For several years already the DS8000 series has featured two types of space efficient volumes:
- Track space efficient (TSE) volumes
- Extent space efficient (ESE) volumes
Track space efficient (TSE) volumes are used as target volumes of a FlashCopy Space Efficient operation and require the definition of a repository from which tracks are gradually allocated.
Extent space efficient (ESE) volumes are the actual thin provisioned volumes that can be used by host applications.The dynamic allocation increments for ESE logical volumes is based on the same existing 1 GB extents used for standard volumes.
With the introduction of R 7.2, there is now an option to create an ESE Repository for the ESE logical volumes
While it may seem simpler to create ESE logical volumes without a repository, there are some benefits to using a repository.
The available space to store data on the ESE volumes is protected in the pool, and cannot be used for standard volumes when creating new ESE volumes.
The ‘opratio’ (over provisioned ratio - that is the ratio between the total virtual capacity of all ESE logical volumes and the available physical extents in the pool) is fixed. This is most important if you were to add a new standard volume to the extent pool, the opratio would not change.
If adding physical capacity to the extent pool, the percentage of the size of the repository to the size of the extent pool is maintained. Therefore when capacity is added to the extent pool, the size of the ESE repository is automatically increased. In this scenario the opratio will decrease as a result.
- Future functions might require that a repository exist.
To read more about ESE repository and new DSCLI controls for ESE volumes, refer to the new draft version of the IBM RedPaper, DS8000 Thin Provisioning, REDP-4554