1. A quality of service control method, comprising: collecting, by a storage system, information about processing capabilities of hard disks in the storage system; dividing, by the storage system, a cache into multiple cache tiers based on the collected information, wherein each cache tier corresponds to one or more hard disks in the storage system; determining, by the storage system, that data in a first cache tier of the multiple cache tiers reaches a preset threshold; writing, by the storage system, in response to the determining, the data in the first cache tier into one or more hard disks corresponding to the first cache tier.