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qemu、rbd-nbd等客户端都是使用librbd进行ceph rbd卷的IO访问，如果要深入理解librbd，那么自己写一个client来访问rbd卷（控制操作、IO操作），肯定是个不错的学习方法。

#include 
   
    #include 
    
     #include 
     
      #include 
      
       #include 
       
        rbd_image_t init_image(rados_ioctx_t io_ctx) {    int ret = 0;    // 5. open rbd image    rbd_image_t image;    const char *image_name = "ee0c15655d44599a82a6bc0848b4aaf8";    ret = rbd_open(io_ctx, image_name, &image, NULL);    if (ret < 0) {        printf("couldn't open rbd image! err %d\n", ret);        return NULL;    } else {        printf("opened an rbd image: sotest\n");    }    return image;}int get_rbd_size(rbd_image_t image) {    int ret = 0;    uint64_t tsize = 0, size = 0;    // 6. get rbd image size    ret = rbd_get_size(image, &size);    if (ret < 0) {        printf("couldn't get image size! err %d\n", ret);        return EXIT_FAILURE;    } else {        printf("The size of the image is: %dMB\n", size/1024/1024);    }    tsize = size;    return tsize;}void rbd_finish_aiocb(rbd_completion_t c, void *arg){    // int ret = rbd_aio_wait_for_complete(c);    int ret = rbd_aio_get_return_value(c);    rbd_aio_release(c);    // for aio read callback, the read data should be copied here to caller    printf("aio callback: %d, %s\n", ret, (const char*)arg);}ssize_t io_write(rbd_image_t image, const char *buff) {    int off = 128;    int len = strlen(buff);    ssize_t ret;    ret = rbd_write(rbd_image_t image, uint64_t ofs, size_t len,                      const char *buf);    if (ret < 0) {        printf("write to image failed %s\n", ret);        return ret;    }    printf("write %s to image end\n", buff);    return ret;}int aio_write(rbd_image_t image, const char *buff) {    int off = 128;    rbd_completion_t c;    int ret = rbd_aio_create_completion((void *)buff, (rbd_callback_t) rbd_finish_aiocb, &c);    if (ret < 0) {        printf("create callback failed %s\n", ret);        return ret;    }    int len = strlen(buff);    ret = rbd_aio_write(image, off, len, buff, c);    if (ret < 0) {        printf("write to image failed %s\n", ret);        return ret;    }    printf("write %s to image end\n", buff);    return ret;}ssize_t io_read(rbd_image_t image, char *buff) {    int off = 128;    int len = 10;    ssize_t ret;    ret = rbd_read(image, off, len, buff);    if (ret < 0) {        printf("read from image failed %s\n", ret);        return ret;    }    printf("read from image end\n");    return ret;}int aio_read(rbd_image_t image, char *buff) {    int off = 128;    int len = 10;    rbd_completion_t c;    int ret = rbd_aio_create_completion(buff, (rbd_callback_t) rbd_finish_aiocb, &c);    if (ret < 0) {        printf("create callback failed %s\n", ret);        return ret;    }    memset(buff, 0, 128);    ret = rbd_aio_read(image, off, len, buff, c);    if (ret < 0) {        printf("read from image failed %s\n", ret);        return ret;    }    printf("read from image end\n");    return ret;}int main (int argc, const char **argv){        int ret;        char buff[128] = {0};        uint64_t size = 0;        /* Declare the cluster handle and required arguments. */        rados_t cluster;        char cluster_name[] = "ceph";        char user_name[] = "client.zstack";        uint64_t flags = 0;        /* Initialize the cluster handle with the "ceph" cluster name and the "client.admin" user */        int err;        err = rados_create2(&cluster, cluster_name, user_name, flags);        if (err < 0) {                fprintf(stderr, "%s: Couldn't create the cluster handle! %s\n", argv[0], strerror(-err));                exit(EXIT_FAILURE);        } else {                printf("\nCreated a cluster handle.\n");        }        /* Read a Ceph configuration file to configure the cluster handle. */        err = rados_conf_read_file(cluster, "/etc/ceph/ceph.conf");        if (err < 0) {                fprintf(stderr, "%s: cannot read config file: %s\n", argv[0], strerror(-err));                exit(EXIT_FAILURE);        } else {                printf("\nRead the config file.\n");        }        /* Read command line arguments */        err = rados_conf_parse_argv(cluster, argc, argv);        if (err < 0) {                fprintf(stderr, "%s: cannot parse command line arguments: %s\n", argv[0], strerror(-err));                exit(EXIT_FAILURE);        } else {                printf("\nRead the command line arguments.\n");        }        /* Connect to the cluster */        err = rados_connect(cluster);        if (err < 0) {                fprintf(stderr, "%s: cannot connect to cluster: %s\n", argv[0], strerror(-err));                exit(EXIT_FAILURE);        } else {                printf("\nConnected to the cluster.\n");        }        rados_ioctx_t io_ctx;        char *poolname = "data";        err = rados_ioctx_create(cluster, poolname, &io_ctx);        if (err < 0) {                fprintf(stderr, "%s: cannot open rados pool %s: %s\n", argv[0], poolname, strerror(-err));                rados_shutdown(cluster);                exit(EXIT_FAILURE);        } else {                printf("\nCreated I/O context.\n");        }    rbd_image_t image = init_image(io_ctx);    if (!image) {        perror("init_image");        rados_ioctx_destroy(io_ctx);        rados_shutdown(cluster);        return EXIT_FAILURE;    }    size = get_rbd_size(image);    printf("image size: %d\n", size);    sprintf(buff, "%s", "abcd123efg");    //aio_write(image, buff);    aio_read(image, buff);    // 7. close image, io context and rados object    ret = rbd_close(image);    if (ret < 0) {        printf("couldn't close rbd image! err %d\n", ret);        return EXIT_FAILURE;    } else {        printf("closed rbd image: sotest\n");    }    rados_ioctx_destroy(io_ctx);    rados_shutdown(cluster);    return 0;}
       
      
     
    
   

编译： gcc -g test.c -o rbdtest -lrados -lrbd

[root@10-96-75-15 tmp]# ./ceph-clientCreated a cluster handle.Read the config file.Read the command line arguments.Connected to the cluster.Created I/O context.opened an rbd image: sotestThe size of the image is: 20480MBimage size: 0read from image endaio callback: 10,closed rbd image: sotest

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