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17.1.2.5 Choosing a Method for Data Snapshots

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If the master database contains existing data it is necessary to copy this data to each slave. There are different ways to dump the data from the master database. The following sections describe possible options.

To select the appropriate method of dumping the database, choose between these options:

  • Use the mysqldump tool to create a dump of all the databases you want to replicate. This is the recommended method, especially when using InnoDB.

  • If your database is stored in binary portable files, you can copy the raw data files to a slave. This can be more efficient than using mysqldump and importing the file on each slave, because it skips the overhead of updating indexes as the INSERT statements are replayed. With storage engines such as InnoDB this is not recommended.

17.1.2.5.1 Creating a Data Snapshot Using mysqldump

To create a snapshot of the data in an existing master database, use the mysqldump tool. Once the data dump has been completed, import this data into the slave before starting the replication process.

The following example dumps all databases to a file named dbdump.db, and includes the --master-data option which automatically appends the CHANGE MASTER TO statement required on the slave to start the replication process: 

shell> mysqldump --all-databases --master-data > dbdump.db
Note

If you do not use --master-data, then it is necessary to lock all tables in a separate session manually. See Section 17.1.2.4, “Obtaining the Replication Master Binary Log Coordinates”.

It is possible to exclude certain databases from the dump using the mysqldump tool. If you want to choose which databases to include in the dump, do not use --all-databases. Choose one of these options:

  • Exclude all the tables in the database using --ignore-table option.

  • Name only those databases which you want dumped using the --databases option.

Note

By default, if GTIDs are in use on the master (gtid_mode=ON), mysqldump includes the GTIDs from the gtid_executed set on the master in the dump output to add them to the gtid_purged set on the slave. If you are dumping only specific databases or tables, it is important to note that the value that is included by mysqldump includes the GTIDs of all transactions in the gtid_executed set on the master, even those that changed suppressed parts of the database, or other databases on the server that were not included in the partial dump. Check the description for mysqldump's --set-gtid-purged option to find the outcome of the default behavior for the MySQL Server versions you are using, and how to change the behavior if this outcome is not suitable for your situation.

For more information, see Section 4.5.4, “mysqldump — A Database Backup Program”.

To import the data, either copy the dump file to the slave, or access the file from the master when connecting remotely to the slave.

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17.1.2.5.2 Creating a Data Snapshot Using Raw Data Files

This section describes how to create a data snapshot using the raw files which make up the database. Employing this method with a table using a storage engine that has complex caching or logging algorithms requires extra steps to produce a perfect “point in time” snapshot: the initial copy command could leave out cache information and logging updates, even if you have acquired a global read lock. How the storage engine responds to this depends on its crash recovery abilities.

If you use InnoDB tables, you can use the mysqlbackup command from the MySQL Enterprise Backup component to produce a consistent snapshot. This command records the log name and offset corresponding to the snapshot to be used on the slave. MySQL Enterprise Backup is a commercial product that is included as part of a MySQL Enterprise subscription. See Section 30.2, “MySQL Enterprise Backup Overview” for detailed information.

This method also does not work reliably if the master and slave have different values for ft_stopword_file, ft_min_word_len, or ft_max_word_len and you are copying tables having full-text indexes.

Assuming the above exceptions do not apply to your database, use the cold backup technique to obtain a reliable binary snapshot of InnoDB tables: do a slow shutdown of the MySQL Server, then copy the data files manually.

To create a raw data snapshot of MyISAM tables when your MySQL data files exist on a single file system, you can use standard file copy tools such as cp or copy, a remote copy tool such as scp or rsync, an archiving tool such as zip or tar, or a file system snapshot tool such as dump. If you are replicating only certain databases, copy only those files that relate to those tables. For InnoDB, all tables in all databases are stored in the system tablespace files, unless you have the innodb_file_per_table option enabled.

The following files are not required for replication:

  • Files relating to the mysql database.

  • The master info repository file master.info, if used; the use of this file is now deprecated (see Section 17.2.4, “Replication Relay and Status Logs”).

  • The master's binary log files, with the exception of the binary log index file if you are going to use this to locate the master binary log coordinates for the slave.

  • Any relay log files.

Depending on whether you are using InnoDB tables or not, choose one of the following:

If you are using InnoDB tables, and also to get the most consistent results with a raw data snapshot, shut down the master server during the process, as follows:

  1. Acquire a read lock and get the master's status. See Section 17.1.2.4, “Obtaining the Replication Master Binary Log Coordinates”.

  2. In a separate session, shut down the master server: 

    shell> mysqladmin shutdown

  3. Make a copy of the MySQL data files. The following examples show common ways to do this. You need to choose only one of them: 

    shell> tar cf /tmp/db.tar ./data
shell> zip -r /tmp/db.zip ./data
shell> rsync --recursive ./data /tmp/dbdata

  4. Restart the master server.

If you are not using InnoDB tables, you can get a snapshot of the system from a master without shutting down the server as described in the following steps:

  1. Acquire a read lock and get the master's status. See Section 17.1.2.4, “Obtaining the Replication Master Binary Log Coordinates”.

  2. Make a copy of the MySQL data files. The following examples show common ways to do this. You need to choose only one of them: 

    shell> tar cf /tmp/db.tar ./data
shell> zip -r /tmp/db.zip ./data
shell> rsync --recursive ./data /tmp/dbdata

  3. In the client where you acquired the read lock, release the lock:

    1. mysql> UNLOCK TABLES;

Once you have created the archive or copy of the database, copy the files to each slave before starting the slave replication process.


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Document created the 26/06/2006, last modified the 26/10/2018
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