1. A method for issuing a partition balancing subtask in a storage system, wherein the storage system comprises a controller and a plurality of nodes, wherein each node of the plurality of nodes stores a partition, and wherein the method comprises: generating, by the controller, a first set of a partition balancing subtask, wherein the first set comprises a first partition balancing subtask and a second partition balancing subtask, wherein the first partition balancing subtask records an identification of a first partition, an identification of a source node to which the first partition belongs and an identification of a destination node to which the first partition needs to be migrated, wherein the second partition balancing subtask records an identification of a second partition, an identification of a source node to which the second partition belongs and an identification of a destination node to which the second partition needs to be migrated, wherein the first partition balancing subtask is used to migrate the first partition from the source node to which the first partition belongs to the destination node to which the first partition needs to be migrated, and wherein the second partition balancing subtask is used to migrate the second partition from the source node to which the second partition belongs to the destination node to which the second partition needs to be migrated; determining, by the controller, the first partition balancing subtask from an issued second set of a partition balancing subtask; deleting, by the controller, the first partition balancing subtask from the first set; and issuing, by the controller, according to the identification of the destination node to which the second partition needs to be migrated, the second partition balancing subtask to the destination node to which the second partition needs to be migrated.