Adding concept info for erasure coding

Change-Id: If3264d05312133e50e7d6034b80a9075182d139c
Partial-bug: #1357139

doc/admin-guide-cloud/source/objectstorage.rst

@@ -13,6 +13,7 @@ Object Storage
    objectstorage_arch.rst
    objectstorage_replication.rst
    objectstorage_auditors.rst
+   objectstorage_EC.rst
    objectstorage_account_reaper.rst
    objectstorage_tenant_specific_image_storage.rst
    objectstorage-monitoring.rst

doc/admin-guide-cloud/source/objectstorage_EC.rst

@@ -0,0 +1,30 @@
+==============
+Erasure coding
+==============
+
+Erasure coding is a set of algorithms that allows the reconstruction of
+missing data from a set of original data. In theory, erasure coding uses
+less capacity with similar durability characteristics as replicas.
+From an application perspective, erasure coding support is transparent.
+Object Storage (swift) implements erasure coding as a Storage Policy.
+See `Storage Policies
+<http://docs.openstack.org/developer/swift/overview_policies.html>`_
+for more details.
+
+There is no external API related to erasure coding. Create a container using a
+Storage Policy; the interaction with the cluster is the same as any
+other durability policy. Because support implements as a Storage Policy,
+you can isolate all storage devices that associate with your cluster's
+erasure coding capability. It is entirely possible to share devices between
+storage policies, but for erasure coding it may make more sense to use
+not only separate devices but possibly even entire nodes dedicated for erasure
+coding.
+
+.. important::
+   The erasure code support in Object Storage is considered beta in Kilo.
+   Most major functionality is included, but it has not been tested or
+   validated at large scale. This feature relies on `ssync` for durability.
+   We recommend deployers do extensive testing and not deploy production
+   data using an erasure code storage policy.
+   If any bugs are found during testing, please report them to
+   https://bugs.launchpad.net/swift