Understanding the disk layout of ext2 file system in Linux

In the Linux operating system, there are many file systems used, among which the ext2 file system is the most basic and common one. Understanding the disk layout of the ext2 file system is important to understanding how the file system works and how data is stored. In this article, we will introduce the disk layout of the ext2 file system, including super block, block group descriptor table, inode table, data block, etc., and provide specific code examples to help readers better understand.

1. Super Block

In the ext2 file system, the super block is one of the metadata of the file system and is used to record the basic information of the entire file system. The superblock is usually located at the beginning of the disk and is used to describe the state and structure of the entire file system. The following is a simplified example of the super block structure:

struct ext2_super_block {
    uint32_t s_inodes_count;      // 文件系统中inode总数
    uint32_t s_blocks_count;      // 文件系统中块总数
    uint32_t s_free_blocks_count; // 空闲块数量
    uint32_t s_free_inodes_count; // 空闲inode数量
    // 其他字段
    // ...
};

The super block contains information such as the total number of inodes, the total number of blocks, the number of free blocks, and the number of free inodes in the file system. By reading the superblock, you can obtain the overall information and status of the file system.

2. Block Group Descriptor Table

In the ext2 file system, the disk is divided into several block groups (Block Group). Each block group contains several data blocks, inodes, and block group descriptor tables. The block group descriptor table stores the basic information of each block group, such as the starting block number of the inode table, the starting block number of the free block bitmap, etc. The following is a simplified example of the block group descriptor table structure:

struct ext2_group_desc {
    uint32_t bg_block_bitmap;  // 空闲块位图的起始块号
    uint32_t bg_inode_bitmap;  // inode位图的起始块号
    uint32_t bg_inode_table;   // inode表的起始块号
    // 其他字段
    // ...
};

Through the block group descriptor table, the location of the key data structure in each block group can be found, which facilitates the management and operation of the file system.

3. Inode Table

In the ext2 file system, each file and directory corresponds to an inode node. The inode node records the attribute information of the file (such as file size, permissions, timestamps, etc.) and the pointers to file data blocks. The inode table stores information about all inode nodes and also contains free inode bitmaps to manage the allocation and release of inodes. The following is a simplified inode structure example:

struct ext2_inode {
    mode_t i_mode;              // 文件类型和权限
    uint32_t i_size;            // 文件大小
    uint32_t i_block[EXT2_N_BLOCKS]; // 文件数据块指针
    // 其他字段
    // ...
};

Through the inode table, you can find the inode node of the file, and then obtain the file's attribute information and data block pointer.

4. Data Blocks

In the ext2 file system, data blocks are used to store the actual data content of the file. Data blocks are allocated to files, and file data are scattered and stored in different data blocks. For small files, the data can be stored directly in the data block pointer in the inode node; for large files, the data will be stored in indirect, double indirect, and triple indirect blocks. The following is a simplified example of the data block pointer structure:

struct ext2_dir_block {
    uint32_t block_ptr[EXT2_PTRS_PER_BLOCK]; // 指向数据块的指针
    // 其他字段
    // ...
};

Data blocks are connected through pointers to form a file's data storage linked list. Data blocks provide access to the actual data content of the file.

Through the above introduction to the disk layout of the ext2 file system, we can better understand the organizational structure and data storage method of the file system. In actual programming, the file system can be operated and managed by reading the super block, block group descriptor table, inode table and data block. I hope this article can help readers gain a deeper understanding of the disk layout of the ext2 file system in Linux.

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