librados的介绍与应用

时间 2019-11-29

标签 librados 介绍应用繁體版

原文原文链接

一个Ceph客户端，经过librados直接与OSD交互，来存储和取出数据。为了与OSD交互，客户端应用必须直接调用librados，链接一个Ceph Monitor。一旦链接好之后，librados会从Monitor处取回一个Cluster map。当客户端的应用想读或者取数据的时候，它会建立一个I/O上下文而且与一个pool绑定。经过这个I/O上下文，客户端将Object的名字提供给librados，而后librados会根据Object的名字和Cluster map计算出相应的PG和OSD的位置。而后客户端就能够读或者写数据。客户端的应用无需知道这个集群的拓扑结构。session

Ceph Storage Cluster handle封装了以下客户端配置，包括：app

1. 给rados_create()使用的User ID和给rados_create2()使用的user name，其中后者更受推荐异步

2. Cephx authentication keyasync

3. Monitor的ID和IP地址函数

4. 日志级别ui

5. 调试级别加密

因此，从你的app使用集群的步骤以下：1. 建立一个cluster handle，它将被用来链接集群；2. 使用这个handle来链接集群。为了链接集群，这个app必须提供一个monitor地址，一个用户名，还有一个authentication key。spa

步骤一配置Ceph cluster handle命令行

那么到底该如何去配置一个handle？Rados提供了一系列方法让你来设置所须要的设置的值。对于monitor和加密key的设置，一个简单的方法就是在ceph.conf文件中设置monitor的地址和keyring的path。举个例子：调试

[global]

mon host = 192.168.1.155

keyring = /etc/ceph/ceph.client.admin.keyring

一旦你创造了一个handle，那么能够经过读取ceph的配置文件来配置handle，也能够经过读取命令行参数或者环境参数来配置handle。

当链接好以后，客户端的app能够调用不少函数来影响整个集群，好比能够作如下事情：

* Get cluster statistics

* Use Pool Operation (exists, create, list, delete)

* Get and set the configuration

Ceph有一个很是强大的能力就是绑定不一样的pool。每一个pool可能拥有不一样数量的PG，对象拷贝数量和拷贝策略。举个例子，一个使用SSD的pool能够被设置为hot pool来存储热数据，一个使用HDD的pool能够被设置用来存储冷数据。

下面是一个配置handle并链接ceph集群的C代码示例：

#include <stdio.h>

#include <string.h>

#include <rados/librados.h>

int main (int argc, char argv**)

{

/* Declare the cluster handle and required arguments. */

rados_t cluster;

char cluster_name[] = "ceph";

char user_name[] = "client.admin";

uint64_t flags;

/* 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");

}

步骤二建立I/O上下文并应用

当你的应用有一个cluster handle以后而且链接上了Ceph集群以后，你能够建立一个I/O上下文来读或者写数据。一个I/O上下文绑定到一个特定pool的链接。I/O上下文的功能包括以下：

1. Write/read data and extended attributes

2. List and iterate over objects and extended attributes

3. Snapshot pools, list snapshots, etc.

Rados容许你同步或者异步地与cluster交互。

来看一段C代码的例子

#include <stdio.h>

#include <string.h>

#include <rados/librados.h>

int main (int argc, const char argv**)

{

* Continued from previous C example, where cluster handle and

* connection are established. First declare an I/O Context.

rados_ioctx_t io;

char *poolname = "data";

err = rados_ioctx_create(cluster, poolname, &io);

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");

}

/* Write data to the cluster synchronously. */

err = rados_write(io, "hw", "Hello World!", 12, 0);

if (err < 0) {

fprintf(stderr, "%s: Cannot write object \"hw\" to pool %s: %s\n", argv[0], poolname, strerror(-err));

rados_ioctx_destroy(io);

rados_shutdown(cluster);

exit(1);

} else {

printf("\nWrote \"Hello World\" to object \"hw\".\n");

}

char xattr[] = "en_US";

err = rados_setxattr(io, "hw", "lang", xattr, 5);

if (err < 0) {

fprintf(stderr, "%s: Cannot write xattr to pool %s: %s\n", argv[0], poolname, strerror(-err));

rados_ioctx_destroy(io);

rados_shutdown(cluster);

exit(1);

} else {

printf("\nWrote \"en_US\" to xattr \"lang\" for object \"hw\".\n");

}

* Read data from the cluster asynchronously.

* First, set up asynchronous I/O completion.

rados_completion_t comp;

err = rados_aio_create_completion(NULL, NULL, NULL, &comp);

if (err < 0) {

fprintf(stderr, "%s: Could not create aio completion: %s\n", argv[0], strerror(-err));

rados_ioctx_destroy(io);

rados_shutdown(cluster);

exit(1);

} else {

printf("\nCreated AIO completion.\n");

}

/* Next, read data using rados_aio_read. */

char read_res[100];

err = rados_aio_read(io, "hw", comp, read_res, 12, 0);

if (err < 0) {

fprintf(stderr, "%s: Cannot read object. %s %s\n", argv[0], poolname, strerror(-err));

rados_ioctx_destroy(io);

rados_shutdown(cluster);

exit(1);

} else {

printf("\nRead object \"hw\". The contents are:\n %s \n", read_res);

}

/* Wait for the operation to complete */

rados_wait_for_complete(comp);

/* Release the asynchronous I/O complete handle to avoid memory leaks. */

rados_aio_release(comp);

char xattr_res[100];

err = rados_getxattr(io, "hw", "lang", xattr_res, 5);

if (err < 0) {

fprintf(stderr, "%s: Cannot read xattr. %s %s\n", argv[0], poolname, strerror(-err));

rados_ioctx_destroy(io);

rados_shutdown(cluster);

exit(1);

} else {

printf("\nRead xattr \"lang\" for object \"hw\". The contents are:\n %s \n", xattr_res);

}

err = rados_rmxattr(io, "hw", "lang");

if (err < 0) {

fprintf(stderr, "%s: Cannot remove xattr. %s %s\n", argv[0], poolname, strerror(-err));

rados_ioctx_destroy(io);

rados_shutdown(cluster);

exit(1);

} else {

printf("\nRemoved xattr \"lang\" for object \"hw\".\n");

}

err = rados_remove(io, "hw");

if (err < 0) {

fprintf(stderr, "%s: Cannot remove object. %s %s\n", argv[0], poolname, strerror(-err));

rados_ioctx_destroy(io);

rados_shutdown(cluster);

exit(1);

} else {

printf("\nRemoved object \"hw\".\n");

}

步骤三关闭session

当你的APP完成了全部要完成的工做之后，这个app应该可以关闭链接而且关闭这个handle。对于异步I/O操做，app还应该保证全部的异步操做已经完成。

rados_ioctx_destroy(io);

rados_shutdown(cluster);