Pools
Use Pools to configure and manage storage pools, internal and external storage, MDisks, and to migrate existing storage to the system.
In general, a pool or storage pool is an allocated amount of capacity that jointly contains all of the data for a specified set of volumes. The system supports two types of pools: parent pools and child pools.
Parent Pools
Parent pools receive their capacity from MDisks. All MDisks in a pool are split into extents of the same size. Volumes are created from the extents that are available in the pool. You can add MDisks to a pool at any time either to 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.
Consider the following general guidelines when you create a parent pool:
- 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 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.
- An MDisk can be associated with just one parent pool.
- 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.
- Volumes are associated with just one pool, except when you migrate between parent pools.
- Volumes that are allocated from a parent pool are striped across all the storage that is placed into that parent pool.
- You can add only MDisks that are in unmanaged mode. When MDisks are added to a parent pool, their mode changes from unmanaged to managed.
You can delete MDisks from a parent pool under the following conditions:
- 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. The MDisks that were in the parent pool are returned to unmanaged mode and can be added to other parent pools. Because the deletion of a parent pool can cause a loss of data, you must force the deletion if volumes are associated with it.
Child Pools
Instead of being created directly from MDisks, child pools are created from existing capacity that is allocated to a parent pool. As with parent pools, volumes can be created that specifically use the capacity that is allocated to the child pool. Child pools are similar to parent pools with similar properties and can be used for volume copy operation.
Child pools are created with fully allocated physical capacity. The capacity of the child pool must be smaller than the free capacity that is available to the parent pool. The allocated capacity of the child pool is no longer reported as the free space of its parent pool.
- Child pools can be created and changed with the command-line interface. You can use the management GUI to view child pools and their properties.
- As with parent pools, you can specify a warning threshold that alerts you when the capacity of the child pool is reaching its upper limit. Use this threshold to ensure that access is not lost when the capacity of the child pool is close to its allocated capacity.
- Ensure that any child pools that are associated with a parent pool have enough capacity for the volumes that are in the child pool before removing MDisks from a parent pool. The system automatically migrates all extents that are used by volumes to other MDisks in the parent pool to ensure data is not lost.
- You cannot shrink the capacity of a child pool below its real capacity. The system uses reserved extents from the parent pool that use multiple extents. The system also resets the warning level when the child pool is shrunk and issues a warning if the level is reached when the capacity is shrunk.
- If the volume is mirrored and the synchronized copies of the volume are all in one pool, the mirrored volume is destroyed when the storage pool is deleted. If the volume is mirrored and there is a synchronized copy in another pool, the volume remains after the pool is deleted.