OpenStack_Swift源代码分析——创建Ring及加入�设备源代码算法具体分析

1 创建Ring 代码具体分析

在OpenStack_Swift——Ring组织架构中我们具体分析了Ring的具体工作过程，以下就Ring中添加�设备，删除设备，已经又一次平衡的实现过程作具体的介绍。

首先看RingBuilder类

    def __init__(self, part_power, replicas, min_part_hours):
        #why 最大 2**32
        if part_power > 32:
            raise ValueError("part_power must be at most 32 (was %d)"
                             % (part_power,))
        if replicas < 1:
            raise ValueError("replicas must be at least 1 (was %.6f)"
                             % (replicas,))
        if min_part_hours < 0:
            raise ValueError("min_part_hours must be non-negative (was %d)"
                             % (min_part_hours,))

        self.part_power = part_power
        self.replicas = replicas
        self.min_part_hours = min_part_hours
        self.parts = 2 ** self.part_power          #分区数量即虚拟节点数量
        self.devs = []                             #用来存放设备的加入�的设备都会加入到devs里
        self.devs_changed = False
        self.version = 0

        self._last_part_moves_epoch = None
        self._last_part_moves = None

        self._last_part_gather_start = 0
        self._remove_devs = []
        self._ring = None

当中devs里面没一个dev含有的属性为：  dev  =  {'weight': 100.0, 'zone': 0, 'ip': '127.0.0.1', 'region': 0, 'parts': 0, 'id': 0, 'meta': 'some meta data', 'device': 'sda1', 'parts_wanted': 96, 'port': 6000}

当中part_wanted 是在加入�设备后设备须要的虚拟节点数，parts为设备已经被分配的虚拟节点。

在swift-ring-builder object.builder create 18 3 1

会调用Command类的create方法，详细看此方法：

 def create(self):
        """
        swift-ring-builder object.builder create 18 3 1
swift-ring-builder <builder_file> create <part_power> <replicas>
                                         <min_part_hours>
    Creates <builder_file> with 2^<part_power> partitions and <replicas>.
    <min_part_hours> is number of hours to restrict（限制） moving a partition more
    than once（不止一次）.
        """
        if len(argv) < 6:
            print Commands.create.__doc__.strip()
            exit(EXIT_ERROR)
        builder = RingBuilder(int(argv[3]), float(argv[4]), int(argv[5]))
        backup_dir = pathjoin(dirname(argv[1]), 'backups')
        try:
            mkdir(backup_dir)
        except OSError as err:
            if err.errno != EEXIST:
                raise
        builder.save(pathjoin(backup_dir, '%d.' % time() + basename(argv[1])))
        builder.save(argv[1])             #序列化
        exit(EXIT_SUCCESS)

在第一次创建时，首先会推断/etc/swift下是否有object.builder文件，没有须要创建此文件，并依据命令中个的 18 3 1 来实例化RingBuilder，最后将builder序列化。

2 为Ring加入�设备

创建了.builder文件后，就须要加入�设备，以下为加入�一个设备的命令：swift-ring-builder object.builder add z2-127.0.0.1:6020/sdb2 100

当中 add 指明为加入�设备 z2 为zone2，127.0.0.1:6020/sdb2为设备ip地址，绑定的port以及设备中存储文件的磁盘，100为设备的权重

以下看加入�设备的详细实现：

这种方法在main方法中药推断/etc/swift下是否有object.builder文件，在第一步create中我们已经创建了此文件，如今仅仅须要将他反序列化回来并用存的数据来实例化RingBuilder类,然后调用Commands类的add方法：

 def add(self):
        """
swift-ring-builder <builder_file> add
    [r<region>]z<zone>-<ip>:<port>[R<r_ip>:<r_port>]/<device_name>_<meta>
     <weight>
    [[r<region>]z<zone>-<ip>:<port>[R<r_ip>:<r_port>]/<device_name>_<meta>
     <weight>] ...

    Where <r_ip> and <r_port> are replication ip and port.

or

swift-ring-builder <builder_file> add
    [--region <region>] --zone <zone> --ip <ip> --port <port>
    --replication-ip <r_ip> --replication-port <r_port>
    --device <device_name> --meta <meta> --weight <weight>

    Adds devices to the ring with the given information. No partitions will be
    assigned to the new device until after running 'rebalance'. This is so you
    can make multiple device changes and rebalance them all just once.
        """
        if len(argv) < 5 or len(argv) % 2 != 1:
            print Commands.add.__doc__.strip()
            exit(EXIT_ERROR)

        for new_dev in _parse_add_values(argv[3:]):
            for dev in builder.devs:
                if dev is None:
                    continue
                if dev['ip'] == new_dev['ip'] and \
                        dev['port'] == new_dev['port'] and \
                        dev['device'] == new_dev['device']:
                    print 'Device %d already uses %s:%d/%s.' % \
                          (dev['id'], dev['ip'], dev['port'], dev['device'])
                    print "The on-disk ring builder is unchanged.\n"
                    exit(EXIT_ERROR)
            dev_id = builder.add_dev(new_dev)         #调用RingBuilder类的add_dev方法
            print('Device %s with %s weight got id %s' %
                  (format_device(new_dev), new_dev['weight'], dev_id))

        builder.save(argv[1])
        exit(EXIT_SUCCESS)

将新加入�的设备会加入到devs[]中，当中会调用add_dev,以下看此方法的详细实现

 def add_dev(self, dev):
        """
        Add a device to the ring. This device dict should have a minimum of the
        following keys:

        ======  ===============================================================
        id      unique integer identifier amongst devices. Defaults to the next
                id if the 'id' key is not provided in the dict
        weight  a float of the relative weight of this device as compared to
                others; this indicates how many partitions the builder will try
                to assign to this device
        region  integer indicating which region the device is in
        zone    integer indicating which zone the device is in; a given
                partition will not be assigned to multiple devices within the
                same (region, zone) pair if there is any alternative
        ip      the ip address of the device
        port    the tcp port of the device
        device  the device's name on disk (sdb1, for example)
        meta    general use 'extra' field; for example: the online date, the
                hardware description
        ======  ===============================================================

        .. note::
            This will not rebalance the ring immediately as you may want to
            make multiple changes for a single rebalance.

        :param dev: device dict

        :returns: id of device
        dev  =  {'weight': 100.0, 'zone': 0, 'ip': '127.0.0.1', 'region': 0, 'parts': 0, 'id': 0, 'meta': 'some meta data', 'device': 'sda1', 'parts_wanted': 96, 'port': 6000}
        """
        if 'id' not in dev:
            dev['id'] = 0
            if self.devs:
                dev['id'] = max(d['id'] for d in self.devs if d) + 1
        if dev['id'] < len(self.devs) and self.devs[dev['id']] is not None:
            raise exceptions.DuplicateDeviceError(
                'Duplicate device id: %d' % dev['id'])
        # Add holes to self.devs to ensure self.devs[dev['id']] will be the dev
        while dev['id'] >= len(self.devs):
            self.devs.append(None)
        dev['weight'] = float(dev['weight'])
        dev['parts'] = 0
        self.devs[dev['id']] = dev
        self._set_parts_wanted()         #设置part_wanted
        self.devs_changed = True
        self.version += 1
        return dev['id']

此方法主要是获得weight,并为设备dev中的id赋值，以及part_wanted赋值。当中part_wanted值的计算为：

((self.parts * self.replicas / sum(d['weight'] for d in self._iter_devs())) * dev['weight']) - dev['parts']

parts=2**power(此处在create方法中的18) replicas为备份数，此处为3

即part_wanted=（(虚拟节点数*备份数/全部已加入�的节点的weight和)*当前设备的weight）-已被分配给此设备的虚拟节点数

至此 创建Ring和为Ring加入�设备解说完毕，再完毕这两个步骤后，就须要平衡环，并为replica2part2dev（备份到分区到设备的映射）赋值，平衡算法是Ring 的核心算法，在下一篇文章中详细解说。

因为本人水平有限，文中难免出现理解错误，敬请指正、交流，谢谢！