Oversubscription in thin provisioning

Cinder spec: Over Subscription in Thin Provisioning https://review.openstack.org/#/c/129342/12/specs/kilo/over-subscription-in-thin-provisioning.rst
cinder bp: Over subscription in thin provisioning https://blueprints.launchpad.net/cinder/+spec/over-subscription-in-thin-provisioning

所谓 Oversubscription in thin provisioning，就相当于是 Thin Provisioning Storage Pool 的超分比限制，防止 Thin Provisioning Storage Pool 被无限放大。对应的配置项是 max_over_subscription_ratio，默认值为 20.0。e.g.

[lvm-1]

volume_group = centos

volume_driver = cinder.volume.drivers.lvm.LVMVolumeDriver

volume_backend_name = lvm-1

iscsi_helper = tgtadm

iscsi_protocol = iscsi

lvm_max_over_subscription_ratio = 25.0

功能生效的地方是 Cinder Scheduler Filter capacity_filter，代码如下：

# cinder/scheduler/filters/capacity_filter.py

        # Only evaluate using max_over_subscription_ratio if

        # thin_provisioning_support is True. Check if the ratio of

        # provisioned capacity over total capacity has exceeded over

        # subscription ratio.

        if (thin and backend_state.thin_provisioning_support and

                backend_state.max_over_subscription_ratio >= 1):

            provisioned_ratio = ((backend_state.provisioned_capacity_gb +

                                  requested_size) / total)

            LOG.debug("Checking provisioning for request of %s GB. "

                      "Backend: %s", requested_size, backend_state)

            if provisioned_ratio > backend_state.max_over_subscription_ratio:

                msg_args = {

                    "provisioned_ratio": provisioned_ratio,

                    "oversub_ratio": backend_state.max_over_subscription_ratio,

                    "grouping": grouping,

                    "grouping_name": backend_state.backend_id,

                }

                LOG.warning(

                    "Insufficient free space for thin provisioning. "

                    "The ratio of provisioned capacity over total capacity "

                    "%(provisioned_ratio).2f has exceeded the maximum over "

                    "subscription ratio %(oversub_ratio).2f on %(grouping)s "

                    "%(grouping_name)s.", msg_args)

                return False

如果 provisioned_ratio > backend_state.max_over_subscription_ratio 为 True 则表示当前 provisioned 的比率已经大于 Oversubscription in thin provisioning 设定的比率了，所以当前 Cinder Backend 无法继续创建新的 Volume。

那么，provisioned_ratio 是怎么得到的呢？代码如下：

provisioned_ratio = ((backend_state.provisioned_capacity_gb + requested_size) / total)

首先弄清楚这几个变量的含义及取值：

# cinder/utils.py

    # provisioned_capacity_gb is the apparent total capacity of

    # all the volumes created on a backend, which is greater than

    # or equal to allocated_capacity_gb, which is the apparent

    # total capacity of all the volumes created on a backend

    # in Cinder. Using allocated_capacity_gb as the default of

    # provisioned_capacity_gb if it is not set.

    allocated_capacity_gb = capability.get('allocated_capacity_gb', 0)

    provisioned_capacity_gb = capability.get('provisioned_capacity_gb',

                                             allocated_capacity_gb)

    thin_provisioning_support = capability.get('thin_provisioning_support',

                                               False)

    total_capacity_gb = capability.get('total_capacity_gb', 0)

    free_capacity_gb = capability.get('free_capacity_gb', 0)

官方说明：

total_capacity: This is an existing parameter already reported by the driver. It is the total physical capacity. Example: Assume backend A has a total physical capacity of 100G.
available_capacity: This is an existing parameter already reported by the driver. It is the real physical capacity available to be used. Example: Assume backend A has a total physical capacity of 100G. There are 10G thick luns and 20G thin luns (10G out of the 20G thin luns are written). In this case, available_capacity = 100 - 10 -10 = 80G.
used_capacity: This parameter is calculated by the difference between total_capacity and available_capacity. It is used below for calculating used ratio.
volume_size: This is an existing parameter. It is the size of the volume to be provisioned.
provisioned_capacity: This is a new parameter. It is the apparent allocated space indicating how much capacity has been provisioned. Example: User A created 2x10G volumes in Cinder from backend A, and user B created 3x10G volumes from backend A directly, without using Cinder. Assume those are all the volumes provisioned on backend A. The total provisioned_capacity will be 50G and that is what the driver should be reporting.
allocated_capacity: This is an existing parameter. Cinder uses this to keep track of how much capacity has been allocated through Cinder. Example: Using the same example above for provisioned_capacity, the allocated_capacity will be 20G because that is what has been provisioned through Cinder. allocated_capacity is documented here to differentiate from the new parameter provisioned_capacity.

简要说明：

allocated_capacity_gb：实际已分配的容量。
provisioned_capacity_gb：已置备的容量，大于或等于 allocated_capacity_gb。

注：所谓已置备是存储领域的专业术语，表示逻辑上的已经分配出去的虚拟容量，可能是精简置备的也可能是厚置备的。需要与实际已分配的容量作一个区分。
total_capacity_gb：实际的总容量。
free_capacity_gb：实际剩余的容量。

在 LVM Driver 中的含义：

allocated_capacity_gb: 在 cinder 中分配的 cinder volume 总大小
free_capacity_gb: vg 中空闲的容量
provisioned_capacity_gb： vg 上面分配的总大小，因为文件稀疏（Thin）的问题，这个值可能很大
total_capacity_gb: vg 容量的总大小
max_over_subscription_ratio: 最大超配比

回过头来在看看这条计算公式：

provisioned_ratio = ((backend_state.provisioned_capacity_gb + requested_size) / total)

provisioned_ratio 就是当前已经被置备的容量的比率，包括精简置备或厚置备的情况，也就是我们不希望它太过于大的比率。理应小于 max_over_subscription_ratio。

但有一个问题需要注意，在 LVMDriver 中使用 Thin provisioning 时，provisioned_capacity_gb 马上就等于 VG 的总容量。你会发现虽然你还没有创建任何 Volume，但 provisioned_capacity_gb 就已经等于 VG 的 Size 了。代码如下：

# cinder/volume/drivers/lvm.py

        if self.configuration.lvm_mirrors > 0:

            total_capacity =\

                self.vg.vg_mirror_size(self.configuration.lvm_mirrors)

            free_capacity =\

                self.vg.vg_mirror_free_space(self.configuration.lvm_mirrors)

            provisioned_capacity = round(

                float(total_capacity) - float(free_capacity), 2)

        elif self.configuration.lvm_type == 'thin':

            total_capacity = self.vg.vg_thin_pool_size

            free_capacity = self.vg.vg_thin_pool_free_space

            provisioned_capacity = self.vg.vg_provisioned_capacity

        else:

            total_capacity = self.vg.vg_size

            free_capacity = self.vg.vg_free_space

            provisioned_capacity = round(

                float(total_capacity) - float(free_capacity), 2)

这是因为 Cinder 假设，当你使用 LVM Thin provisioning 的时候，那么整个 VG 都应该是 Thin provisioning 的，不存在 Thin 和 Thick 混合的情况，否则无法正确进行容量的计算。因此，当我们使用 LVM Thin provisioning 时，切记要划分一个干净的 VG 给 LVM Backend，否则就会出现资源计算错误的问题。

巴特西

Cinder LVM Oversubscription in thin provisioning

目录

文章目录

Oversubscription in thin provisioning

最新文章

热门文章