Rebalancing the usage level among parity groups

If multiple parity groups that contain LDEVs used as pool-VOLs exist, rebalancing can improve biased usage rates in parity groups. Rebalancing is performed as if each parity group were a single pool-VOL. After rebalancing, the usage rates of LDEVs in a parity group can not be balanced, but the usage rate in the entire pool is balanced.

The usage level among parity groups is automatically rebalanced when these operations are in progress:

• Expanding pool capacity

• Shrinking pool capacity

• Reclaiming zero pages

• Reclaiming zero pages in a page release request issued by the host with the Write Same command, for example.

• Performing tier relocations

 In pools created by pool-VOLs that have accelerated compression enabled, the rebalancing operation is performed with the parity group's used capacity in consideration. As a result, after the rebalancing operation is performed and completed, the capacity of the pool-VOL may not be reduced.

If you expand the pool capacity, Dynamic Provisioning moves data to the added space on a per-page basis. When the data is moved, the usage rate among parity groups of the pool-VOLs is rebalanced.

Host I/O performance can decrease when data is moved. If you do not want to have the usage level of parity groups automatically balanced, call the customer support.

You can see the rebalancing progress of the usage level among parity groups in the View Pool Management Status window (see View Pool Management Status window). Dynamic Provisioning automatically stops balancing the usage levels among parity groups if the cache memory is not redundant or if the pool is almost full.