When you want to decrease the capacity of, or shrink, a pool, you delete one or more pool volumes (pool-VOLs) from the pool. When a pool-VOL is removed from a pool, all of the used pages in the pool-VOL are moved to other pool-VOLs in the pool. If you delete the pool-VOL with the pool's system area, the used capacity and the management area are moved to other pool-VOLs in the pool. A pool must include one or more pool-VOLs.

For a pool to which a DP-VOL with the data direct mapping attribute disabled belongs, if a pool-VOL is released after the pool shrinking, the released pool-VOLs (LDEVs) are blocked. If the pool-VOLs (LDEVs) are blocked, format them before using them. If the blocked pool-VOL is an external volume, use Normal Format when formatting the volume. If the pool-VOL being deleted is an external volume and is disconnected during deletion, reconnect the external volume and then retry deleting the pool-VOL.

If you delete the pool-VOL with system area, a different system area pool-VOL will be assigned automatically according to the priority shown in the following table. If multiple pool-VOLs of the same drive type exist, the priority of each is determined by internal index of the storage system.

Notes on shrinking a Dynamic Provisioning pool

You cannot delete a pool-VOL from a Dynamic Provisioning pool under the following conditions:

• If the pool-VOL is deleted, the total of the used pool capacity will exceed the pool threshold.
• If the pool-VOL is deleted, the subscription rate of the total DP-VOL capacity including the control information will exceed the subscription limit.
• If the pool-VOL with system area is deleted, the free space in the pool will be less than 4.2 GB.
• In the case that pool-VOLs assigned to the accelerated compression-enabled parity group are deleted, the pool-VOLs cannot be deleted if the used capacity reserved for writing (after the deletion of pool-VOLs) exceeds the threshold due to deleting pool-VOLs.

  The used capacity reserved for writing (after deleting pool-VOLs) is calculated as follows:

  Used capacity reserved for writing (after deleting pool-VOLs) = Used capacity reserved for writing (before deleting pool-VOLs) + Total used capacity of pool-VOLs to be deleted × FMC saving ratio 
  			 

  The used capacity reserved for writing (after deleting pool-VOLs) is larger than the used capacity reserved for writing (before deleting pool-VOLs). Because data stored in pool-VOLs belonging to the accelerated compression-enabled parity group is migrated in the following parity groups due to the shrinking of pool:

  • Parity group with accelerated compression is not supported (for example, SAS drives)
  • Parity group with accelerated compression is disabled

Notes on shrinking a Dynamic Tiering pool

• You cannot delete a pool-VOL from a Dynamic Tiering pool under the following conditions:

  • If the pool-VOL is deleted, the total of the used pool capacity will exceed the pool threshold.
  • If the pool-VOL is deleted, the subscription rate of the total DP-VOL capacity including the control information will exceed the subscription limit.
  • If the pool-VOL with system area is deleted, the free space in the pool will be less than 4.2 GB.
• When the pool-VOL is deleted, the pages contained in the deleted pool-VOL transfer to another pool-VOL in the same tier. If the used capacity in the tier exceeds Rate of Free Space Newly Allocated to, the overflowing pages transfer to another tier.
• When pool-VOLs in the tier are empty, the appropriate tier is deleted.
• Deleting the pool-VOL stops the tier relocation. The process resumes after the pool-VOL is deleted.

Notes on shrinking a Thin Image pool

You cannot delete a pool-VOL from a Thin Image pool under the following conditions:

• If the pool-VOL is deleted, the used capacity of the pool will exceed the pool threshold.
• If the pool-VOL with system area is deleted, the free space in the pool will be less than 4.2 GB.