About decreasing pool capacity

You can decrease pool capacity by deleting pool-VOLs.

When you remove a pool-VOL from a pool, the used pages in the pool-VOL are moved to other pool-VOLs.

For the pool that owns DP-VOL with a disabled data direct mapping attribute, if pool-VOL is released after the pool shrinking, the released pool-VOLs (LDEVs) will be blocked. If they 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 the external volume and is disconnected during deletion, reconnect the external volume and then retry deleting the pool-VOL.

You can decrease pool capacity for up to eight tasks at the same time. Do not execute a Command Control Interface command to decrease the capacity of the pool while its capacity is already being decreased.

You cannot decrease pool capacity when:

• Creating the pool.

• Deleting the pool.

• Increasing the pool.

• Decreasing the pool.

• Recovering the pool.

• Stopping decreasing the pool.

• Changing the threshold.

• Reclaiming zero pages.

• Creating DP-VOLs.

• Increasing DP-VOL capacity.

If the shrink pool operation has abnormally ended, it might be caused by one or more of the following:

• Maintenance of a cache memory is performed while the pool capacity is being decreased.

• Errors occur on a cache memory while the pool capacity is being decreased.

• The I/Os load to DP-VOLs related to the pool is high.

• DP-VOLs related to the pool are being blocked.

If the shrink pool operation has abnormally ended, perform one or more of the following operations:

• Restore a cache memory, and then perform the shrink pool operation again.

• When the I/Os load to DP-VOLs related to the pool is low, perform the shrink pool operation again.

• Delete or format DP-VOLs related to the pool, and then perform the shrink pool operation again.

 You cannot perform the following operations on a pool while the pool volume capacity is shrinking. Wait until shrinking completes or stop the shrinking process.

• Expand Pool

• Shrink Pools

• Edit Pools

• Restore Pools

If you delete the pool-VOL with the pool's system area, the used capacity and the management area will move to other pool volumes. If you delete the pool-VOL with system area, a different system area pool-VOL is assigned automatically according to the priority shown in the following table. A pool must include one or more pool-VOLs.

If multiple pool-VOLs of the same data drive type exist, the priority of each is determined by internal index of the storage system.

If pool capacity is decreased soon after creating a pool or adding a pool-VOL, processing can take a while to complete.

Notes on using Dynamic Provisioning

You cannot delete a pool-VOL under the following conditions.

• If the pool-VOL is deleted, the used capacity of the pool volume exceeds the pool threshold.

• If the pool-VOL is deleted, the subscription rate of the total V-VOL capacity, including the control information , exceeds the subscription limit. For details about the formula used to calculate the required pages for one DP-VOL including the control information, see Pool subscription limit.

• If the pool-VOL with system area is deleted, more than 4.2 GB of free space is necessary in the pool.

In the case that pool-VOLs assigned from the accelerated compression-enabled parity group are deleted, pool-VOLs cannot be deleted if the used capacity reserved for writing (after the deletion of Pool-VOLs) exceeds the threshold due to the deletion of Pool-VOLs.

The used capacity reserved for data-writing (after the deletion of Pool-VOLs) is calculated as follows:

Used capacity reserved for data-writing (after the deletion of Pool-VOLs) = Used capacity reserved for data-writing (before the deletion of Pool-VOLs) + Total used capacity of Pool-VOLs to be deleted * FMC saving ratio

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

• Parity groups on which the accelerated compression is not supported (For instance, SAS drives)

• accelerated compression disabled-parity groups

Notes on using Dynamic Tiering or active flash

• You cannot delete a pool-VOL under the following conditions.

• If the pool-VOL is deleted, the used capacity of the pool volume exceeds the pool threshold.

• If the pool-VOL is deleted, the subscription rate of the total V-VOL capacity, including the control information, exceeds the subscription limit. For details about the formula used to calculate the required pages for one DP-VOL including the control information, see Pool subscription limit.

• If the pool-VOL with system area is deleted, more than 4.2 GB of free space is necessary in the pool.

• 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 tier relocation. The process resumes after the pool-VOL is deleted.

Notes on using Thin Image

You cannot delete a pool-VOL under the following conditions.

• If the pool-VOL is deleted, the used capacity of the pool volume exceeds the pool threshold.

• If the pool-VOL with system area is deleted, more than 4.2 GB of free space is necessary in the pool.