increase the number of extents that are available for new volume copies or to expand existing  volume copies. The system automatically balances volume extents between the MDisks to  provide the best performance to the volumes.  You cannot use the data migration function to migrate volumes between Parent Pools that  have different extent sizes. However, you can use volume mirroring to move data to a Parent  Pool that has a different extent size. Consider choosing extent size wisely according to you  future needs. Small extents limit your overall usable capacity as well as using a larger extent  size can waste storage. When you create or manage a Parent Pool, consider the following general guidelines: Before you remove MDisks from a Parent Pool, ensure that the Parent Pool has enough  capacity for any Child Pools that are associated with the Parent Pool.  GLYPH<SM590000> Ensure that all MDisks that are allocated to the same tier of a Parent Pool are the same  RAID type. Allocating MDisks within the same tier ensures that a single failure of a  physical disk does not take the entire pool offline. For example, if you have three RAID-5  arrays in one pool and add a non-RAID disk to this pool, you lose access to all the data  that is striped across the pool if the non-RAID disk fails. Similarly, for performance  reasons, you must not mix RAID types. The performance of all volumes is reduced to the  lowest achiever in the tier.  GLYPH<SM590000> An MDisk can be associated with just one Parent Pool. GLYPH<SM590000> You can specify a warning capacity for a pool. A warning event is generated when the  amount of space that is used in the pool exceeds the warning capacity. The warning  threshold is especially useful with thin-provisioned volumes that are configured to  automatically use space from the pool. GLYPH<SM590000> Volumes are associated with just one pool, except when you migrate between Parent  Pools. GLYPH<SM590000> Volumes that are allocated from a Parent Pool are striped across all the storage that is  placed into that Parent Pool. This also enables nondisruptive migration of data from one  storage system to another storage system and helps simplify the decommissioning  process if you want to decommission a storage system later.  GLYPH<SM590000> You can only add MDisks that are in un-managed mode. When MDisks are added to a  Parent Pool, their mode changes from un-managed to managed. GLYPH<SM590000> You can delete MDisks from a Parent Pool under the following conditions: – Volumes are not using any of the extents that are on the MDisk. – Enough free extents are available elsewhere in the pool to move any extents that are in  use from this MDisk. – The system ensures that all extents that are used by volumes in the Child Pool are  migrated to other MDisks in the Parent Pool to ensure that data is not lost.  GLYPH<SM590000> You can delete an array MDisk from a Parent Pool when:  – Volumes are not using any of the extents that are on the MDisk. – Enough free extents are available elsewhere in the Parent Pool to move any extents  that are in use from this MDisk. GLYPH<SM590000> If the Parent Pool is deleted, you cannot recover the mapping that existed between extents  that are in the pool or the extents that the volumes use. If the Parent Pool has associated  Child Pools, then you must delete the Child Pools first and return its extents to the Parent  Pool. After the Child Pools are deleted, you can delete the Parent Pool.  Chapter 4. Storage Pools  57