Moving InnoDB Table Between Servers

This is a small article telling how to move a innoDB table from one MySQL server to another.

Moving MyISAM table from one server to another is very straight forward process. MyISAM table generates 3 files at file system level.

  • .frm file contains table structure/definition
  • .MYD file contains table data
  • .MYI file contains index data

We can simply copy these 3 files to another MySQL server installation under desired database and it will show up in “SHOW TABLES” command.

But things are different in InnoDB, because InnoDB also stores metadata in default tablespace represented by ibdata* datafiles in data directory.

InnoDB table has 2 types of files generated

  • .frm file contains table structure/definition
  • .ibd file contains data of table as well as index

Apart from above files, we also have metadata like table ID, database name and transaction information about tables in default tablespace (ibdata1 datafile).

This makes InnoDB table special and, simply copying .frm and .ibd files from source server to destination server will not migrate/copy InnoDB table to different server.

We need to do additional steps to complete the copy/migration. Please follow below steps to copy InnoDB table from one server to another.

 

Step 1) On destination database, create required database if not available.

So lets say, I want to move one table called flavors from fctest database on source server to destination server.

For that to happen, I have to make sure that on destination server, I have fctest database created. If its not created, create the same.

You need to do this, even if you want to move this table to some other existing database on destination server.

We have to first move the table to fctest database on destination (same as source) and then we will rename the same to desired database.

On destination server:

root [deo] >show databases;
+--------------------+
| Database           |
+--------------------+
| information_schema |
| deo                |
| menagerie          |
| mysql              |
| performance_schema |
| test               |
+--------------------+
6 rows in set (0.00 sec)

I don’t have fctest database. Creating the same.

root [deo] >create database fctest;
Query OK, 1 row affected (0.00 sec)

root [deo] >
root [deo] >use fctest;
Database changed
root [fctest] >

Step 2) Create empty table structure same as source table

Once you create the database fctest, create empty table flavors whose structure/definition is same as source.

You can get the definition from source using “SHOW CREATE TABLE” command

On Source server:

mysql> show create table flavors;
+---------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table   | Create Table                                                                                                                                                                                                                                                                                                                                                                                       |
+---------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| flavors | CREATE TABLE `flavors` (
  `flavor_id` int(11) NOT NULL AUTO_INCREMENT,
  `build_type` varchar(255) NOT NULL DEFAULT '',
  `variant` varchar(255) NOT NULL DEFAULT '',
  `threads` varchar(255) NOT NULL DEFAULT '',
  `first_event_id` int(11) NOT NULL DEFAULT '0',
  `last_event_id` int(11) DEFAULT NULL,
  PRIMARY KEY (`flavor_id`)
) ENGINE=InnoDB AUTO_INCREMENT=30 DEFAULT CHARSET=latin1 |
+---------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

Create the same on destination server

On Destination server:

root [fctest] >CREATE TABLE flavors (
    ->   flavor_id int(11) NOT NULL AUTO_INCREMENT,
    ->   build_type varchar(255) NOT NULL DEFAULT '',
    ->   variant varchar(255) NOT NULL DEFAULT '',
    ->   threads varchar(255) NOT NULL DEFAULT '',
    ->   first_event_id int(11) NOT NULL DEFAULT '0',
    ->   last_event_id int(11) DEFAULT NULL,
    ->   PRIMARY KEY (flavor_id))
    ->  ENGINE=InnoDB;
Query OK, 0 rows affected (0.00 sec)
root [fctest] >show tables;
+------------------+
| Tables_in_fctest |
+------------------+
| flavors          |
+------------------+
1 row in set (0.00 sec)

Step 3) Discard tablespace on destination server

This step is required so that MySQL will discard current .ibd file generated for this table.

root [fctest] >ALTER TABLE flavors DISCARD TABLESPACE;
Query OK, 0 rows affected (0.00 sec)

Step 4) Copy .frm and .ibd file from source server to destination server

At this point copy the .frm and .ibb files from source server to destination server.

Before copying these files, make sure that no transactions are happening on this table (which you are migrating).

Safest way is to use “FLUSH TABLE <table_name> WITH READ LOCK” on source database which will flush table data and takes read only lock so that no DML will happen on the table.

While you keep that session active, copy the .frm and .ibd files to destination server.

Step 5) Import tablespace on destination server

Once you copy .frm and .ibd files to destination server, import the tablespace, which will make required metadata imported into default tablespace and accept the .ibd file.

root [fctest] >ALTER TABLE flavors IMPORT TABLESPACE;
Query OK, 0 rows affected (0.00 sec)
root [fctest] >show tables;
+------------------+
| Tables_in_fctest |
+------------------+
| flavors          |
+------------------+
1 row in set (0.00 sec)
root [fctest] >select count(1) from flavors;
+----------+
| count(1) |
+----------+
|       29 |
+----------+
1 row in set (0.00 sec)

Step 6) Rename the table to desired database

Since we wanted to move this table to deo database on destination server, we can simply run below rename command and it will move the tables to deo database.

root [fctest] >rename table fctest.flavors to deo.flavors;
Query OK, 0 rows affected (0.00 sec)

you can later drop fctest database on destination server.

Hope this helps !!

Disabling GTID in MySQL replication

There could be situations where we have to disable GTID in our replication setup.
We can follow below steps to do so. No need to change any GTID related parameters. We just have to change slave configuration and bounce slave. No need to touch master.

Step 1) Confirm that you are using GTID

If you check processlist on master, you can see GTID is enabled

master> show processlist \G
*************************** 1. row ***************************
     Id: 51
   User: replicate
   Host: master-1.db.example.com:43420
     db: NULL
Command: Binlog Dump GTID
   Time: 81316
  State: Master has sent all binlog to slave; waiting for binlog to be updated
   Info: NULL

Check the “command” above and it says “Binlog Dump GTID

 

Step 2) Stop slave, reset master_auto_position to 0


mysql> stop slave;
Query OK, 0 rows affected (0.00 sec)

mysql> change master to master_auto_position=0;
Query OK, 0 rows affected (0.00 sec)

Step 3) Check slave status for master log file and position

mysql> show slave status \G
*************************** 1. row ***************************
               Slave_IO_State:
                  Master_Host: master-1.db.example.com
                  Master_User: replicate
                  Master_Port: 3306
                Connect_Retry: 5
              Master_Log_File: bin_log.000003
          Read_Master_Log_Pos: 191
               Relay_Log_File: relay_log.000001
                Relay_Log_Pos: 4
        Relay_Master_Log_File: bin_log.000003
             Slave_IO_Running: No
            Slave_SQL_Running: No
              Replicate_Do_DB:
          Replicate_Ignore_DB:
           Replicate_Do_Table:
       Replicate_Ignore_Table:
       ...
       ...
           Master_SSL_Crlpath:
           Retrieved_Gtid_Set:
            Executed_Gtid_Set: 8c593930-b4da-11e3-9e2d-b499ba05473c:1-7,
cc4192b6-a60d-11e4-8301-80c16e2272cc:1-168
                Auto_Position: 0
1 row in set (0.01 sec)

 

Step 4) Use change master to point to master log file and position


mysql> change master to master_host='master-1.db.example.com' , master_port = 3306 , master_user = 'replicate' , master_password='welcome' , master_log_file = 'bin_log.000003' , master_log_pos = 191, master_connect_retry = 5;
Query OK, 0 rows affected, 2 warnings (0.00 sec)

mysql> show warnings;
+-------+------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Level | Code | Message                                                                                                                                                                                                                                                                              |
+-------+------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Note  | 1759 | Sending passwords in plain text without SSL/TLS is extremely insecure.                                                                                                                                                                                                               |
| Note  | 1760 | Storing MySQL user name or password information in the master info repository is not secure and is therefore not recommended. Please consider using the USER and PASSWORD connection options for START SLAVE; see the 'START SLAVE Syntax' in the MySQL Manual for more information. |
+-------+------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
2 rows in set (0.00 sec)

 

Step 5) start slave


mysql> start slave;
Query OK, 0 rows affected (0.00 sec)

show slave status will still show GTIDs but its not using GTIDs now

mysql> show slave status \G
*************************** 1. row ***************************
               Slave_IO_State: Waiting for master to send event
                  Master_Host: master-1.db.example.com
                  Master_User: replicate
                  Master_Port: 3306
                Connect_Retry: 5
              Master_Log_File: bin_log.000003
          Read_Master_Log_Pos: 465
               Relay_Log_File: relay_log.000002
                Relay_Log_Pos: 586
        Relay_Master_Log_File: bin_log.000003
             Slave_IO_Running: Yes
            Slave_SQL_Running: Yes
              Replicate_Do_DB:
              ...
              ...
     Last_SQL_Error_Timestamp:
               Master_SSL_Crl:
           Master_SSL_Crlpath:
           Retrieved_Gtid_Set: 8c593930-b4da-11e3-9e2d-b499ba05473c:8
            Executed_Gtid_Set: 8c593930-b4da-11e3-9e2d-b499ba05473c:1-8,
cc4192b6-a60d-11e4-8301-80c16e2272cc:1-168
                Auto_Position: 0
1 row in set (0.00 sec)

 

We can confirm that GTID is not getting used by checking processlist on master

master>show processlist \G
*************************** 1. row ***************************
     Id: 1
   User: root
   Host: localhost
     db: deo
Command: Query
   Time: 0
  State: init
   Info: show processlist
*************************** 2. row ***************************
     Id: 3
   User: replicate
   Host: master-1.db.example.com:59631
     db: NULL
Command: Binlog Dump
   Time: 24
  State: Master has sent all binlog to slave; waiting for binlog to be updated
   Info: NULL
2 rows in set (0.00 sec)

If you check “command” for row 2, you will notice GTID is gone.

Hope this helps !!

Semi-synchronous Replication in MySQL

If we talk about MySQL replication, we always default to asynchronous replication. By asynchronous, we mean master will keep doing transactions while they are getting replicated to slave via IO thread asynchronously. In this replication, its possible that slave will fall behind but that does not impact master in anyway.
Problem with this replication is data-loss. Example: In case slave is falling behind and master host crashes, some of the transactions may not get transfered/replicated to slave. Following 2 options exists in this scenario

  1. Wait for master host to come up. Which may take around 20-25 mins
  2. Promote slave to master with some data loss

Nether options sounds good on production database. That’s why we have semi-synchronous replication.

Purpose of semi-synchronous replication is twofold

  1. Reduce replication lag and keep slave in sync with master
  2. Ensure no data-loss in case master crashes

Semi-synchronous replication has been in place from MySQL 5.5 onwards in the form of plugin. We just have to install the plugin and then we can enable the same. But again this semi-synchronous replication available in MySQL 5.5 and 5.6 does not guarantee “no data loss” completely for the reasons we will see later but it keeps slave in sync with master.

MySQL 5.7 Oracle has made some enhanced semi-synchronous replication which they call “loss-less semi-synchronous replication”. This guarantees that there is no data loss in case master crashes.

We will take a look at how semi-synchronous replication works, how to install the plugin and whats the enhancement in MySQL 5.7 to prevent data loss.

How Semi-Synchronous replication works?

As discussed above, there are 2 different algorithm for semi-synchronous replication.

  1. Normal semi-synchronous replication available in mysql 5.5 and 5.6 – does not guarantee “no data loss”
  2. Loss-less semi-synchronous replication available in mysql 5.7 – guarantee “no data loss”

lets take a look at each and how it works

1) Normal semi-synchronous replication in MySQL 5.5 and 5.6

If we talk above true synchronous replication – In truly synchronous replication, when you commit a transaction, the commit does not complete on master until all replicas have also committed successfully and acknowledged to master.

In MySQL’s semi-synchronous replication when transaction is committed on master, its written to binary log and innodb transaction log and commit is complete. At this point the data has not even reached to any of the slave but on master commit is complete. The client issuing the commit is still on hold and is not told that commit has complete.
At this point slave IO thread pulls transaction data from binary logs and updated relay logs, flush relay logs to disk and acknowledges master that slave has received and written the data. Only after receiving ack from at least 1 slave, MySQL confirms the client that transaction is committed.

There is a problem with above functionality. MySQL is only delaying acknowledge of commit to client. In reality its committing the transaction on master way before that (before even sending data to slave). Since committing of transaction is done before waiting for ack from semi-sync slave, so the committed rows are visible from applications, even though semi-sync slaves may not have received the data. If master is crashed and none of the slaves received the data, the data is lost but applications may have seen them. This is called phantom reads, and in many cases it’s problematic.

2) Loss-less semi-synchronous replication in MySQL 5.7

loss-less semi-synchronous replication in MySQL 5.7 answers above problem. Only change that’s done in this semi-synchronous replication is that commit on primary is delayed until master receives acknowledgement from only of the semi-synchronous slave that data is received and written by the slave. So when committed data is visible from applications, one of the semi-sync slaves have received that. Phantom read risk is much smaller: if both master and the latest semi-sync slave are down at the same time, data is lost. But it’s much less likely to happen compared to normal semi-sync.

To avoid data loss and phantom reads, Normal Semi-sync can’t meet your expectations. Using Loss-Less Semi-sync is needed.
With Loss-Less Semi-Synchronous replication, committed data should be on one of the slaves, so you can recover from the latest slave. You can always do fast failover here.

The acknowledgement from the slave means, it has received the transaction and applied it to the relay log and flushed it to disk. It doesn’t mean the data has actually gone into the database.

Just to make this statement clearer, it means the transaction was written to the relay log, and the relay log was flushed to disk.
There is still the need for the SQL thread to pick it up and apply it to the database. It’s very unlikely not to complete this task, but that way we’re getting a bit more performance.

Implementing semi-synchronous replication

1) check if semi-synchronous plugin is installed using “show plugins;”


mysql> show plugins;
+----------------------------+----------+--------------------+---------+---------+
| Name                       | Status   | Type               | Library | License |
+----------------------------+----------+--------------------+---------+---------+
| binlog                     | ACTIVE   | STORAGE ENGINE     | NULL    | GPL     |
| mysql_native_password      | ACTIVE   | AUTHENTICATION     | NULL    | GPL     |
| mysql_old_password         | ACTIVE   | AUTHENTICATION     | NULL    | GPL     |
| sha256_password            | ACTIVE   | AUTHENTICATION     | NULL    | GPL     |
| MRG_MYISAM                 | ACTIVE   | STORAGE ENGINE     | NULL    | GPL     |
| MyISAM                     | ACTIVE   | STORAGE ENGINE     | NULL    | GPL     |
| MEMORY                     | ACTIVE   | STORAGE ENGINE     | NULL    | GPL     |
| CSV                        | ACTIVE   | STORAGE ENGINE     | NULL    | GPL     |
| InnoDB                     | ACTIVE   | STORAGE ENGINE     | NULL    | GPL     |
| INNODB_TRX                 | ACTIVE   | INFORMATION SCHEMA | NULL    | GPL     |
| INNODB_LOCKS               | ACTIVE   | INFORMATION SCHEMA | NULL    | GPL     |
...
...
| PERFORMANCE_SCHEMA         | ACTIVE   | STORAGE ENGINE     | NULL    | GPL     |
| BLACKHOLE                  | ACTIVE   | STORAGE ENGINE     | NULL    | GPL     |
| FEDERATED                  | DISABLED | STORAGE ENGINE     | NULL    | GPL     |
| ARCHIVE                    | ACTIVE   | STORAGE ENGINE     | NULL    | GPL     |
| partition                  | ACTIVE   | STORAGE ENGINE     | NULL    | GPL     |
+----------------------------+----------+--------------------+---------+---------+
42 rows in set (0.01 sec)

You should be looking for “rpl_semi_sync_master” name.
In my case plugin was not installed.

Optionally you can also check system variables


mysql> show variables like 'rpl_semi_sync%';
Empty set (0.00 sec)

Nothing. That means semi-synchronous functionality is not installed.

2) Install semi-synchronous plugin

You need to install “semisync_master.so” on master and “semisync_slave.so” on slave

on Master:


mysql> INSTALL PLUGIN rpl_semi_sync_master SONAME 'semisync_master.so';
Query OK, 0 rows affected (0.12 sec)

on Slave:


mysql> INSTALL PLUGIN rpl_semi_sync_slave SONAME 'semisync_slave.so';
Query OK, 0 rows affected (0.00 sec)

These .so plugin files are available in <mysql_install_dir>/lib/plugin directory.

“show plugins;” should show the plugin now


mysql> show plugins;
+----------------------------+----------+--------------------+--------------------+---------+
| Name                       | Status   | Type               | Library            | License |
+----------------------------+----------+--------------------+--------------------+---------+
| binlog                     | ACTIVE   | STORAGE ENGINE     | NULL               | GPL     |
| mysql_native_password      | ACTIVE   | AUTHENTICATION     | NULL               | GPL     |
| mysql_old_password         | ACTIVE   | AUTHENTICATION     | NULL               | GPL     |
| sha256_password            | ACTIVE   | AUTHENTICATION     | NULL               | GPL     |
| MRG_MYISAM                 | ACTIVE   | STORAGE ENGINE     | NULL               | GPL     |
...
...
| rpl_semi_sync_master       | ACTIVE   | REPLICATION        | semisync_master.so | GPL     |
+----------------------------+----------+--------------------+--------------------+---------+
43 rows in set (0.00 sec)

You should also see the variables


mysql> show variables like 'rpl_semi_syn%';
+------------------------------------+-------+
| Variable_name                      | Value |
+------------------------------------+-------+
| rpl_semi_sync_master_enabled       | OFF   |
| rpl_semi_sync_master_timeout       | 10000 |
| rpl_semi_sync_master_trace_level   | 32    |
| rpl_semi_sync_master_wait_no_slave | ON    |
+------------------------------------+-------+
4 rows in set (0.00 sec)

3) Enable semi-synchronous replication

On master:


mysql> SET GLOBAL rpl_semi_sync_master_enabled = 1;
Query OK, 0 rows affected (0.03 sec)

mysql> SET GLOBAL rpl_semi_sync_master_timeout = 10000;
Query OK, 0 rows affected (0.00 sec)

rpl_semi_sync_master_timeout is the timeout in millisec. We are setting a timeout of 10 sec.

On slave:


mysql> SET GLOBAL rpl_semi_sync_slave_enabled = 1;
Query OK, 0 rows affected (0.00 sec)

mysql> stop slave io_thread;
Query OK, 0 rows affected (0.00 sec)

mysql> start slave io_thread;
Query OK, 0 rows affected (0.00 sec)

we have to bounce IO_THREAD on slave so that it will connect back to master as semi-synchronous slave.

Monitoring Semi-synchronous replication

We have few status variables that we can monitor to check if semi-synchronous replication is working as expected or not.

If you check status variables on master, it shows following status variables.


mysql> SHOW STATUS LIKE 'Rpl_semi_sync%';
+--------------------------------------------+-------+
| Variable_name                              | Value |
+--------------------------------------------+-------+
| Rpl_semi_sync_master_clients               | 1     |
| Rpl_semi_sync_master_net_avg_wait_time     | 0     |
| Rpl_semi_sync_master_net_wait_time         | 0     |
| Rpl_semi_sync_master_net_waits             | 0     |
| Rpl_semi_sync_master_no_times              | 0     |
| Rpl_semi_sync_master_no_tx                 | 0     |
| Rpl_semi_sync_master_status                | ON    |
| Rpl_semi_sync_master_timefunc_failures     | 0     |
| Rpl_semi_sync_master_tx_avg_wait_time      | 0     |
| Rpl_semi_sync_master_tx_wait_time          | 0     |
| Rpl_semi_sync_master_tx_waits              | 0     |
| Rpl_semi_sync_master_wait_pos_backtraverse | 0     |
| Rpl_semi_sync_master_wait_sessions         | 0     |
| Rpl_semi_sync_master_yes_tx                | 0     |
+--------------------------------------------+-------+
14 rows in set (0.00 sec)

On replication slave we have only 1 variable Rpl_semi_sync_slave_status.

Among the variables mentioned above, 3 varaibles are important for us to watch

Rpl_semi_sync_master_status -> Whether semi-synchronous replication currently is operational on the master
Rpl_semi_sync_master_yes_tx -> The number of commits that were acknowledged successfully by a slave
Rpl_semi_sync_master_no_tx  -> The number of commits that were not acknowledged successfully by a slave

Currently we see that Rpl_semi_sync_master_status is ON, that means the semi-synchronous replication is working fine.

Lets create a table in “deo” database and insert few records.

On master:


mysql> use deo;
Reading table information for completion of table and column names
You can turn off this feature to get a quicker startup with -A

Database changed
mysql> create table test (col1 smallint not null);
Query OK, 0 rows affected (0.14 sec)

mysql> set autocommit=1;
Query OK, 0 rows affected (0.00 sec)

mysql> insert into test values (1),(2),(3);
Query OK, 3 rows affected (0.02 sec)
Records: 3  Duplicates: 0  Warnings: 0

mysql> select * from test;
+------+
| col1 |
+------+
|    1 |
|    2 |
|    3 |
+------+
3 rows in set (0.00 sec)
mysql> show status like 'Rpl_semi_sync%';
+--------------------------------------------+-------+
| Variable_name                              | Value |
+--------------------------------------------+-------+
| Rpl_semi_sync_master_clients               | 1     |
| Rpl_semi_sync_master_net_avg_wait_time     | 1356  |
| Rpl_semi_sync_master_net_wait_time         | 4070  |
| Rpl_semi_sync_master_net_waits             | 3     |
| Rpl_semi_sync_master_no_times              | 0     |
| Rpl_semi_sync_master_no_tx                 | 0     |
| Rpl_semi_sync_master_status                | ON    |
| Rpl_semi_sync_master_timefunc_failures     | 0     |
| Rpl_semi_sync_master_tx_avg_wait_time      | 1379  |
| Rpl_semi_sync_master_tx_wait_time          | 4139  |
| Rpl_semi_sync_master_tx_waits              | 3     |
| Rpl_semi_sync_master_wait_pos_backtraverse | 0     |
| Rpl_semi_sync_master_wait_sessions         | 0     |
| Rpl_semi_sync_master_yes_tx                | 3     |
+--------------------------------------------+-------+
14 rows in set (0.00 sec)

If you see above, we have 3 transactions done (corrosponding to 3 inserts since autocommit was on) and all are replicated synchronously on slave.

Rpl_semi_sync_master_yes_tx has become 3 and Rpl_semi_sync_master_no_tx is still 0.

Lets stop replication IO thread so that changes in master will not get replicated on slave

on slave:


mysql> stop slave io_thread;
Query OK, 0 rows affected (0.00 sec)

Now, lets insert a record in test table in master

On master:


mysql> insert into test values (4);
Query OK, 1 row affected (10.01 sec)

mysql> show status like 'Rpl_semi_sync%';
+--------------------------------------------+-------+
| Variable_name                              | Value |
+--------------------------------------------+-------+
| Rpl_semi_sync_master_clients               | 1     |
| Rpl_semi_sync_master_net_avg_wait_time     | 1022  |
| Rpl_semi_sync_master_net_wait_time         | 4090  |
| Rpl_semi_sync_master_net_waits             | 4     |
| Rpl_semi_sync_master_no_times              | 1     |
| Rpl_semi_sync_master_no_tx                 | 1     |
| Rpl_semi_sync_master_status                | OFF   |
| Rpl_semi_sync_master_timefunc_failures     | 0     |
| Rpl_semi_sync_master_tx_avg_wait_time      | 1379  |
| Rpl_semi_sync_master_tx_wait_time          | 4139  |
| Rpl_semi_sync_master_tx_waits              | 3     |
| Rpl_semi_sync_master_wait_pos_backtraverse | 0     |
| Rpl_semi_sync_master_wait_sessions         | 0     |
| Rpl_semi_sync_master_yes_tx                | 3     |
+--------------------------------------------+-------+
14 rows in set (0.00 sec)

If you see, Rpl_semi_sync_master_no_tx is updated to 1 and Rpl_semi_sync_master_status is updated to off.
It means that replication is changed back to asynchronous.

Also note that insert statement took 10 secs to complete, because semi-sync replication timeout was set to 10 sec (rpl_semi_sync_master_timeout = 10000)

We can now start the slave IO thread and it will be back to semi-synchronous replication

on slave:


mysql> start slave io_thread;
Query OK, 0 rows affected (0.00 sec)

mysql> select * from test;
+------+
| col1 |
+------+
|    1 |
|    2 |
|    3 |
|    4 |
+------+
4 rows in set (0.00 sec)

On master:


mysql> show status like 'Rpl_semi_sync%';
+--------------------------------------------+-------+
| Variable_name                              | Value |
+--------------------------------------------+-------+
| Rpl_semi_sync_master_clients               | 1     |
| Rpl_semi_sync_master_net_avg_wait_time     | 1626  |
| Rpl_semi_sync_master_net_wait_time         | 8131  |
| Rpl_semi_sync_master_net_waits             | 5     |
| Rpl_semi_sync_master_no_times              | 1     |
| Rpl_semi_sync_master_no_tx                 | 1     |
| Rpl_semi_sync_master_status                | ON    |
| Rpl_semi_sync_master_timefunc_failures     | 0     |
| Rpl_semi_sync_master_tx_avg_wait_time      | 1379  |
| Rpl_semi_sync_master_tx_wait_time          | 4139  |
| Rpl_semi_sync_master_tx_waits              | 3     |
| Rpl_semi_sync_master_wait_pos_backtraverse | 0     |
| Rpl_semi_sync_master_wait_sessions         | 0     |
| Rpl_semi_sync_master_yes_tx                | 3     |
+--------------------------------------------+-------+
14 rows in set (0.00 sec)

So after starting slave IO thread, it recovered immediately and semi-synchronous replication enabled automatically.

Reference:

http://yoshinorimatsunobu.blogspot.in/2014/04/semi-synchronous-replication-at-facebook.html
http://dev.mysql.com/doc/refman/5.5/en/replication-semisync.html
http://www.percona.com/blog/2012/01/19/how-does-semisynchronous-mysql-replication-work/

Hope this help !!