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: 

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 

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: 

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 

## Total volume virtual capacity 

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. 

## Space for Easy Tier 

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. 

## Extent space efficient volumes 

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. 

