10.2.1 Concepts Volumes can be configured as thin-provisioned or fully allocated. Both of them can be  configured in standard pools and data reduction pools. In IBM Storwize V7000, each volume has  virtual capacity  and  real capacity  parameters.  Virtual capacity  is the volume storage capacity that is available to a host Operating System  (OS) and is used by it to create a file system.  Real capacity  is the storage capacity that is  allocated to a volume from a pool, it shows the actual amount of space that is used on  physical storage. Fully allocated volumes are created with the same amount of  real capacity  and  virtual  capacity . This type uses no storage efficiency features.  A thin-provisioned volume presents a different capacity to mapped hosts than the capacity  that the volume uses in the storage pool, so  real  and  virtual capacities  might not be equal.  The virtual capacity of a thin-provisioned volume is typically significantly larger than its real  capacity. As more information is written by the host to the volume, more of the real capacity is  used. The system identifies read operations to unwritten parts of the virtual capacity and  returns zeros to the server without using any real capacity. The autoexpand feature prevents a thin-provisioned volume from using up its capacity and  going offline. As a thin-provisioned volume uses capacity, the autoexpand feature maintains a  fixed amount of unused real capacity, called the  contingency capacity . For thin-provisioned  volumes in standard pools, the autoexpand feature can be turned on and off. For  thin-provisioned volumes in data reduction pools, the autoexpand feature is always enabled. A thin-provisioned volume can be converted non-disruptively to a fully allocated volume, or  vice versa, by using the volume mirroring function. For example, you can add a  thin-provisioned copy to a fully allocated primary volume and then remove the fully allocated  copy from the volume after they are synchronized. The fully allocated to thin-provisioned migration procedure uses a zero-detection algorithm so  that grains that contain all zeros do not cause any real capacity to be used. Usually, if the  Storwize V7000 is to detect zeros on the volume, you must use software on the host side to  write zeros to all unused space on the disk or file system. For more information about creating thin-provisioned volumes, see Chapter 7, “Volumes” on  page 241. In a standard pool, the system uses the real capacity to store data that is written to the  volume, and metadata that describes the thin-provisioned configuration of the volume.  Metadata uses volume real capacity and usually needs less than 0.5% of virtual capacity to  store its data.  This means that if your host used 100% of virtual capacity, some extra space is required on  your storage pool to store thin provisioning metadata. In a worst case, the real size of a thin  provisioned volume can be 100.5% of its virtual capacity.  In a data reduction pool, metadata for a thin provisioned volume is stored separately from  user data, and does not count in the volumes real capacity. 10.2.2 Implementation Metadata 416  Implementing the IBM Storwize V7000 with IBM Spectrum Virtualize V8.2.1