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+..
+ This work is licensed under a Creative Commons Attribution 3.0 Unported
+ License.
+
+ http://creativecommons.org/licenses/by/3.0/legalcode
+
+==========================================
+Storage Capacity Balance Strategy
+==========================================
+
+https://blueprints.launchpad.net/watcher/+spec/storage-capacity-balance
+
+As of now, Watcher optimizes only compute nodes.
+Storage optimization is also an important feature for centralized storage
+(non distributed storage).
+
+This spec will add Storage Capacity Balance Strategy to balance the
+storage capacity, which can be also considered a way to balance the
+storage workload. And we can use existing goal(workload_balancing) and
+action(volume_migrate) for this storage capacity balance.
+
+Problem description
+===================
+
+In current Data Center, the capacity of storage back-end may be not balanced,
+some are extremely high, some are idle. This situation will degrade the
+performance of I/O Read/Write, which will finally affect the QoS.
+This problem can be solved by storage capacity balance strategy.
+
+This strategy migrates volumes based on the capacity utilization of the cinder
+pools. It makes decision to migrate a volume whenever a pool's capacity
+utilization % is higher than the specified threshold. The migration of a
+volume should make the capacity utilization of the pool where it locates
+lower than the storage capacity utilization threshold.
+
+Use Cases
+----------
+
+As an administrator, I want to be able to trigger an audit that controls the
+storage capacity utilization below a certain threshold.
+
+Proposed change
+===============
+
+* Extend base strategy classes to add one new strategy - " Storage Capacity
+  Balance Strategy"
+
+* Use Cinder client to get all volumes with status in available or in-use
+  and no snapshots, and to get all pools except the pools listed as
+  exclude_pools in the configuration file.
+
+* Group volume pools into two categories: underload or overload pools
+  according to threshold:
+
+  .. code-block:: python
+
+      under_pools = list(filter(lambda p: float(p.total_capacity_gb) -
+      float(p.free_capacity_gb) < float(p.total_capacity_gb) * threshold, pools))
+
+      over_pools = list(filter(lambda p: float(p.total_capacity_gb) -
+      float(p.free_capacity_gb) >= float(p.total_capacity_gb) * threshold, pools))
+
+* Determine migrate_volumes, source pools and destination pools based on some
+  factors:
+
+  * whether a volume is mounted to a VM
+
+  * volume size,
+
+  * whether a volume is a mirrored volume
+
+  * priority
+
+Alternatives
+------------
+
+None
+
+Data model impact
+-----------------
+
+None
+
+REST API impact
+---------------
+
+None
+
+Security impact
+---------------
+
+None
+
+Notifications impact
+--------------------
+
+None
+
+Other end user impact
+---------------------
+
+None
+
+Performance Impact
+------------------
+
+None
+
+Other deployer impact
+---------------------
+
+None
+
+Developer impact
+----------------
+
+None
+
+Implementation
+==============
+
+Assignee(s)
+-----------
+
+Primary assignee:
+  <yumeng-bao>,<li-canwei2>
+
+Work Items
+----------
+
+1. Define proper threshold
+
+2. Write the execute function to locate pool overloaded
+
+3. Function to generate actions:volume_migrate or volume_retype.
+
+Dependencies
+============
+
+None
+
+Testing
+=======
+
+Unit and functional test are needed.
+
+Documentation Impact
+====================
+
+Add docs on how to use this strategy.
+
+References
+==========
+
+None
+
+History
+=======
+
+None