An in-depth discussion of the physical storage structure of the Linux ext2 file system

Linux ext2 file system is a file system used on most Linux operating systems. It uses an efficient disk storage structure to manage the storage of files and directories. Before we delve into the physical storage structure of the Linux ext2 file system, we first need to understand some basic concepts.

In the ext2 file system, data is stored in data blocks (blocks), which are the smallest allocable units in the file system. Each data block has a fixed size, usually 1KB, 2KB or 4KB. The file system also divides the data blocks on the disk into groups. Each group contains several data blocks and is described by a group descriptor.

Each group has a group descriptor. The group descriptor contains some important information, such as how many data blocks there are in the group, the starting position of the index node (inode), etc. Inodes are data structures used in the ext2 file system to describe file and directory attributes.

Next, let’s take a deep dive into the physical storage structure of the Linux ext2 file system, and attach some code examples to help better understand.

First, we need to open a Linux terminal and use the following command to create a new ext2 file system:

mkfs.ext2 /dev/sda1

This will create a new ext2 file on the device /dev/sda1 system.

Next, we can use the following command to mount the newly created ext2 file system:

mkdir /mnt/ext2
mount /dev/sda1 /mnt/ext2

Now that we have successfully mounted the ext2 file system, let’s check the file system Physical storage structure.

First, let’s look at the structure of the group descriptor table. The size of each group descriptor is 32 bytes, which contains some key information, such as the number of blocks in the group, the number of free blocks, the number of inodes, etc. The following is a simple C language sample code to read the group descriptor table:

#include <stdio.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/types.h>
#include <sys/stat.h>

#define BLOCK_SIZE 1024
#define GROUP_DESC_SIZE 32

int main() {
    int fd;
    char buf[BLOCK_SIZE];
    
    fd = open("/dev/sda1", O_RDONLY);
    if(fd == -1) {
        perror("open");
        return 1;
    }
    
    lseek(fd, BLOCK_SIZE * 2, SEEK_SET); // Seek to the location of the group descriptor table
    
    read(fd, buf, GROUP_DESC_SIZE); // Read the first group descriptor
    
    for(int i = 0; i < GROUP_DESC_SIZE; i++) {
        printf("%02X ", buf[i]);
    }
    
    close(fd);
    
    return 0;
}

This code opens the /dev/sda1 device and finds the group descriptor at the second data block (block) descriptor table and reads the contents of the first group descriptor. We can view the group descriptor table information by running this code.

In addition, we can also view the structure of the index node. Inodes also have a fixed size, usually 128 bytes or 256 bytes, and are used to describe file and directory details. The following is a simple C language sample code to read the contents of the index node:

#include <stdio.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/types.h>
#include <sys/stat.h>

#define BLOCK_SIZE 1024
#define INODE_SIZE 128

int main() {
    int fd;
    char buf[BLOCK_SIZE];
    
    fd = open("/dev/sda1", O_RDONLY);
    if(fd == -1) {
        perror("open");
        return 1;
    }
    
    lseek(fd, BLOCK_SIZE * 3, SEEK_SET); // Seek to the location of the first inode block
    
    read(fd, buf, BLOCK_SIZE); // Read the entire first inode block
    
    for(int i = 0; i < INODE_SIZE; i++) {
        printf("%02X ", buf[i]);
    }
    
    close(fd);
    
    return 0;
}

This code opens the /dev/sda1 device and finds the first data block at the third data block (block). inode block and read its contents. We can see the structure of the index nodes by running this code.

Through the above code examples and explanations, we have a deeper understanding of the physical storage structure of the Linux ext2 file system. By deeply studying the physical storage structure of the file system, you can better understand how the file system works, which is very helpful for system administrators and developers.

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