MySQL 20 MySQL data directory

One . MySQL8 The main directory structure of

find / -name mysql

Install well MySQL 8 after , Let's look at the following directory structure ：

1.1. Storage path of database file

MySQL Storage path of database file ：/var/lib/mysql/

show variables like 'datadir';

As can be seen from the results , On my computer MySQL The data directory is /var/lib/mysql/ .

1.2. Related command directory

Related command directory ：/usr/bin（mysqladmin、mysqlbinlog、mysqldump Wait for the order ） and /usr/sbin.

1.3. Profile directory

Profile directory ：/usr/share/mysql-8.0（ Commands and configuration files ）,/etc/mysql（ Such as my.cnf）

Two . The relationship between database and file system

2.1. View the default database

Check what databases are currently on my computer ：

SHOW DATABASES;

You can see that there is 4 A database belongs to MySQL Its own system database .

1. mysql:MySQL The core database of the system , It stores MySQL User account and permission information , Some stored procedures 、 Event definition information , Some log information generated during operation , Some help information and time zone information .
2. information_schema:MySQL The database of the system , This database holds MySQL The server Information about all other databases maintained , For example, what tables are there 、 Which views 、 Which triggers 、 Which columns 、 Which indexes . This information is not real user data , But some descriptive information , Sometimes it's called Metadata . In the system database information_schema Provides some information to innodb_sys Table at the beginning , Used to represent internal system tables .
3. performance_schema:MySQL The database of the system , This database mainly stores MySQL Some status information during server operation , Can be used to monitor MySQL Various performance indicators of the service . Including statistics on which statements have been executed recently , How long did it take at each stage of the execution process , Memory usage and other information .
4. sys: MySQL The database of the system , This database is mainly through View In the form of information_schema and performance_schema Combine , Help system administrators and developers monitor MySQL Technical performance .

2.2. The representation of a database in a file system

cd /var/lib/mysql

There are many files and subdirectories in this data directory , except information_schema This system is outside the database , Other databases are Data directory There are corresponding subdirectories under .

2.3. The representation of tables in the file system

2.3.1. InnoDB Storage engine mode

1. Table structure
   To save the table structure , InnoDB stay Data directory Under the corresponding database subdirectory, a special for Description of the file structure , The file name is like this ：

 Table name .frm

Let's say we're in MYSQLTEST Create a database named test Table of ：

 create table test(
 c1 int
 );

That's in the database MYSQLTEST A subdirectory named... Will be created under the corresponding subdirectory test.frm A file used to describe the table structure ..frm The format of the file is the same on different platforms . The suffix is .frm In order to Binary format Stored , It's garbled when we open it directly .

2. Data and indexes in the table
   ① SYSTEM tablespace （system tablespace）
   By default ,InnoDB Will create a data directory named ibdata1 、 The size is 12M The file of , This file is the corresponding SYSTEM tablespace Representation on the file system . How to 12M？ Note that this file is Self expanding file , When it is not enough, it will increase the file size by itself .
   Of course , If you want the system table space to correspond to multiple actual files on the file system , Or just feel the original ibdata1 The file name is ugly , That can be in MySQL Configure the corresponding file paths and their sizes at startup , For example, let's modify it like this my.cnf The configuration file ：

[server]
innodb_data_file_path=data1:512M;data2:512M:autoextend

② Independent table space (file-per-table tablespace)
stay MySQL5.6.6 And later versions ,InnoDB It does not store the data of each table in the system table space by default , But for Each table establishes an independent table space , That is to say, how many tables have we created , There are just as many independent table spaces . Use Independent table space To store table data , A file representing the independent table space will be created under the corresponding subdirectory of the database to which the table belongs , The file name is the same as the table name , Just added one .ibd It's just an extension of , So the full file name is like this ：

 Table name .ibd

such as ： We used Independent table space De storage MYSQLTEST Database based test Words of watch , Then, in the database where the table is located atguigu The directory will be test The table creates these two files ：

test.frm
test.ibd

among test.ibd Files are used to store test Data and indexes in tables .
③ Setting of system table space and independent table space
We can use it by ourselves SYSTEM tablespace still Independent table space To store data , This function is controlled by the startup parameters innodb_file_per_table control , For example, we want to deliberately store the table data in SYSTEM tablespace when , Can be started at MySQL The server is configured like this ：

[server]
innodb_file_per_table=0 # 0： Represents the use of system tablespaces ; 1： Represents the use of independent tablespaces 

④ Other types of tablespaces
With MySQL The development of , In addition to the above two old table spaces , A number of different types of table spaces have been proposed , For example, common table spaces （general tablespace）、 Temporary table space （temporary tablespace） etc. .

2.3.2. MyISAM Storage engine mode

1. Table structure
   In terms of storage table structure , MyISAM and InnoDB equally , Also in Data directory A file dedicated to describing the table structure is created under the corresponding database subdirectory ：

 Table name .frm

2. Data and indexes in the table
   stay MyISAM The indexes in are all Secondary indexes , Of the storage engine Data and index are stored separately Of . Therefore, different files are also used in the file system to store data files and index files , At the same time, the table data are stored in the corresponding database subdirectory . If test Table use MyISAM Storage engine , Then, in the database where it is located MYSQLTEST The directory will be test The table creates these three files ：

test.frm  Storage table structure 
test.MYD  Store the data  (MYData)
test.MYI  Storage index  (MYIndex)

Create a MyISAM surface , Use ENGINE Option to explicitly specify the engine . because InnoDB Is the default engine .

2.4. Summary

give an example ： database a , surface b .

1. If the table b use InnoDB ,data\a There will be 1 A or 2 File ：

• b.frm ： Description table structure file , Field length, etc .
• If the SYSTEM tablespace Mode , Data information and index information are stored in ibdata1 in .
• If the Independent table space Storage mode ,data\a There will also be b.ibd file （ Store data information and index information ）.
  Besides ：
  ① MySQL5.7 Will be in data/a Generated in the directory of db.opt File is used to save the relevant configuration of the database . such as ： Character set 、 Compare the rules . and MySQL8.0 No more db.opt file .
  ② MySQL8.0 Is no longer available separately b.frm, But merge in b.ibd In file .

2. If the table b use MyISAM ,data\a There will be 3 File ：

• MySQL5.7 in ： b.frm ： Description table structure file , Field length, etc .
• MySQL8.0 in b.xxx.sdi ： Description table structure file , Field length, etc .
• b.MYD (MYData)： Data information file , Store data information ( If the independent table storage mode is adopted ).
• b.MYI (MYIndex)： Store index information file .