[MySQL from introduction to mastery] [advanced part] (II) representation of MySQL directory structure and tables in the file system

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1. brief introduction

Install well mysql after , We can go through find / -name mysql Command view mysql File or folder . As shown in the figure below , There must be something marked in red .

Here is a brief introduction to the core directory ：

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

The parts marked with red boxes in the above figure are all MySQL Database in ：
among ： database demodb1,test,testdb1 The three databases are created by users themselves .
database mysql,information_schema,performance_schema,sys All four databases are MySQL The database that comes with it . More on that later .

MySQL When the server program starts, it will load some files in a directory of a certain system of the file system , After that, the data generated during operation will also be stored in some files in this directory , This directory is called the data directory .

Actually, the data directory corresponds to a system variable datadir, We can check the value of this system variable after establishing a connection between the client and the server ;

mysql>  show variables like 'datadir';

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

The installation directory is very important bin Catalog , It stores many commands about controlling client programs and server programs （ Many executables , such as mysql,mysqld etc. ）. The data directory is used to store MySQL Data generated during operation .

1.3. Profile directory

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

2. The relationship between database and file system

image InnoDB、MyISAM Such a storage engine stores tables on disk , The structure used by the operating system to manage the disk is called the file system , So to put it in professional terms is ： image InnoDB、MyISAM Such storage engines store tables on the file system . When we want to read data , These storage engines will read the data from the file system and return it to us , When we want to write data , These storage engines will write the data back to the file system .
Check what databases are currently in my computer ：

mysql> show databases;

among ： database demodb1,test,testdb1 The three databases are created by users themselves .
database mysql,information_schema,performance_schema,sys All four databases are MySQL The database that comes with it .

database	meaning
mysql	Storage MySQL All kinds of information required for the normal operation of the server （ The time zone 、 Master-slave 、 user 、 Authority, etc ）
information_schema	Provides various tables and views for accessing database metadata , Include database 、 surface 、 Field type and access rights, etc
performance_schema	by MySQL Server runtime status provides an underlying monitoring function , Mainly used to collect database server performance parameters
sys	Contains a series of convenient DBA And developers take advantage of performance_schema View of performance database for performance tuning and diagnosis

2.1. mysql A brief description of the library

We can go through show tables Command to view all data tables under a database .

stay mysql In the database user Table most commonly used , This table stores all the available MySQL User information of the server .

2.1. information_schema A brief description of the library

information_schema The main function of the library is to store various tables and views of database metadata , Include database 、 surface 、 Field type and access rights, etc .
We have the execution ahead show databases as well as show tables Command is actually from information_schema Pull data from the corresponding data table in the library .
information_schema The tables in save various database information .

Data sheet	The corresponding order	Store content
ENGINES	show engines;	Data engine
SCHEMATA	show databases;	All databases that the current user can see
TABLES	show tables;	All data tables that the current user can see
COLUMNS	show columns from table_name	All columns under the data table
STATISTICS	show index from table_name	Index under data table
PROCESSLIST	show processlist	Thread under data table

2. The representation of a database in a file system

Use CREATE DATABASE Database name Statement to create a database , What actually happens on the file system ？ Actually , Each database corresponds to a subdirectory under the data directory , Or corresponding to a folder , Whenever you create a new database ,MySQL Will help us do these two things ;

1. Create a subdirectory with the same name as the database under the data directory .
2. Create a subdirectory named... Under the same name as the database name db.opt The file of （ Is limited to MySQL 5.7 And previous versions ）, This file contains various properties of the database , For example, the character set and comparison rules of the database .

2.1. Data and indexes in the table

Reserve knowledge ： InnoDB In fact, the page is used as the basic unit to manage the storage space , The default page size is 16KB.
about InnoDB For storage engines , Each index corresponds to a tree B+ Trees , The B+ Each node of the tree is a data page , Data pages need not be physically contiguous , Because there is a two-way linked list between data pages to maintain the order of these pages .
InnoDB The leaf node of the cluster index of stored complete user records , That is to say, index is data , Data is index .

In order to better manage these pages ,InnoDB Propose a table space or file space （ English name ：table space perhaps file space） The concept of , This table space is an abstract concept , It can correspond to one or more real files on the file system （ The number of files corresponding to different tablespaces may be different ）. Each table space can be divided into many pages , Our table space is stored in some pages under a table space . There are different types of tablespaces here . Data directory /var/lib/mysql.

2.1.1. SYSTEM tablespace （system tablespace）

By default ,InnoDB Will create a data directory named ibdata1、 The size is 12MB The file of , This file is the representation of the corresponding system table space on the file system . How to 12MB？ Note that this file is a 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 system , Or just feel the original ibdata1. We can do it in MySQL Configure the corresponding file paths and their sizes at startup , For example, we can modify my.cnf The configuration file ：

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

In this way MySQL These two will be created after startup 512MB The size of the file system as the table space , Among them autoextend Indicates that the two files will be automatically expanded if they are not enough data2 File size .

2.1.2. 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 data tables , 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 use a separate table space to store testdb1 Database based testtable1 Words of watch , Then, in the database where the table is located testdb1 The directory will be testtable1 The table creates these two files ：

among :testtable1.ibd Files are used to store test Table data and index .

2.1.3. Setting of system table space and independent table space

We can specify for ourselves whether to use a system table space or a separate table space to store data , This function is controlled by the startup parameters innodb_file_per_table control , For example, we want to store the table data in the system table space , 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 

By default ：

explain ：innodb_file_per_table Parameter modification only affects the newly created table , It does not work for data tables that have allocated tablespaces , If we want to move tables that already exist in the system table space to a separate table space , You can use the following syntax ：

ALTER TABLE  Table name  TABLESPACE[=]innodb_file_per_table;

3. summary

Data directory /var/lib/mysql,MySQL The data in the database eventually falls into the disk file system .