What is inode? It will help you understand inode and what help inode provides when creating and deleting files.

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Preface

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inode

         In order to explain clearly inode Let's take a brief look at the file system ：

         Linux ext2 file system , The figure above shows the disk file system diagram （ The kernel memory image must be different ）, Disk is a typical block device , Hard disk partitions are divided into one by one block. One block The size of is determined when formatting , And cannot be changed . for example mke2fs Of -b Options can be set block The size is 1024、2048 or 4096 byte . The startup block in the figure above （Boot Block） The size of is certain ,

• Block Group：ext2 The file system is divided into several partitions according to the size of the partition Block Group. And each Block Group All have the same structural composition .
• Superblock （Super Block）： Store the structure information of the file system itself . The recorded information mainly includes ：bolck and inode Total amount , That is not used block and inode The number of , One block and inode Size , Time of last mount , The last time data was written , Information about other file systems such as the time when the disk was last verified .Super Block The information is corrupted , It can be said that the entire file system structure is destroyed .
• GDT,Group Descriptor Table： Block group descriptor , Describes block group attribute information , Interested students can learn about .
• Block bitmap （Block Bitmap）：Block Bitmap It's recorded in Data Block Which data block in is already occupied , Which data block is not occupied .
• inode Bitmap （inode Bitmap）： Every bit It means a inode Whether it is free or not .
• i Node table ： Storage file properties Such as file size , owner , Last modification time, etc .
• Data area ： Contents of stored documents .

          The idea of storing attributes and data separately seems simple , But how does it actually work ？ We go through touch A new file to see how it works .

To illustrate , Let's simplify the figure above ：

        In order to understand this diagram , Next, I will introduce ：

        Then we can understand better now ls 、 ls -l 、 cat The order is what to do again ：

        Then we can explain the process of creating files ：

         Now that you have created the file , Then this file must have its inode Number , That is from inode Bitmap Apply for one inode Number , And then from inode Table The corresponding position in , Put the current file's inode Fill in the attribute , Then if there is data written in , Then in Block Bitmap Apply for block , And establish  inode and block The correspondence between , Then write the data in , Then the file name of this file and inode No. is filled in the current directory  inode Table in .

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Creating a new file mainly includes the following 4 Operations ：

1. Storage attribute
       The kernel first finds a free i node （ Here is 790231）. The kernel records the file information .
2. Store the data
        The file needs to be stored in three disk blocks , The kernel found a free block ：300,500,800( Write casually ). The first piece of data in the kernel buffer Copied to the 300, Copy the next piece to 500, And so on .
3. Record the distribution
        The contents of the documents are in order 300,500,800 Deposit . Kernel in inode The above block list is recorded in the disk distribution area on the .
4. Add the file name to the directory
       New file name new.txt .
       linux How to record this file in the current directory ？ The kernel will enter (790231,new.txt ) Add to catalog file . File name and inode The correspondence between the file name and the contents and attributes of the file .

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