Explore the Linux caching mechanism: detailed explanation of memory, disk and file system caching

In-depth understanding of Linux cache mechanism: memory cache, disk cache and file system cache

Introduction: In Linux system, cache is an important mechanism for Accelerate data access and improve system performance. This article will delve into the three caching mechanisms in Linux: memory caching, disk caching, and file system caching, and provide specific code examples to help readers better understand and use these caching mechanisms.

1. Memory cache

Memory cache means that the Linux system caches file data on the disk in the memory to reduce frequent reads and writes to the disk, thereby speeding up data access. The memory cache in the Linux system mainly consists of page cache. When an application reads a file, the operating system reads the contents of the file into the page cache and stores them in memory. The next time the file is read, the operating system first checks whether the cache data for the file exists in the page cache. If it exists, it reads it directly from the cache instead of accessing the disk again. This mechanism can significantly improve file access speed.

The following is a simple C code example that shows how to use the memory cache:

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

int main() {
    int fd;
    struct stat sb;
    char *file_data;

    // 打开文件
    fd = open("test.txt", O_RDONLY);
    if (fd == -1) {
        perror("open");
        exit(1);
    }

    // 获取文件大小
    if (fstat(fd, &sb) == -1) {
        perror("fstat");
        exit(1);
    }

    // 将文件映射到内存中
    file_data = mmap(NULL, sb.st_size, PROT_READ, MAP_PRIVATE, fd, 0);
    if (file_data == MAP_FAILED) {
        perror("mmap");
        exit(1);
    }

    // 通过内存访问文件内容
    printf("%s", file_data);

    // 解除内存映射
    if (munmap(file_data, sb.st_size) == -1) {
        perror("munmap");
        exit(1);
    }

    // 关闭文件
    close(fd);

    return 0;
}

The above code uses the mmap function to map the file into memory and access the file content through the pointer file_data. In this way, the contents of the file will be cached in memory, and the file contents can be read directly the next time it is accessed, without the need to access the disk again.

2. Disk cache

In addition to the memory cache, the Linux system also has an important caching mechanism which is the disk cache. Disk caching means that Linux uses part of the memory as a cache for disk I/O to improve the performance of disk access. When an application performs a disk read or write operation, the operating system first caches the data in memory and then writes the data to disk. This mechanism can reduce frequent access to the disk and improve disk read and write efficiency.

The following is a simple C code example that shows how to use the disk cache:

#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <unistd.h>

int main() {
    int fd;
    char buffer[512];

    // 打开文件
    fd = open("test.txt", O_WRONLY | O_CREAT, S_IRUSR | S_IWUSR);
    if (fd == -1) {
        perror("open");
        exit(1);
    }

    // 写入文件
    write(fd, buffer, sizeof(buffer));

    // 刷新文件缓冲
    fsync(fd);

    // 关闭文件
    close(fd);

    return 0;
}

The above code uses the write function to write data to the file, and refreshes the file buffer through the fsync function. In this way, data is first cached in memory and then written to disk uniformly. This mechanism can significantly improve disk write performance.

3. File system cache

The file system cache refers to the cache used by the file system in the Linux system and is used to accelerate file system access. The file system cache mainly consists of file system data structures and metadata (such as file permissions, creation time, etc.). When an application performs file system operations, the operating system caches relevant data in memory to improve file system access speed.

The following is a simple C code example that shows how to use the file system cache:

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

int main() {
    int fd;

    // 打开文件
    fd = open("test.txt", O_RDONLY);
    if (fd == -1) {
        perror("open");
        exit(1);
    }

    // 修改文件权限
    if (fchmod(fd, S_IRUSR | S_IWUSR) == -1) {
        perror("fchmod");
        exit(1);
    }

    // 关闭文件
    close(fd);

    return 0;
}

The above code uses the fchmod function to modify the permissions of the file. In this way, file-related information will be cached in memory and can be used directly in subsequent file accesses, improving the efficiency of file operations.

Conclusion:

This article deeply explores the three caching mechanisms in Linux: memory cache, disk cache and file system cache, and provides specific code examples. By understanding and using these caching mechanisms, you can improve system performance and speed up data access. I hope this article will help readers understand and apply the Linux caching mechanism.

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