About decreasing pool capacity

When you want to decrease or shrink the capacity of a pool, you delete one or more pool volumes (pool-VOLs) by parity group from the 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.

The more cache management devices for V-VOLs in the target pool, the more time is required for the pool shrinking to complete. At least 2 seconds are required for each cache management device. For the number of required cache management devices, see Cache management device requirements.

The total throughputs of page movements due to rebalancing and pool shrinking are limited on a per-storage system basis according to the average processor utilization on the storage system to mitigate the impact on I/O performance. The following table shows the total throughput limits of page movements according to the average processor utilization.

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

• Expand pools
• Shrink pools
• Edit pools
• Restore pools

Pool shrinking cannot be performed while the following operations are performed on the target pool:

• Create pools
• Delete pools
• Expand pools
• Shrink pools
• Restore pools
• Stop the pool shrink operation
• Change the threshold
• Create V-VOLs
• Expand V-VOLs

If the pool shrinking is attempted while SOM 749 is set to ON, the processing stops abnormally. Before the pool shrinking, verify that the SOM 749 setting is OFF.

If a blocked pool-VOL exists in the target pool, the pool capacity cannot be shrunk. Restore the blocked pool-VOL, and then shrink the pool capacity. Even if the pool-VOL in the pool is blocked, when the mapped capacity (MAPPED_PAGE_CAP(MB) for raidcom get ldev) of the pool-VOL is 0, the pool shrinking can be performed by using CCI (the pool-VOL can be removed from the pool).

You can shrink up to eight pools at the same time.

The more cache management devices the V-VOLs in the target pool have, the longer it will take to shrink the pool. It takes at least about two seconds per cache management device. For the number of required cache management devices, see Cache management device requirements.

If you delete the system 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.