In the DS8880, the logical extent structure changed to move to a single volume structure for  all volumes. All volumes now use an improved metadata extent structure, similar to what was  previously used for ESE volumes. This unified extent structure greatly simplifies the internal  management of logical volumes and their metadata. No area is fixed and reserved for volume  metadata, and this capacity is added to the space that is available for use. The metadata is allocated in the Storage Pool when volumes are created, and the space that  is used by metadata is referred to as the  Pool Overhead . Pool Overhead means that the  amount of space that can be allocated by volumes is variable and depends on both the  number of volumes and the logical capacity of these volumes. Tips:  The following characteristics apply to the DS8880: For storage pools with large extents, metadata is also allocated as large extents (1 GiB for FB  pools or 1113 cylinder for CKD pools). Large extents that are allocated for metadata are  subdivided into 16 MiB subextents, which are also referred to as  metadata extents , for FB  volumes, or 21 cylinders for CKD. For extent pools with small extents, metadata extents are  also small extents. Sixty-four metadata subextents are in each large metadata extent for FB,  and 53 metadata subextents are in each large metadata extent for CKD.  For each FB volume that is allocated, an initial 16 MiB metadata subextent or metadata small  extent is allocated, and an additional 16 MiB metadata subextent or metadata small extent is  allocated for every 10 GiB of allocated capacity or portion of allocated capacity. For each CKD volume that is allocated, an initial 21 cylinders metadata subextent or  metadata small extent is allocated, and an additional 21 cylinders metadata subextent or  metadata small extent is allocated for every 11130 cylinders (or 10 3390 Model 1) of allocated  capacity or portion of allocated capacity. For example, a 3390-3 (that is, 3339 cylinders or about 3 GB) or 3390-9 (that is, 10,017  cylinders or 10 GB) volume takes two metadata extents (one metadata extent for the volume  and another metadata extent for any portion of the first 10 GB). A 128 GB FB volume takes  14 metadata extents (one metadata extent for the volume and another 13 metadata extents to  account for the 128 GB). 126  IBM DS8880 Architecture and Implementation GLYPH<SM590000> As you define new logical volumes, you must allow for the fact that this metadata space  continues to use user extent space. GLYPH<SM590000> As volumes are defined, the metadata is immediately allocated and reduces the size of  the available user extents in the storage pool. GLYPH<SM590000> The size of the metadata for a logical volume depends on the number and size of the  logical volumes that are being defined. GLYPH<SM590000> Metadata is variable in size. Metadata is used only as logical volumes are defined.