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: This formula overestimates the space that is used by the metadata by a small amount  because it assumes wastage on every volume. However, the precise size of each volume  does not need to be known. Two examples are explained: For systems that can support up to 4 PB of virtual capacity when using small extents, you  should not allocate more than 2 PB because Easy Tier is not able to monitor the capacity  beyond 2 PB. Note that the extra 2 PB of virtual capacity can still be used for Safeguarded  Copy backup capacity because this backup capacity is not monitored by Easy Tier. If you need to allocate more than 2 PB of virtual capacity, use large extents.  The Easy Tier function of DS8000 monitors access to extents transparently to host  applications. Easy Tier moves extents to higher or lower tiers depending on the access  frequency or heat of an extent. To be able to move around extents, some free space or free  extents must be available in an extent pool. You can manually have an eye on this, but a better  option is to let the system reserve some space for Easy Tier extent movements. The  chsi command can be used to change the  ETSRMode  parameter to  Enabled . This setting causes the  storage system to reserve some capacity for Easy Tier. Obviously, when this mode is  enabled, capacity for new volume allocations is reduced.  Volumes can be created as thin-provisioned or fully provisioned. Space for a thin-provisioned volume is allocated when a write occurs. More precisely, it is  allocated when a destage from the cache occurs and insufficient free space is left on the  currently allocated extent.  Therefore, thin provisioning allows a volume to exist that is larger than the physical space in  the extent pool that it belongs to. This approach allows the “host” to work with the volume at  its defined capacity, even though insufficient physical space might exist to fill the volume with  data.  128  IBM DS8880 Architecture and Implementation Extent space efficient volumes Space for Easy Tier Total volume virtual capacity  GLYPH<SM590000> FB Pool Overhead = (Number of volumes  2 + total volume extents/10)/64 and rounded  up to the nearest integer GLYPH<SM590000> CKD Pool Overhead = (Number of volumes  2 + total volume extents/10)/53 and rounded  up to the nearest integer GLYPH<SM590000> A CKD storage pool has 6,190 extents in which you expect to allocate all of the space on  700 volumes. Your Pool Overhead is 39 extents by using this calculation: (700 x 2 + 6190/10)/53 = 38.09 GLYPH<SM590000> An FB storage pool has 6,190 extents in which you expect to use thin provisioning and  allocate up to 12,380 extents (2:1 overprovisioning) on 100 volumes. Your Pool Overhead  is 23 extents by using this calculation: (100 x 2 + 12380/10)/64 = 22.46