1. A method for issuing a partition balancing subtask, applied to a controller, wherein the controller and at least two nodes form a storage system, and wherein each node comprises a partition, the method comprises: generating a second partition balancing subtask set after a second partition balancing task is received, wherein the second partition balancing subtask set comprises a partition balancing subtask, and wherein the partition balancing subtask records: a migration partition, a node to which the migration partition belongs, and a destination node; searching a current partition balancing subtask set; deleting a repeated partition balancing subtask between the second partition balancing subtask set and the current partition balancing subtask set; and issuing the partition balancing subtask in the second partition balancing subtask set after the repeated partition balancing subtask is deleted to the destination node recorded in the partition balancing subtask; wherein the generating the second partition balancing subtask set according to the second partition balancing task comprises: calculating target partition distribution information of the storage system after the second partition balancing task is received; searching a current partition distribution information of the storage system; obtaining a partition distribution information variation according to the current partition distribution information and the target partition distribution information, wherein the current partition distribution information records correspondence between each node and partitions on the node, wherein the target partition distribution information records correspondence between each node and the partitions on the node after partition migration is performed, and wherein the partition distribution information variation records the migration partition that needs to be migrated for the current partition distribution information to become the target partition distribution information and the destination node of the migration partition; selecting the node to which the migration partition belongs; and generating the second partition balancing subtask set according to the node to which the migration partition belongs and the partition distribution information variation.