Understanding inode

inode Is a data structure in the file system .
notes ： The files mentioned here are hard disk files , Instead of being loaded into memory and managed by the process

Disk composition

A disk is a mechanical device , For example, a mechanical hard disk is a kind of disk .（ Solid state drives are not disks ）

Disks can store information , It can be divided into three areas to store information .

1. Different disk surfaces
2. cylinder
3. A sector

The cylinder divides the disk into many concentric circles , Different concentric circles are different cylinders .
The sector is on the same concentric circle , There are many small pieces , It's called sector .

Mapping of disk storage

File systems are managing physical memory , Abstract management . The hard disk will be divided into a linear structure . And this is a linear structure Block management

After layer upon layer , It will eventually be divided into one called block group The partition .

One block group There are many members in it , Mainly about several important .

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inode and data block（ a key ）

The file system is through struct file To manage hardware level files .struct file There's a dentry member , Can pass dentry Member found inode.（ Can be interpreted as struct file You can find inode）

Why find inode Well ？ Because by inode File data can be found .

inode It's a data structure , There are all kinds of documents and information stored inside , Mapping relationship with file data area . namely

map[inode id] = data block

inode bitmap

inode There is a member named inode id. Each file has its own inode id.
Without considering hard links , A file has only one inode id. The system also uses... When searching for files inode id To find out .

We can use ls -i To see the file inode id. The preceding figures are inode id

It can also be used. stat To see more file information , You can see inode id yes 665534

got it inode id, You know the inode bitmap What did you do .
bitmap It's a bitmap , every last bit One for one inode id Are you free , Not being used . Once used, you can no longer use it .

block bitmap

block bitmap It is also a bitmap , every last bit One for one data block Are you free , When you are free, you can put data in it .

inode table

One block group It can store many file information . from inode bitmap I know , One group There will be a lot of inode Need to manage , therefore inode table It's used to store inode Of .

data table

To store the data of a file .

Find a diagram of the file （ a key ）

1. Find... By file name inode id How much is the
2. Through the first inode id find inode stay inode table The location of
3. Then through the mapping relationship , Find the data of this file in data block Those positions of , And get the data

What you need to do to create a new file （ a key ）

1. Find a free inode id, use inode Save the file information of this new file .
2. Save the contents of the file to data block
3. Record the contents of the document data block, And keep it in inode Inside .
4. Add the file name to the directory

Why is there still space , But can't create a file

We know inode id In a group There is a limit , When inode id When it's finished , Even if there is space , Still unable to create file .

Operations required to create a directory

We know , Directory is also a kind of file . So the catalog is also related to inode It matters .

Similar to what is required to create a file , But it's not exactly the same , Because the directory has no data .
The directory data block What is stored inside is the file name under the directory and inode id The mapping relation of .

Summarize all the steps ：
1. Find a free inode id, Use this inode Save the file information of the directory
2. Save the contents of the file to data block Inside （ The contents here are the file name and inode id The mapping relation of ）
3. use inode Record where the data is placed data block in
4. Add the file name to the directory

Actions required to delete files

Deleting a file does not require that data block and inode Empty , Just put this inode id and data block id Become idle . This operation has no cost .

This is why data can be restored after deletion , The reason is that as long as the data is not overwritten , Still there .

Catalog files and inode

There is a phenomenon , When a directory file is unreadable , We can still access the contents of the files in the directory . But when a directory file is not executable , We cannot read or write the contents of the files in the directory .

This sum inode It's also relevant .

When a directory file is unreadable , It's just inode The information inside is unreadable , But we can still map to data block Get the files in the directory inode id To access the file . But if there is no directory x jurisdiction , We can't get through inode Get data block Of the mapping relationship .

Hard links

In general , One file name corresponds to one inode, But the hard link special file can be shared with other files inode.

ln  file   Hard links 

notes : Hard links are not links , It's an ordinary document .

Number of hard links in file information

This 2 Yes is the number of hard links to the file .
Why is the number of hard links in a newly created Directory 2？

Recall the definition of hard links , share inode id The number of files is the number of hard links . Who else shares the same with this directory inode id Well ？？？

We know linux There are two hidden files in each directory . and …, They are the current directory and the parent directory .

Turned out to be . This document and tmp Directories share the same inode id, So the number of hard links is 2.

Soft link

Soft links are a bit like shortcuts . If the executable program is in a deep Directory , It is inconvenient to enter and run （ Or it is inconvenient to use absolute path ） You can create soft links to files in the current directory , Then you can run in the current directory .

ln -s  file   Soft link 

l Soft links

Three times in the file information a c m

Access Last access time
Modify Last modification time of file content
Change Property last modified