8456ch_Virtualization_Concepts.fm Draft Document for Review December 5, 2019 4:33 pm 118  IBM DS8900F Architecture and Implementation Figure 4-12 Metadata allocation For each CKD volume that is provisioned (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). Figure 4-12 shows an illustration of 3 GB and 12 GB FB volumes for a storage pool with large  extents. In an extent pool with small extents, there is no concept of sub-extents. You just have  user extents, unused extents, and metadata extents. Metadata extents with free space can be used for metadata by any volume in the extent pool. With this concept of metadata extents and user extents within an extent pool, some planning  and calculations are required. This is particularly true in a mainframe environment where  often thousands of volumes are defined and often the whole capacity is provisioned  (allocated) during the initial configuration. You have to calculate the capacity used up by the  metadata to get the capacity that can be used for user data. This is only important when fully  provisioned volumes will be used. Thin-provisioned volumes consume no space when  created, only metadata and space will be consumed when data is actually written. For extent pools with small extents, the number of available user data extents can be  estimated as follows: (user extents) = (pool extents) - (number of volumes) - (total virtual  capacity)/10 For extent pools with regular 1 GiB extents, when the details of the volume configuration are  not known, you can estimate the number of metadata extents based on many volumes only.  The calculations are performed as shown: