About decreasing pool capacity

You can decrease pool capacity by deleting pool volumes.

When a pool-VOL is removed from a pool, all the used pages in the pool volume are moved to other pool volumes.

For the pool that owns DP-VOL with a disabled data direct mapping attribute, if a pool volume is released after the pool shrinking, the released pool volumes (LDEVs) will be blocked. If the pool volumes (LDEVs) are blocked, format them before using them.

If the blocked pool volume is an external volume, use Normal Format when formatting the volume.

If the pool volume being deleted is the external volume and is disconnected during deletion, reconnect the external volume and then retry deleting the pool volume.

You can decrease pool capacity for up to eight tasks at the same time. Do not issue a Command Control Interface command to decrease the capacity of a pool whose capacity is already in the process of being decreased.

You cannot decrease pool capacity while doing any of the following to a pool:

• 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 of the following:

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

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

• The I/O load to DP-VOLs related to the pool is too 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 too 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 in the process of shrinking. Wait until shrinking completes or stop the shrinking process.

• Expand Pool

• Shrink Pools

• Edit Pools

• Restore Pools

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

If multiple pool volumes 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 volume, processing may take a while to complete.

Notes on using Dynamic Provisioning

You cannot delete a pool volume under these conditions.

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

• If the pool volume is deleted, the subscription rate of the total DP-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 volume with system area is deleted, more than 4.2 GB of free space is necessary in the pool.

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

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

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

The used capacity reserved for writing (after deleting pool volumes) is larger than the used capacity reserved for writing (before deleting pool volumes). Because data stored in pool volumes 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 instance, SAS drives)

• Parity group with accelerated compression is disabled

Notes on using Dynamic Tiering

• You cannot delete a pool volume under these conditions.

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

• If the pool volume is deleted, the subscription rate of the total DP-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 volume with system area is deleted, more than 4.2 GB of free space is necessary in the pool.

• When the pool volume is deleted, the pages contained in the deleted pool volume transfer to another pool volume 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 volumes in the tier are empty, the appropriate tier is deleted.

• Deleting the pool volume stops the tier relocation. The process resumes after the pool volume is deleted.

Notes on using Thin Image

You cannot delete a pool volume under these conditions.

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

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