6.1 Working with storage pools A managed disk (MDisk) is a logical unit (LU) of physical storage. MDisks are RAID arrays  that are constructed of internal storage disks or flash disk modules, or LUs that are exported  from external storage systems. Storage pools act as a container for MDisks by dividing the  MDisks into extents. Storage pools provision the available capacity from the extents to  volumes. An overview of how storage pools, MDisks, and volumes are related is shown in Figure 6-1.  IBM SAN Volume Controller organizes storage into pools to ease storage management and  make it more efficient. All MDisks in a pool are split into extents of the same size and volumes  are created out of the available extents. The extent size is a property of the storage pool and  cannot be changed after the pool is created. It is possible to add MDisks to a pool to provide  more extents or to remove extents from the pool by deleting the MDisk.  Storage pools can be further divided into subcontainers that are called  child pools . Child  pools are created from existing capacity that is allocated to a parent pool. They have a similar  extent size setting as their parent pool and can be used for volume operation.  When a storage pool is created, you can define it to have the Data reduction feature enabled.  This feature creates a  Data Reduction Pool  (DRP). DRPs increase infrastructure capacity  use by using new efficiency functions. DRPs enable you to automatically de-allocate and  reclaim capacity of thin-provisioned volumes that contain deleted data and enable this  reclaimed capacity to be used by other volumes. DRPs also support data deduplication and  compression features.  For more information about DRP planing and implementation, see  Introduction and  Implementation of Data Reduction Pools and Deduplication , SG24-8430. Throttles can be defined for storage pools to control I/O operations. If a throttle limit is  defined, the system processes the I/O for that object, or delays the processing of the I/O to  free resources for more critical I/O operations. You can create an IOPS, bandwidth limit, or  both.  214  Implementing the IBM System Storage SAN Volume Controller with IBM Spectrum Virtualize V8.2.1 Figure 6-1 Relationship between MDisks, Storage Pools, and Volumes