How to re-sync the Mysql DB if Master and slave ha

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

We are using master-master replication technique of MySQL and if one MySQL server say 1 is removed from the network it reconnects itself after the connection are restored and all the records that were committed in the in the server 2 which was in the network are transferred to the server 1 which has lost the connection after restoration. Slave thread in the MySQL retries to connect to its master after every 60 sec by default. This property can be changed as MySQL ha a flag "master_connect_retry=5" where 5 is in sec. This means that we want a retry after every 5 sec.

But you need to make sure that the server which lost the connection show not make any commit in the database as you get duplicate Key error Error code: 1062

Answer 2

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 3

I created a GitHub repo with an script to solve this problem quickly. Just change a couple variables and run it (First, the script creates a backup of your database).

I hope this help you (and others people too).

How to Reset (Re-Sync) MySQL Master-Slave Replication

Answer 4

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 5

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 6

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\G

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

BR, Juha Vehnia

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