This section describes the characteristics of different types of backups.
Logical Versus Physical (Raw) Backups
Logical backups save information represented as logical database
CREATE TABLE statements) and
INSERT statements or
delimited-text files). Physical backups consist of raw copies of
the directories and files that store database contents.
Logical backup methods have these characteristics:
The backup is done by querying the MySQL server to obtain database structure and content information.
Backup is slower than physical methods because the server must access database information and convert it to logical format. If the output is written on the client side, the server must also send it to the backup program.
Output is larger than for physical backup, particularly when saved in text format.
Backup and restore granularity is available at the server level (all databases), database level (all tables in a particular database), or table level. This is true regardless of storage engine.
The backup does not include log or configuration files, or other database-related files that are not part of databases.
Backups stored in logical format are machine independent and highly portable.
Logical backups are performed with the MySQL server running. The server is not taken offline.
Physical backup methods have these characteristics:
The backup consists of exact copies of database directories
and files. Typically this is a copy of all or part of the
MySQL data directory. Data from
tables cannot be backed up this way because their contents are
not stored on disk.
Physical backup methods are faster than logical because they involve only file copying without conversion.
Output is more compact than for logical backup.
Backup and restore granularity ranges from the level of the
entire data directory down to the level of individual files.
This may or may not provide for table-level granularity,
depending on storage engine. (Each
table corresponds uniquely to a set of files, but an
InnoDB table shares file storage with other
In addition to databases, the backup can include any related files such as log or configuration files.
Backups are portable only to other machines that have identical or similar hardware characteristics.
Backups can be performed while the MySQL server is not running. If the server is running, it is necessary to perform appropriate locking so that the server does not change database contents during the backup.
For restore, files copied at the file system level or with
mysqlhotcopy can be copied back to their
original locations with file system commands;
tables, and ndb_restore restores
Online Versus Offline Backups
Online backups take place while the MySQL server is running so that the database information can be obtained from the server. Offline backups take place while the server is stopped. This distinction can also be described as “hot” versus “cold” backups; a “warm” backup is one where the server remains running but locked against modifying data while you access database files externally.
Online backup methods have these characteristics:
The backup is less intrusive to other clients, which can connect to the MySQL server during the backup and may be able to access data depending on what operations they need to perform.
Care must be taken to impose appropriate locking so that data modifications do not take place that would compromise backup integrity.
Offline backup methods have these characteristics:
Clients can be affected adversely because the server is unavailable during backup.
The backup procedure is simpler because there is no possibility of interference from client activity.
A similar distinction between online and offline applies for recovery operations, and similar characteristics apply. However, it is more likely that clients will be affected for online recovery than for online backup because recovery requires stronger locking. During backup, clients might be able to read data while it is being backed up. Recovery modifies data and does not just read it, so clients must be prevented from accessing data while it is being restored.
Local Versus Remote Backups
A local backup is performed on the same host where the MySQL server runs, whereas a remote backup is done from a different host. For some types of backups, the backup can be initiated from a remote host even if the output is written locally on the server. host.
mysqldump can connect to local or remote
servers. For SQL output (
INSERT statements), local or
remote dumps can be done and generate output on the client.
For delimited-text output (with the
--tab option), data files
are created on the server host.
mysqlhotcopy performs only local backups: It connects to the server to lock it against data modifications and then copies local table files.
SELECT ... INTO
OUTFILE can be initiated from a local or remote
client host, but the output file is created on the server
Physical backup methods typically are initiated locally on the MySQL server host so that the server can be taken offline, although the destination for copied files might be remote.
Some file system implementations enable “snapshots” to be taken. These provide logical copies of the file system at a given point in time, without requiring a physical copy of the entire file system. (For example, the implementation may use copy-on-write techniques so that only parts of the file system modified after the snapshot time need be copied.) MySQL itself does not provide the capability for taking file system snapshots. It is available through third-party solutions such as Veritas, LVM, or ZFS.
Full Versus Incremental Backups
A full backup includes all data managed by a MySQL server at a given point in time. An incremental backup consists of the changes made to the data during a given time span (from one point in time to another). MySQL has different ways to perform full backups, such as those described earlier in this section. Incremental backups are made possible by enabling the server's binary log, which the server uses to record data changes.
Full Versus Point-in-Time (Incremental) Recovery
A full recovery restores all data from a full backup. This restores the server instance to the state that it had when the backup was made. If that state is not sufficiently current, a full recovery can be followed by recovery of incremental backups made since the full backup, to bring the server to a more up-to-date state.
Incremental recovery is recovery of changes made during a given time span. This is also called point-in-time recovery because it makes a server's state current up to a given time. Point-in-time recovery is based on the binary log and typically follows a full recovery from the backup files that restores the server to its state when the backup was made. Then the data changes written in the binary log files are applied as incremental recovery to redo data modifications and bring the server up to the desired point in time.
Data integrity can be compromised if tables become corrupt. MySQL
provides programs for checking
tables and repairing them should problems be found. See
Section 7.6, “
MyISAM Table Maintenance and Crash Recovery”.
Backup Scheduling, Compression, and Encryption
Backup scheduling is valuable for automating backup procedures.
Compression of backup output reduces space requirements, and
encryption of the output provides better security against
unauthorized access of backed-up data. MySQL itself does not
provide these capabilities. ibbackup can
InnoDB backups, and compression or
encryption of backup output can be achieved using file system
utilities. Other third-party solutions may be available.
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