mysql DB if Master and slave have different database incase of Mysql replication?

Question

Mysql Server1 is running as MASTER.
Mysql Server2 is running as SLAVE.

Now DB replication is happening from MASTER to SLAVE.

Server2 is removed from network and re-connect it back after 1 day. After this there is mismatch in database in master and slave.

How to re-sync the DB again as after restoring DB taken from Master to Slave also doesn't solve the problem ?

Answer 1

This is the full step-by-step procedure to resync a master-slave replication from scratch:

At the master:

RESET MASTER;
FLUSH TABLES WITH READ LOCK;
SHOW MASTER STATUS;

And copy the values of the result of the last command somewhere.

Without closing the connection to the client (because it would release the read lock) issue the command to get a dump of the master:

mysqldump -u root -p --all-databases > /a/path/mysqldump.sql

Now you can release the lock, even if the dump hasn't ended yet. To do it, perform the following command in the MySQL client:

UNLOCK TABLES;

Now copy the dump file to the slave using scp or your preferred tool.

At the slave:

Open a connection to mysql and type:

STOP SLAVE;

Load master's data dump with this console command:

mysql -uroot -p < mysqldump.sql

Sync slave and master logs:

RESET SLAVE;
CHANGE MASTER TO MASTER_LOG_FILE='mysql-bin.000001', MASTER_LOG_POS=98;

Where the values of the above fields are the ones you copied before.

Finally, type:

START SLAVE;

To check that everything is working again, after typing:

SHOW SLAVE STATUS;

you should see:

Slave_IO_Running: Yes
Slave_SQL_Running: Yes

That's it!

Answer 2

The documentation for this at the MySQL site is woefully out of date and riddled with foot-guns (such as interactive_timeout). Issuing FLUSH TABLES WITH READ LOCK as part of your export of the master generally only makes sense when coordinated with a storage/filesystem snapshot such as LVM or zfs.

If you are going to use mysqldump, you should rely instead on the --master-data option to guard against human error and release the locks on the master as quickly as possible.

Assume the master is 192.168.100.50 and the slave is 192.168.100.51, each server has a distinct server-id configured, the master has binary logging on and the slave has read-only=1 in my.cnf

To stage the slave to be able to start replication just after importing the dump, issue a CHANGE MASTER command but omit the log file name and position:

slaveserver> CHANGE MASTER TO MASTER_HOST='192.168.100.50', MASTER_USER='replica', MASTER_PASSWORD='asdmk3qwdq1';

Issue the GRANT on the master for the slave to use:

masterserver> GRANT REPLICATION SLAVE ON *.* TO 'replica'@'192.168.100.51' IDENTIFIED BY 'asdmk3qwdq1';

Export the master (in screen) using compression and automatically capturing the correct binary log coordinates:

mysqldump --master-data --all-databases --flush-privileges | gzip -1 > replication.sql.gz

Copy the replication.sql.gz file to the slave and then import it with zcat to the instance of MySQL running on the slave:

zcat replication.sql.gz | mysql

Start replication by issuing the command to the slave:

slaveserver> START SLAVE;

Optionally update the /root/.my.cnf on the slave to store the same root password as the master.

If you are on 5.1+, it is best to first set the master's binlog_format to MIXED or ROW. Beware that row logged events are slow for tables which lack a primary key. This is usually better than the alternative (and default) configuration of binlog_format=statement (on master), since it is less likely to produce the wrong data on the slave.

If you must (but probably shouldn't) filter replication, do so with slave options replicate-wild-do-table=dbname.% or replicate-wild-ignore-table=badDB.% and use only binlog_format=row

This process will hold a global lock on the master for the duration of the mysqldump command but will not otherwise impact the master.

If you are tempted to use mysqldump --master-data --all-databases --single-transaction (because you only using InnoDB tables), you are perhaps better served using MySQL Enterprise Backup or the open source implementation called xtrabackup (courtesy of Percona)

Answer 3

Unless you are writing directly to the slave (Server2) the only problem should be that Server2 is missing any updates that have happened since it was disconnected. Simply restarting the slave with "START SLAVE;" should get everything back up to speed.

Answer 4

I think, Maatkit utilits helps for you! You can use mk-table-sync. Please see this link: http://www.maatkit.org/doc/mk-table-sync.html

Answer 5

I am very late to this question, however I did encounter this problem and, after much searching, I found this information from Bryan Kennedy: http://plusbryan.com/mysql-replication-without-downtime

On Master take a backup like this:
mysqldump --skip-lock-tables --single-transaction --flush-logs --hex-blob --master-data=2 -A > ~/dump.sql

Now, examine the head of the file and jot down the values for MASTER_LOG_FILE and MASTER_LOG_POS. You will need them later: head dump.sql -n80 | grep "MASTER_LOG"

Copy the "dump.sql" file over to Slave and restore it: mysql -u mysql-user -p < ~/dump.sql

Connect to Slave mysql and run a command like this: CHANGE MASTER TO MASTER_HOST='master-server-ip', MASTER_USER='replication-user', MASTER_PASSWORD='slave-server-password', MASTER_LOG_FILE='value from above', MASTER_LOG_POS=value from above; START SLAVE;

To check the progress of Slave: SHOW SLAVE STATUS;

If all is well, Last_Error will be blank, and Slave_IO_State will report “Waiting for master to send event”. Look for Seconds_Behind_Master which indicates how far behind it is. YMMV. :)

Answer 6

Here is what I typically do when a mysql slave gets out of sync. I have looked at mk-table-sync but thought the Risks section was scary looking.

On Master:

SHOW MASTER STATUS

The outputted columns (File, Position) will be of use to us in a bit.

On Slave:

STOP SLAVE

Then dump the master db and import it to the slave db.

Then run the following:

CHANGE MASTER TO
  MASTER_LOG_FILE='[File]',
  MASTER_LOG_POS=[Position];
START SLAVE;

Where [File] and [Position] are the values outputted from the "SHOW MASTER STATUS" ran above.

Hope this helps!

Answer 7

Following up on David's answer...

Using SHOW SLAVE STATUS\G will give human-readable output.

Answer 8

Here is a complete answer that will hopefully help others...

---

I want to setup mysql replication using master and slave, and since the only thing I knew was that it uses log file(s) to synchronize, if the slave goes offline and gets out of sync, in theory it should only need to connect back to its master and keep reading the log file from where it left off, as user malonso mentioned.

So here are the test result after configuring the master and slave as mentioned by: http://dev.mysql.com/doc/refman/5.0/en/replication-howto.html ...

Provided you use the recommended master/slave configuration and don't write to the slave, he and I where right (as far as mysql-server 5.x is concerned). I didn't even need to use "START SLAVE;", it just caught up to its master. But there is a default 88000 something retries every 60 second so I guess if you exhaust that you might have to start or restart the slave. Anyways, for those like me who wanted to know if having a slave going offline and back up again requires manual intervention.. no, it doesn't.

Maybe the original poster had corruption in the log-file(s)? But most probably not just a server going off-line for a day.

---

pulled from /usr/share/doc/mysql-server-5.1/README.Debian.gz which probably makes sense to non debian servers as well:

* FURTHER NOTES ON REPLICATION
===============================
If the MySQL server is acting as a replication slave, you should not
set --tmpdir to point to a directory on a memory-based filesystem or to
a directory that is cleared when the server host restarts. A replication
slave needs some of its temporary files to survive a machine restart so
that it can replicate temporary tables or LOAD DATA INFILE operations. If
files in the temporary file directory are lost when the server restarts,
replication fails.

you can use something sql like: show variables like 'tmpdir'; to find out.

Answer 9

Adding to the popular answer to include this error:

"ERROR 1200 (HY000): The server is not configured as slave; fix in config file or with CHANGE MASTER TO",

Replication from slave in one shot:

In one terminal window:

mysql -h <Master_IP_Address> -uroot -p

After connecting,

RESET MASTER;
FLUSH TABLES WITH READ LOCK;
SHOW MASTER STATUS;

The status appears as below: Note that position number varies!

+------------------+----------+--------------+------------------+
| File             | Position | Binlog_Do_DB | Binlog_Ignore_DB |
+------------------+----------+--------------+------------------+
| mysql-bin.000001 |      98  | your_DB      |                  |
+------------------+----------+--------------+------------------+

Export the dump similar to how he described "using another terminal"!

Exit and connect to your own DB(which is the slave):

mysql -u root -p

The type the below commands:

STOP SLAVE;

Import the Dump as mentioned (in another terminal, of course!) and type the below commands:

RESET SLAVE;
CHANGE MASTER TO 
  MASTER_HOST = 'Master_IP_Address', 
  MASTER_USER = 'your_Master_user', // usually the "root" user
  MASTER_PASSWORD = 'Your_MasterDB_Password', 
  MASTER_PORT = 3306, 
  MASTER_LOG_FILE = 'mysql-bin.000001', 
  MASTER_LOG_POS = 98; // In this case

Once logged, set the server_id parameter (usually, for new / non-replicated DBs, this is not set by default),

set global server_id=4000;

Now, start the slave.

START SLAVE;
SHOW SLAVE STATUS\G;

The output should be the same as he described.

  Slave_IO_Running: Yes
  Slave_SQL_Running: Yes

Note: Once replicated, the master and slave share the same password!

Answer 10

Master:

mysqldump -u root -p --all-databases --master-data | gzip > /tmp/dump.sql.gz  

scp master:/tmp/dump.sql.gz slave:/tmp/ Move dump file to slave server

Slave:

STOP SLAVE;

zcat /tmp/dump.sql.gz | mysql -u root -p

START SLAVE;
SHOW SLAVE STATUS;  

NOTE:
On master you can run SET GLOBAL expire_logs_days = 3 to keep binlogs for 3 days in case of slave issues.

Answer 11

Rebuilding the slave using LVM

Here is the method we use to rebuild MySQL slaves using Linux LVM. This guarantees a consistent snapshot while requiring very minimal downtime on your master.

Set innodb max dirty pages percent to zero on the master MySQL server. This will force MySQL to write all the pages to the disk which will significantly speed up the restart.

set global innodb_max_dirty_pages_pct = 0;

To monitor the number of dirty pages run the command

mysqladmin ext -i10 | grep dirty

Once the number stop decreasing you have reach the point to continue. Next reset the master to clear the old bin logs / relay logs:

RESET MASTER;

Execute lvdisplay to get LV Path

lvdisplay

Output will look like this

--- Logical volume ---
LV Path                /dev/vg_mysql/lv_data
LV Name                lv_data
VG Name                vg_mysql

Shutdown the master database with command

service mysql stop

Next take a snaphot, mysql_snapshot will be the new logical volume name. If binlogs are place on the OS drive those need to be snapshot as well.

lvcreate --size 10G --snapshot --name mysql_snapshot /dev/vg_mysql/lv_data

Start master again with command

service mysql start

Restore dirty pages setting to the default

set global innodb_max_dirty_pages_pct = 75;

Run lvdisplay again to make sure the snapshot is there and visible

lvdisplay

Output:

--- Logical volume ---
LV Path                /dev/vg_mysql/mysql_snapshot
LV Name                mysql_snapshot
VG Name                vg_mysql

Mount the snapshot

mkdir /mnt/mysql_snapshot
mount /dev/vg_mysql/mysql_snapshot /mnt/mysql_snapshot

If you have an existing MySQL slave running you need to stop it

service mysql stop

Next you need to clear MySQL data folder

cd /var/lib/mysql
rm -fr *

Back to master. Now rsync the snapshot to the MySQL slave

rsync --progress -harz /mnt/mysql_snapshot/ targethostname:/var/lib/mysql/

Once rsync has completed you may unmount and remove the snapshot

umount /mnt/mysql_snapshot
lvremove -f /dev/vg_mysql/mysql_snapshot

Create replication user on the master if the old replication user doesn't exist or password is unknown

GRANT REPLICATION SLAVE on *.* to 'replication'@'[SLAVE IP]' identified by 'YourPass';

Verify that /var/lib/mysql data files are owned by the mysql user, if so you can omit the following command:

chown -R mysql:mysql /var/lib/mysql

Next record the binlog position

ls -laF | grep mysql-bin

You will see something like

..
-rw-rw----     1 mysql mysql  1073750329 Aug 28 03:33 mysql-bin.000017
-rw-rw----     1 mysql mysql  1073741932 Aug 28 08:32 mysql-bin.000018
-rw-rw----     1 mysql mysql   963333441 Aug 28 15:37 mysql-bin.000019
-rw-rw----     1 mysql mysql    65657162 Aug 28 16:44 mysql-bin.000020

Here the master log file is the highest file number in sequence and bin log position is the file size. Record these values:

master_log_file=mysql-bin.000020
master_log_post=65657162

Next start the slave MySQL

service mysql start

Execute change master command on the slave by executing the following:

CHANGE MASTER TO 
master_host="10.0.0.12", 
master_user="replication", 
master_password="YourPass", 
master_log_file="mysql-bin.000020", 
master_log_pos=65657162; 

Finally start the slave

SLAVE START;

Check slave status:

SHOW SLAVE STATUS;

Make sure Slave IO is running and there are no connection errors. Good luck!

I recently wrote this on my blog which is found here... There are few more details there but the story is the same.

http://www.juhavehnia.com/2015/05/rebuilding-mysql-slave-using-linux-lvm.html