Detailed explanation of the physical structure of the ext2 file system under Linux

Detailed explanation of the physical structure of the ext2 file system under Linux

In the Linux system, ext2 is a commonly used file system type. It is A relatively simple and efficient file system. In this article, we will delve into the physical structure of the ext2 file system, including super blocks, group descriptors, inode tables, data blocks, etc., and provide specific code examples to help readers better understand.

1. Super Block

Super block is one of the most important data structures in the ext2 file system. It stores the overall information of the file system. Such as the total number of inodes, the total number of data blocks, the number of inodes in each group, the number of data blocks, etc. The following is an example of the structure of a super block:

struct ext2_super_block {
    __le32 s_inodes_count; // inode总数
    __le32 s_blocks_count; // 数据块总数
    __le32 s_inodes_per_group; // 每组的inode数量
    __le32 s_blocks_per_group; // 每组的数据块数量
    // 其他字段省略
};

2. Group Descriptor

Group Descriptor stores the metadata of each group (group) Information, including inode bitmap, data block bitmap, inode table starting block number, data block starting block number, etc. The following is an example of the structure of a group descriptor:

struct ext2_group_desc {
    __le32 bg_inode_bitmap; // inode位图块号
    __le32 bg_block_bitmap; // 数据块位图块号
    __le32 bg_inode_table; // inode表的起始块号
    __le16 bg_free_blocks_count; // 空闲数据块数量
    __le16 bg_free_inodes_count; // 空闲inode数量
    // 其他字段省略
};

3. Inode Table

inode table stores metadata information of files or directories, such as Size, permissions, owner, timestamp, etc. Each file or directory corresponds to an index node (inode) in the inode table. The following is an example of the structure of each inode in the inode table:

struct ext2_inode {
    __le16 i_mode; // 文件类型和权限
    __le32 i_size; // 文件大小
    __le32 i_blocks; // 数据块数量
    __le32 i_block[15]; // 数据块号数组
    // 其他字段省略
};

4. Data Block

Data block is where the actual file content is stored, ext2 The file system uses indirect addressing to manage data blocks. A data block is composed of several sectors, and a sector is the smallest storage unit in the file system. The following is an example of the structure of a data block:

struct ext2_data_block {
    char data[1024]; // 数据块大小为1KB
};

5. Code example

The following is a simple sample program for reading super block information in an ext2 file system:

#include <stdio.h>
#include <fcntl.h>
#include <ext2fs/ext2_fs.h>

int main() {
    int fd = open("/dev/sda1", O_RDONLY);
    if (fd == -1) {
        perror("open");
        return 1;
    }

    struct ext2_super_block sb;
    lseek(fd, 1024, SEEK_SET); // 超级块位于偏移1024字节处
    read(fd, &sb, sizeof(sb));

    printf("Inode总数：%u
", sb.s_inodes_count);
    printf("数据块总数：%u
", sb.s_blocks_count);
    // 输出其他超级块信息

    close(fd);
    return 0;
}

Conclusion

This article provides a detailed analysis of the physical structure of the ext2 file system under Linux, including important parts such as super block, group descriptor, inode table and data block, and provides relevant code examples to help readers Get a deeper understanding of the internals of the ext2 file system. I hope this article can be helpful to readers.

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