Rechercher dans le manuel MySQL

17.3.1.3 Backing Up a Master or Slave by Making It Read Only

Contents

It is possible to back up either master or slave servers in a replication setup by acquiring a global read lock and manipulating the read_only system variable to change the read-only state of the server to be backed up:

  1. Make the server read-only, so that it processes only retrievals and blocks updates.

  2. Perform the backup.

  3. Change the server back to its normal read/write state.

Note

The instructions in this section place the server to be backed up in a state that is safe for backup methods that get the data from the server, such as mysqldump (see Section 4.5.4, “mysqldump — A Database Backup Program”). You should not attempt to use these instructions to make a binary backup by copying files directly because the server may still have modified data cached in memory and not flushed to disk.

The following instructions describe how to do this for a master server and for a slave server. For both scenarios discussed here, suppose that you have the following replication setup:

  • A master server M1

  • A slave server S1 that has M1 as its master

  • A client C1 connected to M1

  • A client C2 connected to S1

In either scenario, the statements to acquire the global read lock and manipulate the read_only variable are performed on the server to be backed up and do not propagate to any slaves of that server.

Scenario 1: Backup with a Read-Only Master

Put the master M1 in a read-only state by executing these statements on it:

  1. mysql> FLUSH TABLES WITH READ LOCK;
  2. mysql> SET GLOBAL read_only = ON;

While M1 is in a read-only state, the following properties are true:

  • Requests for updates sent by C1 to M1 will block because the server is in read-only mode.

  • Requests for query results sent by C1 to M1 will succeed.

  • Making a backup on M1 is safe.

  • Making a backup on S1 is not safe. This server is still running, and might be processing the binary log or update requests coming from client C2.

While M1 is read only, perform the backup. For example, you can use mysqldump.

After the backup operation on M1 completes, restore M1 to its normal operational state by executing these statements:

  1. mysql> SET GLOBAL read_only = OFF;
  2. mysql> UNLOCK TABLES;

Although performing the backup on M1 is safe (as far as the backup is concerned), it is not optimal for performance because clients of M1 are blocked from executing updates.

This strategy applies to backing up a master server in a replication setup, but can also be used for a single server in a nonreplication setting.

Scenario 2: Backup with a Read-Only Slave

Put the slave S1 in a read-only state by executing these statements on it:

  1. mysql> FLUSH TABLES WITH READ LOCK;
  2. mysql> SET GLOBAL read_only = ON;

While S1 is in a read-only state, the following properties are true:

  • The master M1 will continue to operate, so making a backup on the master is not safe.

  • The slave S1 is stopped, so making a backup on the slave S1 is safe.

These properties provide the basis for a popular backup scenario: Having one slave busy performing a backup for a while is not a problem because it does not affect the entire network, and the system is still running during the backup. In particular, clients can still perform updates on the master server, which remains unaffected by backup activity on the slave.

While S1 is read only, perform the backup. For example, you can use mysqldump.

After the backup operation on S1 completes, restore S1 to its normal operational state by executing these statements:

  1. mysql> SET GLOBAL read_only = OFF;
  2. mysql> UNLOCK TABLES;

After the slave is restored to normal operation, it again synchronizes to the master by catching up with any outstanding updates from the binary log of the master.


Find a PHP function

Document created the 26/06/2006, last modified the 26/10/2018
Source of the printed document:https://www.gaudry.be/en/mysql-rf-replication-solutions-backups-read-only.html

The infobrol is a personal site whose content is my sole responsibility. The text is available under CreativeCommons license (BY-NC-SA). More info on the terms of use and the author.

References

  1. View the html document Language of the document:en Manuel MySQL : https://dev.mysql.com/

These references and links indicate documents consulted during the writing of this page, or which may provide additional information, but the authors of these sources can not be held responsible for the content of this page.
The author This site is solely responsible for the way in which the various concepts, and the freedoms that are taken with the reference works, are presented here. Remember that you must cross multiple source information to reduce the risk of errors.

Contents Haut