Table of Contents
It is important to back up your databases so that you can recover your data and be up and running again in case problems occur, such as system crashes, hardware failures, or users deleting data by mistake. Backups are also essential as a safeguard before upgrading a MySQL installation, and they can be used to transfer a MySQL installation to another system or to set up replica servers.
MySQL offers a variety of backup strategies from which you can choose the methods that best suit the requirements for your installation. This chapter discusses several backup and recovery topics with which you should be familiar:
Types of backups: Logical versus physical, full versus incremental, and so forth.
Methods for creating backups.
Recovery methods, including point-in-time recovery.
Backup scheduling, compression, and encryption.
Table maintenance, to enable recovery of corrupt tables.
Resources related to backup or to maintaining data availability include the following:
Customers of MySQL Enterprise Edition can use the MySQL Enterprise Backup product for backups. For an overview of the MySQL Enterprise Backup product, see Section 29.2, “MySQL Enterprise Backup Overview”.
A forum dedicated to backup issues is available at https://forums.mysql.com/list.php?28.
The syntax of the SQL statements described here is given in Chapter 13, SQL Statements.
For additional information about
backup procedures, see Section 15.18.1, “InnoDB Backup”.
Replication enables you to maintain identical data on multiple servers. This has several benefits, such as enabling client query load to be distributed over servers, availability of data even if a given server is taken offline or fails, and the ability to make backups with no impact on the source by using a replica. See Chapter 17, Replication.
MySQL InnoDB Cluster is a collection of products that work together to provide a high availability solution. A group of MySQL servers can be configured to create a cluster using MySQL Shell. The cluster of servers has a single source, called the primary, which acts as the read-write source. Multiple secondary servers are replicas of the source. A minimum of three servers are required to create a high availability cluster. A client application is connected to the primary via MySQL Router. If the primary fails, a secondary is automatically promoted to the role of primary, and MySQL Router routes requests to the new primary.
NDB Cluster provides a high-availability, high-redundancy version of MySQL adapted for the distributed computing environment. See Chapter 22, MySQL NDB Cluster 8.0, which provides information about MySQL NDB Cluster 8.0.
This section describes the characteristics of different types of backups.
Physical backups consist of raw copies of the directories and files that store database contents. This type of backup is suitable for large, important databases that need to be recovered quickly when problems occur.
Logical backups save information represented as logical database
CREATE TABLE statements) and
INSERT statements or
delimited-text files). This type of backup is suitable for smaller
amounts of data where you might edit the data values or table
structure, or recreate the data on a different machine
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.
Physical backup methods are faster than logical because they involve only file copying without conversion.
Output is more compact than for logical backup.
Because backup speed and compactness are important for busy, important databases, the MySQL Enterprise Backup product performs physical backups. For an overview of the MySQL Enterprise Backup product, see Section 29.2, “MySQL Enterprise Backup Overview”.
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. For example,
InnoDB tables can each be in a separate
file, or share file storage with other
InnoDB tables; each
MyISAM table corresponds uniquely to a set
In addition to databases, the backup can include any related files such as log or configuration files.
MEMORY tables is tricky to back
up this way because their contents are not stored on disk.
(The MySQL Enterprise Backup product has a feature where you
can retrieve data from
MEMORY tables during
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. MySQL Enterprise Backup does this locking automatically for tables that require it.
Physical backup tools include the
mysqlbackup of MySQL Enterprise Backup for
InnoDB or any other tables, or file
system-level commands (such as cp,
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.
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. The MySQL Enterprise Backup product does such locking automatically.
Offline backup methods have these characteristics:
Clients can be affected adversely because the server is unavailable during backup. For that reason, such backups are often taken from a replica that can be taken offline without harming availability.
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 for clients to be affected by online recovery than by 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.
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.
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.
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.
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. For
InnoDB tables, this is not a typical
issue. For programs to check
tables and repair them if problems are found, see
Section 7.6, “MyISAM Table Maintenance and Crash Recovery”.
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. The MySQL Enterprise Backup product
InnoDB backups, and compression or
encryption of backup output can be achieved using file system
utilities. Other third-party solutions may be available.
This section summarizes some general methods for making backups.
Customers of MySQL Enterprise Edition can use the
Backup product to do
physical backups of entire
instances or selected databases, tables, or both. This product
includes features for
Backing up the physical database files makes restore much faster
than logical techniques such as the
InnoDB tables are copied using a
hot backup mechanism.
InnoDB tables should represent a
substantial majority of the data.) Tables from other storage
engines are copied using a warm
backup mechanism. For an overview of the MySQL Enterprise
Backup product, see Section 29.2, “MySQL Enterprise Backup Overview”.
MyISAM tables can be backed up by copying table files
*.MYI files, and
*.sdi files). To get a consistent
backup, stop the server or lock and flush the relevant tables:
tbl_listWITH READ LOCK;
You need only a read lock; this enables other clients to continue to query the tables while you are making a copy of the files in the database directory. The flush is needed to ensure that the all active index pages are written to disk before you start the backup. See Section 13.3.6, “LOCK TABLES and UNLOCK TABLES Statements”, and Section 188.8.131.52, “FLUSH Statement”.
You can also create a binary backup simply by copying the table
files, as long as the server isn't updating anything. (But note
that table file copying methods do not work if your database
InnoDB tables. Also, even if the
server is not actively updating data,
may still have modified data cached in memory and not flushed to
For an example of this backup method, refer to the export and import example in Section 13.2.5, “IMPORT TABLE Statement”.
To create a text file containing a table's data, you can use
SELECT * INTO OUTFILE
'. The file is created
on the MySQL server host, not the client host. For this statement,
the output file cannot already exist because permitting files to
be overwritten constitutes a security risk. See
Section 13.2.10, “SELECT Statement”. This method works for any kind of data
file, but saves only table data, not the table structure.
Another way to create text data files (along with files containing
CREATE TABLE statements for the
backed up tables) is to use mysqldump with the
--tab option. See
Section 7.4.3, “Dumping Data in Delimited-Text Format with mysqldump”.
MySQL supports incremental backups using the binary log. The binary log files provide you with the information you need to replicate changes to the database that are made subsequent to the point at which you performed a backup. Therefore, to allow a server to be restored to a point-in-time, binary logging must be enabled on it, which is the default setting for MySQL 8.0 ; see Section 5.4.4, “The Binary Log”.
At the moment you want to make an incremental backup (containing
all changes that happened since the last full or incremental
backup), you should rotate the binary log by using
FLUSH LOGS. This done, you need to
copy to the backup location all binary logs which range from the
one of the moment of the last full or incremental backup to the
last but one. These binary logs are the incremental backup; at
restore time, you apply them as explained in
Section 7.5, “Point-in-Time (Incremental) Recovery”. The next time you do a
full backup, you should also rotate the binary log using
FLUSH LOGS or mysqldump
--flush-logs. See Section 4.5.4, “mysqldump — A Database Backup Program”.
If you have performance problems with a server while making backups, one strategy that can help is to set up replication and perform backups on the replica rather than on the source. See Section 17.4.1, “Using Replication for Backups”.
If you are backing up a replica, you should back up its connection
metadata repository and applier metadata repository (see
Section 17.2.4, “Relay Log and Replication Metadata Repositories”) when you back up the replica's
databases, regardless of the backup method you choose. This
information is always needed to resume replication after you
restore the replica's data. If your replica is replicating
LOAD DATA statements, you should
also back up any
SQL_LOAD-* files that exist
in the directory that the replica uses for this purpose. The
replica needs these files to resume replication of any interrupted
LOAD DATA operations. The location
of this directory is the value of the
variable. If the server was not started with that variable set,
the directory location is the value of the
tmpdir system variable.
If you have to restore
MyISAM tables that have
become corrupt, try to recover them using
REPAIR TABLE or myisamchk
-r first. That should work in 99.9% of all cases. If
myisamchk fails, see
Section 7.6, “MyISAM Table Maintenance and Crash Recovery”.
If you are using a Veritas file system, you can make a backup like this:
Similar snapshot capabilities may be available in other file systems, such as LVM or ZFS.
This section discusses a procedure for performing backups that enables you to recover data after several types of crashes:
Operating system crash
File system crash
Hardware problem (hard drive, motherboard, and so forth)
The example commands do not include options such as
--password for the
mysqldump and mysql client
programs. You should include such options as necessary to enable
client programs to connect to the MySQL server.
Assume that data is stored in the
storage engine, which has support for transactions and automatic
crash recovery. Assume also that the MySQL server is under load at
the time of the crash. If it were not, no recovery would ever be
For cases of operating system crashes or power failures, we can
assume that MySQL's disk data is available after a restart. The
InnoDB data files might not contain consistent
data due to the crash, but
InnoDB reads its
logs and finds in them the list of pending committed and
noncommitted transactions that have not been flushed to the data
InnoDB automatically rolls back those
transactions that were not committed, and flushes to its data
files those that were committed. Information about this recovery
process is conveyed to the user through the MySQL error log. The
following is an example log excerpt:
InnoDB: Database was not shut down normally. InnoDB: Starting recovery from log files... InnoDB: Starting log scan based on checkpoint at InnoDB: log sequence number 0 13674004 InnoDB: Doing recovery: scanned up to log sequence number 0 13739520 InnoDB: Doing recovery: scanned up to log sequence number 0 13805056 InnoDB: Doing recovery: scanned up to log sequence number 0 13870592 InnoDB: Doing recovery: scanned up to log sequence number 0 13936128 ... InnoDB: Doing recovery: scanned up to log sequence number 0 20555264 InnoDB: Doing recovery: scanned up to log sequence number 0 20620800 InnoDB: Doing recovery: scanned up to log sequence number 0 20664692 InnoDB: 1 uncommitted transaction(s) which must be rolled back InnoDB: Starting rollback of uncommitted transactions InnoDB: Rolling back trx no 16745 InnoDB: Rolling back of trx no 16745 completed InnoDB: Rollback of uncommitted transactions completed InnoDB: Starting an apply batch of log records to the database... InnoDB: Apply batch completed InnoDB: Started mysqld: ready for connections
For the cases of file system crashes or hardware problems, we can assume that the MySQL disk data is not available after a restart. This means that MySQL fails to start successfully because some blocks of disk data are no longer readable. In this case, it is necessary to reformat the disk, install a new one, or otherwise correct the underlying problem. Then it is necessary to recover our MySQL data from backups, which means that backups must already have been made. To make sure that is the case, design and implement a backup policy.
To be useful, backups must be scheduled regularly. A full backup
(a snapshot of the data at a point in time) can be done in MySQL
with several tools. For example,
Backup can perform a
physical backup of
an entire instance, with optimizations to minimize overhead and
avoid disruption when backing up
files; mysqldump provides online
logical backup. This
discussion uses mysqldump.
Assume that we make a full backup of all our
InnoDB tables in all databases using the
following command on Sunday at 1 p.m., when load is low:
mysqldump --all-databases --master-data --single-transaction > backup_sunday_1_PM.sql
This backup operation acquires a global read lock on all tables
at the beginning of the dump (using
TABLES WITH READ LOCK). As soon as this lock has been
acquired, the binary log coordinates are read and the lock is
released. If long updating statements are running when the
FLUSH statement is issued, the
backup operation may stall until those statements finish. After
that, the dump becomes lock-free and does not disturb reads and
writes on the tables.
It was assumed earlier that the tables to back up are
InnoDB tables, so
--single-transaction uses a
consistent read and guarantees that data seen by
mysqldump does not change. (Changes made by
other clients to
InnoDB tables are not seen
by the mysqldump process.) If the backup
operation includes nontransactional tables, consistency requires
that they do not change during the backup. For example, for the
MyISAM tables in the
database, there must be no administrative changes to MySQL
accounts during the backup.
Full backups are necessary, but it is not always convenient to create them. They produce large backup files and take time to generate. They are not optimal in the sense that each successive full backup includes all data, even that part that has not changed since the previous full backup. It is more efficient to make an initial full backup, and then to make incremental backups. The incremental backups are smaller and take less time to produce. The tradeoff is that, at recovery time, you cannot restore your data just by reloading the full backup. You must also process the incremental backups to recover the incremental changes.
To make incremental backups, we need to save the incremental
changes. In MySQL, these changes are represented in the binary
log, so the MySQL server should always be started with the
--log-bin option to enable that
log. With binary logging enabled, the server writes each data
change into a file while it updates data. Looking at the data
directory of a MySQL server that has been running for some days,
we find these MySQL binary log files:
-rw-rw---- 1 guilhem guilhem 1277324 Nov 10 23:59 gbichot2-bin.000001 -rw-rw---- 1 guilhem guilhem 4 Nov 10 23:59 gbichot2-bin.000002 -rw-rw---- 1 guilhem guilhem 79 Nov 11 11:06 gbichot2-bin.000003 -rw-rw---- 1 guilhem guilhem 508 Nov 11 11:08 gbichot2-bin.000004 -rw-rw---- 1 guilhem guilhem 220047446 Nov 12 16:47 gbichot2-bin.000005 -rw-rw---- 1 guilhem guilhem 998412 Nov 14 10:08 gbichot2-bin.000006 -rw-rw---- 1 guilhem guilhem 361 Nov 14 10:07 gbichot2-bin.index
Each time it restarts, the MySQL server creates a new binary log
file using the next number in the sequence. While the server is
running, you can also tell it to close the current binary log
file and begin a new one manually by issuing a
FLUSH LOGS SQL statement or with
a mysqladmin flush-logs command.
mysqldump also has an option to flush the
.index file in the data directory
contains the list of all MySQL binary logs in the directory.
The MySQL binary logs are important for recovery because they form the set of incremental backups. If you make sure to flush the logs when you make your full backup, the binary log files created afterward contain all the data changes made since the backup. Let's modify the previous mysqldump command a bit so that it flushes the MySQL binary logs at the moment of the full backup, and so that the dump file contains the name of the new current binary log:
mysqldump --single-transaction --flush-logs --master-data=2 \
--all-databases > backup_sunday_1_PM.sql
After executing this command, the data directory contains a new
binary log file,
option causes the server to flush its logs. The
--master-data option causes
mysqldump to write binary log information to
its output, so the resulting
.sql dump file
includes these lines:
-- Position to start replication or point-in-time recovery from -- CHANGE MASTER TO MASTER_LOG_FILE='gbichot2-bin.000007',MASTER_LOG_POS=4;
Because the mysqldump command made a full backup, those lines mean two things:
The dump file contains all changes made before any changes
written to the
binary log file or higher.
All data changes logged after the backup are not present in
the dump file, but are present in the
gbichot2-bin.000007 binary log file or
On Monday at 1 p.m., we can create an incremental backup by
flushing the logs to begin a new binary log file. For example,
executing a mysqladmin flush-logs command
gbichot2-bin.000008. All changes
between the Sunday 1 p.m. full backup and Monday 1 p.m. are
incremental backup is important, so it is a good idea to copy it
to a safe place. (For example, back it up on tape or DVD, or
copy it to another machine.) On Tuesday at 1 p.m., execute
another mysqladmin flush-logs command. All
changes between Monday 1 p.m. and Tuesday 1 p.m. are written in
gbichot2-bin.000008 (which also should be
copied somewhere safe).
The MySQL binary logs take up disk space. To free up space, purge them from time to time. One way to do this is by deleting the binary logs that are no longer needed, such as when we make a full backup:
mysqldump --single-transaction --flush-logs --master-data=2 \
--all-databases --delete-master-logs > backup_sunday_1_PM.sql
Deleting the MySQL binary logs with mysqldump
--delete-master-logs can be dangerous if your server
is a replication source server, because replicas might not yet
fully have processed the contents of the binary log. The
description for the
LOGS statement explains what should be verified
before deleting the MySQL binary logs. See
Section 184.108.40.206, “PURGE BINARY LOGS Statement”.
Now, suppose that we have a catastrophic unexpected exit on Wednesday at 8 a.m. that requires recovery from backups. To recover, first we restore the last full backup we have (the one from Sunday 1 p.m.). The full backup file is just a set of SQL statements, so restoring it is very easy:
mysql < backup_sunday_1_PM.sql
At this point, the data is restored to its state as of Sunday 1
p.m.. To restore the changes made since then, we must use the
incremental backups; that is, the
gbichot2-bin.000008 binary log files. Fetch
the files if necessary from where they were backed up, and then
process their contents like this:
mysqlbinlog gbichot2-bin.000007 gbichot2-bin.000008 | mysql
We now have recovered the data to its state as of Tuesday 1
p.m., but still are missing the changes from that date to the
date of the crash. To not lose them, we would have needed to
have the MySQL server store its MySQL binary logs into a safe
location (RAID disks, SAN, ...) different from the place where
it stores its data files, so that these logs were not on the
destroyed disk. (That is, we can start the server with a
--log-bin option that specifies a
location on a different physical device from the one on which
the data directory resides. That way, the logs are safe even if
the device containing the directory is lost.) If we had done
this, we would have the
file (and any subsequent files) at hand, and we could apply them
using mysqlbinlog and
mysql to restore the most recent data changes
with no loss up to the moment of the crash:
mysqlbinlog gbichot2-bin.000009 ... | mysql
For more information about using mysqlbinlog to process binary log files, see Section 7.5, “Point-in-Time (Incremental) Recovery”.
In case of an operating system crash or power failure,
InnoDB itself does all the job of recovering
data. But to make sure that you can sleep well, observe the
Always tun the MySQL server with binary logging enabled (that is the default setting for MySQL 8.0). If you have such safe media, this technique can also be good for disk load balancing (which results in a performance improvement).
This section describes how to use mysqldump to produce dump files, and how to reload dump files. A dump file can be used in several ways:
As a backup to enable data recovery in case of data loss.
As a source of data for setting up replicas.
As a source of data for experimentation:
To make a copy of a database that you can use without changing the original data.
To test potential upgrade incompatibilities.
mysqldump writes SQL statements to the
standard output. This output consists of
CREATE statements to create dumped objects
(databases, tables, stored routines, and so forth), and
INSERT statements to load data into tables.
The output can be saved in a file and reloaded later using
mysql to recreate the dumped objects.
Options are available to modify the format of the SQL
statements, and to control which objects are dumped.
mysqldump produces two output files for
each dumped table. The server writes one file as tab-delimited
text, one line per table row. This file is named
in the output directory. The server also sends a
CREATE TABLE statement for the
table to mysqldump, which writes it as a
in the output directory.
By default, mysqldump writes information as SQL statements to the standard output. You can save the output in a file:
mysqldump --all-databases > dump.sql
To dump only specific databases, name them on the command line
and use the
mysqldump --databases db1 db2 db3 > dump.sql
--databases option causes
all names on the command line to be treated as database names.
Without this option, mysqldump treats the
first name as a database name and those following as table
statements prior to the dump output for each database. This
ensures that when the dump file is reloaded, it creates each
database if it does not exist and makes it the default database
so database contents are loaded into the same database from
which they came. If you want to cause the dump file to force a
drop of each database before recreating it, use the
--add-drop-database option as
well. In this case, mysqldump writes a
DROP DATABASE statement preceding
CREATE DATABASE statement.
To dump a single database, name it on the command line:
mysqldump --databases test > dump.sql
In the single-database case, it is permissible to omit the
mysqldump test > dump.sql
When you reload the dump file, you must specify a default database name so that the server knows which database to reload.
For reloading, you can specify a database name different from the original name, which enables you to reload the data into a different database.
If the database to be reloaded does not exist, you must create it first.
To dump only specific tables from a database, name them on the command line following the database name:
mysqldump test t1 t3 t7 > dump.sql
By default, if GTIDs are in use on the server where you create
the dump file (
mysqldump includes a
@@GLOBAL.gtid_purged statement in the output to add
the GTIDs from the
gtid_executed set on the source
server to the
on the target server. 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 source
server, even those that changed suppressed parts of the
database, or other databases on the server that were not
included in the partial dump. If you only replay one partial
dump file on the target server, the extra GTIDs do not cause any
problems with the future operation of that server. However, if
you replay a second dump file on the target server that contains
the same GTIDs (for example, another partial dump from the same
source server), any
statement in the second dump file fails. To avoid this issue,
either set the mysqldump option
COMMENTED to output the second dump file
without an active
statement, or remove the statement manually before replaying the
To reload a dump file written by mysqldump
that consists of SQL statements, use it as input to the
mysql client. If the dump file was created by
mysqldump with the
--databases option, it
CREATE DATABASE and
USE statements and it is not
necessary to specify a default database into which to load the
mysql < dump.sql
Alternatively, from within mysql, use a
mysqladmin create db1
Then specify the database name when you load the dump file:
mysql db1 < dump.sql
Alternatively, from within mysql, create the database, select it as the default database, and load the dump file:
CREATE DATABASE IF NOT EXISTS db1;mysql>
For Windows PowerShell users: Because the "<" character is
reserved for future use in PowerShell, an alternative approach
is required, such as using quotes
cmd.exe /c "mysql
This section describes how to use mysqldump to create delimited-text dump files. For information about reloading such dump files, see Section 7.4.4, “Reloading Delimited-Text Format Backups”.
If you invoke mysqldump with the
option, it uses
dir_name as the
output directory and dumps tables individually in that directory
using two files for each table. The table name is the base name
for these files. For a table named
files are named
CREATE TABLE statement
for the table. The
.txt file contains the
table data, one line per table row.
The following command dumps the contents of the
db1 database to files in the
mysqldump --tab=/tmp db1
.txt files containing table data are
written by the server, so they are owned by the system account
used for running the server. The server uses
SELECT ... INTO
OUTFILE to write the files, so you must have the
FILE privilege to perform this
operation, and an error occurs if a given
.txt file already exists.
It is best that
--tab be used
only for dumping a local server. If you use it with a remote
must exist on both the local and remote hosts, and the
.txt files are written by the server in the
remote directory (on the server host), whereas the
.sql files are written by
mysqldump in the local directory (on the
For mysqldump --tab, the server by default
writes table data to
.txt files one line
per row with tabs between column values, no quotation marks
around column values, and newline as the line terminator. (These
are the same defaults as for
SELECT ... INTO
To enable data files to be written using a different format, mysqldump supports these options:
The string for separating column values (default: tab).
The character within which to enclose column values (default: no character).
The character within which to enclose non-numeric column values (default: no character).
The character for escaping special characters (default: no escaping).
The line-termination string (default: newline).
Depending on the value you specify for any of these options, it
might be necessary on the command line to quote or escape the
value appropriately for your command interpreter. Alternatively,
specify the value using hex notation. Suppose that you want
mysqldump to quote column values within
double quotation marks. To do so, specify double quote as the
value for the
option. But this character is often special to command
interpreters and must be treated specially. For example, on
Unix, you can quote the double quote like this:
On any platform, you can specify the value in hex:
It is common to use several of the data-formatting options
together. For example, to dump tables in comma-separated values
format with lines terminated by carriage-return/newline pairs
\r\n), use this command (enter it on a
mysqldump --tab=/tmp --fields-terminated-by=,
--fields-enclosed-by='"' --lines-terminated-by=0x0d0a db1
Should you use any of the data-formatting options to dump table data, you need to specify the same format when you reload data files later, to ensure proper interpretation of the file contents.
For backups produced with mysqldump --tab,
each table is represented in the output directory by an
.sql file containing the
CREATE TABLE statement for the
table, and a
.txt file containing the table
data. To reload a table, first change location into the output
directory. Then process the
.sql file with
mysql to create an empty table and process
.txt file to load the data into the
mysql db1 < t1.sqlshell>
mysqlimport db1 t1.txt
LOAD DATA INFILE 't1.txt' INTO TABLE t1;
If you used any data-formatting options with
mysqldump when you initially dumped the
table, you must use the same options with
DATA to ensure proper interpretation of the data file
--fields-enclosed-by='"' --lines-terminated-by=0x0d0a db1 t1.txt
LOAD DATA INFILE 't1.txt' INTO TABLE t1
FIELDS TERMINATED BY ',' FIELDS ENCLOSED BY '"'
LINES TERMINATED BY '\r\n';
This section surveys techniques that enable you to use mysqldump to solve specific problems:
How to make a copy a database
How to copy a database from one server to another
How to dump stored programs (stored procedures and functions, triggers, and events)
How to dump definitions and data separately
mysqldump db1 > dump.sqlshell>
mysqladmin create db2shell>
mysql db2 < dump.sql
On Server 1:
mysqldump --databases db1 > dump.sql
Copy the dump file from Server 1 to Server 2.
On Server 2:
mysql < dump.sql
--databases with the
mysqldump command line causes the dump file
CREATE DATABASE and
USE statements that create the
database if it does exist and make it the default database for
the reloaded data.
Alternatively, you can omit
--databases from the
mysqldump command. Then you need to create
the database on Server 2 (if necessary) and specify it as the
default database when you reload the dump file.
On Server 1:
mysqldump db1 > dump.sql
On Server 2:
mysqladmin create db1shell>
mysql db1 < dump.sql
Several options control how mysqldump handles stored programs (stored procedures and functions, triggers, and events):
--triggers option is
enabled by default so that when tables are dumped, they are
accompanied by any triggers they have. The other options are
disabled by default and must be specified explicitly to dump
the corresponding objects. To disable any of these options
explicitly, use its skip form:
--no-data option tells
mysqldump not to dump table data, resulting
in the dump file containing only statements to create the
tables. Conversely, the
tells mysqldump to suppress
CREATE statements from the output, so that
the dump file contains only table data.
For example, to dump table definitions and data separately for
test database, use these commands:
mysqldump --no-data test > dump-defs.sqlshell>
mysqldump --no-create-info test > dump-data.sql
mysqldump --no-data --routines --events test > dump-defs.sql
When contemplating a MySQL upgrade, it is prudent to install the newer version separately from your current production version. Then you can dump the database and database object definitions from the production server and load them into the new server to verify that they are handled properly. (This is also useful for testing downgrades.)
On the production server:
mysqldump --all-databases --no-data --routines --events > dump-defs.sql
On the upgraded server:
mysql < dump-defs.sql
Because the dump file does not contain table data, it can be processed quickly. This enables you to spot potential incompatibilities without waiting for lengthy data-loading operations. Look for warnings or errors while the dump file is being processed.
After you have verified that the definitions are handled properly, dump the data and try to load it into the upgraded server.
On the production server:
mysqldump --all-databases --no-create-info > dump-data.sql
On the upgraded server:
mysql < dump-data.sql
Now check the table contents and run some test queries.
Point-in-time recovery refers to recovery of data changes up to a given point in time. Typically, this type of recovery is performed after restoring a full backup that brings the server to its state as of the time the backup was made. (The full backup can be made in several ways, such as those listed in Section 7.2, “Database Backup Methods”.) Point-in-time recovery then brings the server up to date incrementally from the time of the full backup to a more recent time.
This section explains the general idea of using the binary log to perform a point-in-time-recovery. The next section, Section 7.5.2, “Point-in-Time Recovery Using Event Positions”, explains the operation in details with an example.
Many of the examples in this and the next section use the
mysql client to process binary log output
produced by mysqlbinlog. If your binary log
\0 (null) characters, that output
cannot be parsed by mysql unless you invoke
it with the
The source of information for point-in-time recovery is the set of binary log files generated subsequent to the full backup operation. Therefore, to allow a server to be restored to a point-in-time, binary logging must be enabled on it, which is the default setting for MySQL 8.0 (see Section 5.4.4, “The Binary Log”).
To restore data from the binary log, you must know the name and
location of the current binary log files. By default, the server
creates binary log files in the data directory, but a path name
can be specified with the
--log-bin option to place the
files in a different location. To see a listing of all binary
log files, use this statement:
SHOW BINARY LOGS;
To determine the name of the current binary log file, issue the following statement:
SHOW MASTER STATUS;
The mysqlbinlog utility converts the events in the binary log files from binary format to text so that they can be viewed or applied. mysqlbinlog has options for selecting sections of the binary log based on event times or position of events within the log. See Section 4.6.8, “mysqlbinlog — Utility for Processing Binary Log Files”.
Applying events from the binary log causes the data modifications they represent to be reexecuted. This enables recovery of data changes for a given span of time. To apply events from the binary log, process mysqlbinlog output using the mysql client:
binlog_files| mysql -u root -p
If binary log files have been encrypted, which can be done from
MySQL 8.0.14 onwards, mysqlbinlog cannot read
them directly as in the above example, but can read them from
the server using the
-R) option. For example:
mysqlbinlog --read-from-remote-server --host=
host_name--port=3306 --user=root --password --ssl-mode=required
binlog_files| mysql -u root -p
Here, the option
--ssl-mode=required has been
used to ensure that the data from the binary log files is
protected in transit, because it is sent to
mysqlbinlog in an unencrypted format.
Viewing log contents can be useful when you need to determine event times or positions to select partial log contents prior to executing events. To view events from the log, send mysqlbinlog output into a paging program:
Alternatively, save the output in a file and view the file in a text editor:
Saving the output in a file is useful as a preliminary to
executing the log contents with certain events removed, such as
DROP TABLE. You can
delete from the file any statements not to be executed before
executing its contents. After editing the file, apply the
contents as follows:
mysql -u root -p < tmpfile
If you have more than one binary log to apply on the MySQL server, the safe method is to process them all using a single connection to the server. Here is an example that demonstrates what may be unsafe:
mysqlbinlog binlog.000001 | mysql -u root -p # DANGER!!shell>
mysqlbinlog binlog.000002 | mysql -u root -p # DANGER!!
Processing binary logs this way using different connections to
the server causes problems if the first log file contains a
TABLE statement and the second log contains a
statement that uses the temporary table. When the first
mysql process terminates, the server drops
the temporary table. When the second mysql
process attempts to use the table, the server reports
To avoid problems like this, use a single connection to apply the contents of all binary log files that you want to process. Here is one way to do so:
mysqlbinlog binlog.000001 binlog.000002 | mysql -u root -p
Another approach is to write the whole log to a single file and then process the file:
mysqlbinlog binlog.000001 > /tmp/statements.sqlshell>
mysqlbinlog binlog.000002 >> /tmp/statements.sqlshell>
mysql -u root -p -e "source /tmp/statements.sql"
When writing to a dump file while reading back from a binary log
containing GTIDs (see Section 17.1.3, “Replication with Global Transaction Identifiers”), use
--skip-gtids option with
mysqlbinlog, like this:
mysqlbinlog --skip-gtids binlog.000001 > /tmp/dump.sqlshell>
mysqlbinlog --skip-gtids binlog.000002 >> /tmp/dump.sqlshell>
mysql -u root -p -e "source /tmp/dump.sql"
The last section, Section 7.5.1, “Point-in-Time Recovery Using Binary Log”, explains the general idea of using the binary log to perform a point-in-time-recovery. The section explains the operation in details with an example.
As an example, suppose that around 20:06:00 on March 11, 2020, an SQL statement was executed that deleted a table. You can perform a point-in-time recovery to restore the server up to its state right before the table deletion. These are some sample steps to achieve that:
Restore the last full backup created before the
point-in-time of interest (call it
tp, which is
20:06:00 on March 11, 2020 in our example). When finished,
note the binary log position up to which you have restored
the server for later use, and restart the server.
While the last binary log position recovered is also displayed by InnoDB after the restore and server restart, that is not a reliable means for obtaining the ending log position of your restore, as there could be DDL events and non-InnoDB changes that have taken place after the time reflected by the displayed position. Your backup and restore tool should provide you with the last binary log position for your recovery: for example, if you are using mysqlbinlog for the task, check the stop position of the binary log replay; if you are using MySQL Enterprise Backup, the last binary log position has been saved in your backup. See Point-in-Time Recovery.
Find the precise binary log event position corresponding to
the point in time up to which you want to restore your
database. In our example, given that we know the rough time
where the table deletion took place
tp), we can find
the log position by checking the log contents around that
time using the mysqlbinlog utility. Use
to specify a short time period around
tp, and then look
for the event in the output. For example:
mysqlbinlog --start-datetime="2020-03-11 20:05:00" \ --stop-datetime="2020-03-11 20:08:00" --verbose \
/var/lib/mysql/bin.123456 | grep -C 15 "DROP TABLE"/*!80014 SET @@session.original_server_version=80019*//*!*/; /*!80014 SET @@session.immediate_server_version=80019*//*!*/; SET @@SESSION.GTID_NEXT= 'ANONYMOUS'/*!*/; # at 232 #200311 20:06:20 server id 1 end_log_pos 355 CRC32 0x2fc1e5ea Query thread_id=16 exec_time=0 error_code=0 SET TIMESTAMP=1583971580/*!*/; SET @@session.pseudo_thread_id=16/*!*/; SET @@session.foreign_key_checks=1, @@session.sql_auto_is_null=0, @@session.unique_checks=1, @@session.autocommit=1/*!*/; SET @@session.sql_mode=1168113696/*!*/; SET @@session.auto_increment_increment=1, @@session.auto_increment_offset=1/*!*/; /*!\C utf8mb4 *//*!*/; SET @@session.character_set_client=255,@@session.collation_connection=255,@@session.collation_server=255/*!*/; SET @@session.lc_time_names=0/*!*/; SET @@session.collation_database=DEFAULT/*!*/; /*!80011 SET @@session.default_collation_for_utf8mb4=255*//*!*/; DROP TABLE `pets`.`cats` /* generated by server */ /*!*/; # at 355 #200311 20:07:48 server id 1 end_log_pos 434 CRC32 0x123d65df Anonymous_GTID last_committed=1 sequence_number=2 rbr_only=no original_committed_timestamp=1583971668462467 immediate_commit_timestamp=1583971668462467 transaction_length=473 # original_commit_timestamp=1583971668462467 (2020-03-11 20:07:48.462467 EDT) # immediate_commit_timestamp=1583971668462467 (2020-03-11 20:07:48.462467 EDT) /*!80001 SET @@session.original_commit_timestamp=1583971668462467*//*!*/; /*!80014 SET @@session.original_server_version=80019*//*!*/; /*!80014 SET @@session.immediate_server_version=80019*//*!*/; SET @@SESSION.GTID_NEXT= 'ANONYMOUS'/*!*/; # at 434 #200311 20:07:48 server id 1 end_log_pos 828 CRC32 0x57fac9ac Query thread_id=16 exec_time=0 error_code=0 Xid = 217 use `pets`/*!*/; SET TIMESTAMP=1583971668/*!*/; /*!80013 SET @@session.sql_require_primary_key=0*//*!*/; CREATE TABLE dogs
From the output of mysqlbinlog, the
DROP TABLE `pets`.`cats` statement can be
found in the segment of the binary log between the line
# at 232 and
# at 355,
which means the statement takes place
after the log position 232, and the log
is at position 355 after the
Only use the
options to help you find the actual event positions of
interest. Using the two options to specify the range of
binary log segment to apply is not recommended: there is a
higher risk of missing binary log events when using the
Apply the events in binary log file to the server, starting with the log position your found in step 1 (assume it is 155) and ending at the position you have found in step 2 that is before your point-in-time of interest (which is 232):
mysqlbinlog --start-position=155 --stop-position=232 /var/lib/mysql/bin.123456 \
| mysql -u root -p
The command recovers all the transactions from the starting
position until just before the stop position. Because the
output of mysqlbinlog includes
SET TIMESTAMP statements before each SQL
statement recorded, the recovered data and related MySQL
logs reflect the original times at which the transactions
Your database has now been restored to the point-in-time of
right before the table
Beyond the point-in-time recovery that has been finished, if
you also want to reexecute all the statements
after your point-in-time of interest,
use mysqlbinlog again to apply all the
the server. We noted in step 2 that after the statement we
wanted to skip, the log is at position 355; we can use it
option, so that any statements after the position are
mysqlbinlog --start-position=355 /var/lib/mysql/bin.123456 \
| mysql -u root -p
Your database has been restored the latest statement recorded in the binary log file, but with the selected event skipped.
This section discusses how to use myisamchk to
check or repair
MyISAM tables (tables that have
.MYI files for
storing data and indexes). For general
myisamchk background, see
Section 4.6.4, “myisamchk — MyISAM Table-Maintenance Utility”. Other table-repair information can be
found at Section 2.11.13, “Rebuilding or Repairing Tables or Indexes”.
You can use myisamchk to check, repair, or optimize database tables. The following sections describe how to perform these operations and how to set up a table maintenance schedule. For information about using myisamchk to get information about your tables, see Section 220.127.116.11, “Obtaining Table Information with myisamchk”.
Even though table repair with myisamchk is quite secure, it is always a good idea to make a backup before doing a repair or any maintenance operation that could make a lot of changes to a table.
myisamchk operations that affect indexes can
indexes to be rebuilt with full-text parameters that are
incompatible with the values used by the MySQL server. To avoid
this problem, follow the guidelines in
Section 18.104.22.168, “myisamchk General Options”.
MyISAM table maintenance can also be done using
the SQL statements that perform operations similar to what
myisamchk can do:
For additional information about these statements, see Section 13.7.3, “Table Maintenance Statements”.
These statements can be used directly or by means of the mysqlcheck client program. One advantage of these statements over myisamchk is that the server does all the work. With myisamchk, you must make sure that the server does not use the tables at the same time so that there is no unwanted interaction between myisamchk and the server.
This section describes how to check for and deal with data corruption in MySQL databases. If your tables become corrupted frequently, you should try to find the reason why. See Section B.3.3.3, “What to Do If MySQL Keeps Crashing”.
For an explanation of how
MyISAM tables can
become corrupted, see Section 16.2.4, “MyISAM Table Problems”.
If you run mysqld with external locking disabled (which is the default), you cannot reliably use myisamchk to check a table when mysqld is using the same table. If you can be certain that no one can access the tables using mysqld while you run myisamchk, you only have to execute mysqladmin flush-tables before you start checking the tables. If you cannot guarantee this, you must stop mysqld while you check the tables. If you run myisamchk to check tables that mysqld is updating at the same time, you may get a warning that a table is corrupt even when it is not.
If the server is run with external locking enabled, you can use myisamchk to check tables at any time. In this case, if the server tries to update a table that myisamchk is using, the server waits for myisamchk to finish before it continues.
If you use myisamchk to repair or optimize tables, you must always ensure that the mysqld server is not using the table (this also applies if external locking is disabled). If you do not stop mysqld, you should at least do a mysqladmin flush-tables before you run myisamchk. Your tables may become corrupted if the server and myisamchk access the tables simultaneously.
When performing crash recovery, it is important to understand
tbl_name in a database corresponds to
the three files in the database directory shown in the following
Each of these three file types is subject to corruption in various ways, but problems occur most often in data files and index files.
myisamchk works by creating a copy of the
.MYD data file row by row. It ends the
repair stage by removing the old
and renaming the new file to the original file name. If you use
myisamchk does not create a temporary
.MYD file, but instead assumes that the
.MYD file is correct and generates only a
new index file without touching the
file. This is safe, because myisamchk
automatically detects whether the
is corrupt and aborts the repair if it is. You can also specify
--quick option twice to
myisamchk. In this case,
myisamchk does not abort on some errors (such
as duplicate-key errors) but instead tries to resolve them by
.MYD file. Normally the use
--quick options is
useful only if you have too little free disk space to perform a
normal repair. In this case, you should at least make a backup
of the table before running myisamchk.
To check a
MyISAM table, use the following
This finds 99.99% of all errors. What it cannot find is
corruption that involves only the data
file (which is very unusual). If you want to check a table,
you should normally run myisamchk without
options or with the
-s (silent) option.
This finds 99.999% of all errors. It first checks all index entries for errors and then reads through all rows. It calculates a checksum for all key values in the rows and verifies that the checksum matches the checksum for the keys in the index tree.
This does a complete and thorough check of all data
-e means “extended check”).
It does a check-read of every key for each row to verify
that they indeed point to the correct row. This may take a
long time for a large table that has many indexes. Normally,
myisamchk stops after the first error it
finds. If you want to obtain more information, you can add
-v (verbose) option. This causes
myisamchk to keep going, up through a
maximum of 20 errors.
This is like the previous command, but the
-i option tells
myisamchk to print additional statistical
In most cases, a simple myisamchk command with no arguments other than the table name is sufficient to check a table.
The discussion in this section describes how to use
Symptoms of corrupted tables include queries that abort unexpectedly and observable errors such as these:
Can't find file
Unexpected end of file
Record file is crashed
nnn from table handler
To get more information about the error, run
nnn is the error number. The
following example shows how to use perror to
find the meanings for the most common error numbers that
indicate a problem with a table:
perror 126 127 132 134 135 136 141 144 145MySQL error code 126 = Index file is crashed MySQL error code 127 = Record-file is crashed MySQL error code 132 = Old database file MySQL error code 134 = Record was already deleted (or record file crashed) MySQL error code 135 = No more room in record file MySQL error code 136 = No more room in index file MySQL error code 141 = Duplicate unique key or constraint on write or update MySQL error code 144 = Table is crashed and last repair failed MySQL error code 145 = Table was marked as crashed and should be repaired
Note that error 135 (no more room in record file) and error 136
(no more room in index file) are not errors that can be fixed by
a simple repair. In this case, you must use
ALTER TABLE to increase the
AVG_ROW_LENGTH table option values:
If you do not know the current table option values, use
SHOW CREATE TABLE.
For the other errors, you must repair your tables. myisamchk can usually detect and fix most problems that occur.
The repair process involves up to three stages, described here. Before you begin, you should change location to the database directory and check the permissions of the table files. On Unix, make sure that they are readable by the user that mysqld runs as (and to you, because you need to access the files you are checking). If it turns out you need to modify files, they must also be writable by you.
This section is for the cases where a table check fails (such as those described in Section 7.6.2, “How to Check MyISAM Tables for Errors”), or you want to use the extended features that myisamchk provides.
The myisamchk options used for table maintenance with are described in Section 4.6.4, “myisamchk — MyISAM Table-Maintenance Utility”. myisamchk also has variables that you can set to control memory allocation that may improve performance. See Section 22.214.171.124, “myisamchk Memory Usage”.
If you are going to repair a table from the command line, you must first stop the mysqld server. Note that when you do mysqladmin shutdown on a remote server, the mysqld server is still available for a while after mysqladmin returns, until all statement-processing has stopped and all index changes have been flushed to disk.
Stage 1: Checking your tables
You have to repair only those tables for which myisamchk announces an error. For such tables, proceed to Stage 2.
If you get unexpected errors when checking (such as
of memory errors), or if myisamchk
crashes, go to Stage 3.
Stage 2: Easy safe repair
First, try myisamchk -r -q
-q means “quick recovery mode”). This
attempts to repair the index file without touching the data
file. If the data file contains everything that it should and
the delete links point at the correct locations within the data
file, this should work, and the table is fixed. Start repairing
the next table. Otherwise, use the following procedure:
Make a backup of the data file before continuing.
Use myisamchk -r
-r means “recovery mode”).
This removes incorrect rows and deleted rows from the data
file and reconstructs the index file.
If the preceding step fails, use myisamchk
tbl_name. Safe recovery
mode uses an old recovery method that handles a few cases
that regular recovery mode does not (but is slower).
If you get unexpected errors when repairing (such as
out of memory errors), or if
myisamchk crashes, go to Stage 3.
Stage 3: Difficult repair
You should reach this stage only if the first 16KB block in the index file is destroyed or contains incorrect information, or if the index file is missing. In this case, it is necessary to create a new index file. Do so as follows:
Move the data file to a safe place.
Use the table description file to create new (empty) data and index files:
Copy the old data file back onto the newly created data file. (Do not just move the old file back onto the new file. You want to retain a copy in case something goes wrong.)
If you are using replication, you should stop it prior to performing the above procedure, since it involves file system operations, and these are not logged by MySQL.
Go back to Stage 2. myisamchk -r -q should work. (This should not be an endless loop.)
You can also use the
statement, which performs the whole procedure automatically.
There is also no possibility of unwanted interaction between a
utility and the server, because the server does all the work
when you use
REPAIR TABLE. See
Section 126.96.36.199, “REPAIR TABLE Statement”.
To coalesce fragmented rows and eliminate wasted space that results from deleting or updating rows, run myisamchk in recovery mode:
You can optimize a table in the same way by using the
OPTIMIZE TABLE SQL statement.
OPTIMIZE TABLE does a table
repair and a key analysis, and also sorts the index tree so that
key lookups are faster. There is also no possibility of unwanted
interaction between a utility and the server, because the server
does all the work when you use
TABLE. See Section 188.8.131.52, “OPTIMIZE TABLE Statement”.
myisamchk has a number of other options that you can use to improve the performance of a table:
-a: Perform key distribution analysis. This
improves join performance by enabling the join optimizer to
better choose the order in which to join the tables and
which indexes it should use.
-S: Sort the index blocks. This optimizes
seeks and makes table scans that use indexes faster.
Sort data rows according to a given index. This makes your
data much more localized and may speed up range-based
BY operations that use this index.
For a full description of all available options, see Section 4.6.4, “myisamchk — MyISAM Table-Maintenance Utility”.
It is a good idea to perform table checks on a regular basis
rather than waiting for problems to occur. One way to check and
MyISAM tables is with the
CHECK TABLE and
REPAIR TABLE statements. See
Section 13.7.3, “Table Maintenance Statements”.
Another way to check tables is to use
myisamchk. For maintenance purposes, you can
use myisamchk -s. The
option (short for
causes myisamchk to run in silent mode,
printing messages only when errors occur.
It is also a good idea to enable automatic
MyISAM table checking. For example, whenever
the machine has done a restart in the middle of an update, you
usually need to check each table that could have been affected
before it is used further. (These are “expected crashed
tables.”) To cause the server to check
MyISAM tables automatically, start it with
system variable set. See
Section 5.1.8, “Server System Variables”.
You should also check your tables regularly during normal system
operation. For example, you can run a cron
job to check important tables once a week, using a line like
this in a
35 0 * * 0
This prints out information about crashed tables so that you can examine and repair them as necessary.
To start with, execute myisamchk -s each night on all tables that have been updated during the last 24 hours. As you see that problems occur infrequently, you can back off the checking frequency to once a week or so.
Normally, MySQL tables need little maintenance. If you are
performing many updates to
MyISAM tables with
dynamic-sized rows (tables with
TEXT columns) or have tables with
many deleted rows you may want to defragment/reclaim space from
the tables from time to time. You can do this by using
OPTIMIZE TABLE on the tables in
question. Alternatively, if you can stop the
mysqld server for a while, change location
into the data directory and use this command while the server is
myisamchk -r -s --sort-index --myisam_sort_buffer_size=16M */*.MYI