Ceph RBD编程接口Librbd(C++) -- 映像创建与数据读写

目前我们有两种路径使用Ceph的块存储：
- 利用QEMU/KVM通过librbd与 Ceph 块设备交互，主要为虚拟机提供块存储设备，如下图所示；
- 利用kernel module与Host kernel交互，主要为物理机提供块设备支持。

Librbd 是Ceph提供的块存储接口的抽象，它提供C/C++、Python等多种接口。对于C++，最主要的两个类就是RBD 和 Image。 RBD 主要负责创建、删除、克隆映像等操作，而Image 类负责映像的读写等操作。

准备工作

对于任何客户端应用，都需要首先连接到一个运行良好的Ceph集群。

获取集群句柄

//声明Rados对象，并初始化
librados::Rados rados;
ret = rados.init("admin"); // just use the client.admin keyring
if (ret < 0) { // let's handle any error that might have come back
    std::cerr << "couldn't initialize rados! err " << ret << std::endl;
    ret = EXIT_FAILURE;
    return EXIT_FAILURE;
} else {
    std::cout << "we just set up a rados cluster object" << std::endl;
}

//获取配置文件信息: /etc/ceph/ceph.conf
// 1. 根据命令行参数
/*
ret = rados.conf_parse_argv(argc, argv);
if (ret < 0) {
    // This really can't happen, but we need to check to be a good citizen.
    std::cerr << "failed to parse config options! error " << ret << std::endl;
    ret = EXIT_FAILURE;
    return EXIT_FAILURE;
} else {
    std::cout << "we just parsed our config options" << std::endl;
    // We also want to apply the config file if the user specified
    // one, and conf_parse_argv won't do that for us.
    for (int i = 0; i < argc; ++i) {
   
   
        if ((strcmp(argv[i], "-c") == 0) || (strcmp(argv[i], "--conf") == 0)) {
            ret = rados.conf_read_file(argv[i+1]);
            if (ret < 0) {
                // This could fail if the config file is malformed, but it'd be hard.
                std::cerr << "failed to parse config file " << argv[i+1]
                        << "! error" << ret << std::endl;
                ret = EXIT_FAILURE;
                return EXIT_FAILURE;
            }
            break;
        }
    }
}
*/
// 2. 程序里面指定
ret = rados.conf_read_file("/etc/ceph/ceph.conf");
if (ret < 0) {
    // This could fail if the config file is malformed, but it'd be hard.
    std::cerr << "failed to parse config file! err " << ret << std::endl;
    ret = EXIT_FAILURE;
    return EXIT_FAILURE;
}

连接集群

ret = rados.connect();
if (ret < 0) {
    std::cerr << "couldn't connect to cluster! err " << ret << std::endl;
    ret = EXIT_FAILURE;
    return EXIT_FAILURE;
} else {
    std::cout << "we just connected to the rados cluster" << std::endl;
}

创建I/O上下文环境

如果没有存储池，需要先新建一个存储池。

新建存储池

const char *pool_name = "gnar";
ret = rados.pool_create(pool_name);
if (ret < 0) {
    std::cerr << "couldn't create pool! error " << ret << std::endl;
    ret = EXIT_FAILURE;