1. A method for upgrading storage processors in a storage system comprising a storage processor enclosure comprising a first storage processor comprising a first cache, and a second storage processor comprising a second cache, the second storage processor connected to the first storage processor by a first cache mirroring connection; and a disk enclosure comprising a plurality of disk units forming a plurality of logical units, wherein the disk enclosure is connected to the first storage processor and the second storage processor, the method comprising: performing, by the first storage processor, IO requests on a first one or more logical units of the plurality of logical units; performing, by the second storage processor, IO requests on a second one or more logical units of the plurality of logical units; mirroring, by the first storage processor and the second storage processor, the first cache and the second cache via the first cache mirroring connection; causing, by the storage system, the first storage processor to stop performing the IO requests on the first one or more logical units of the plurality of logical units and the second storage processor to perform the IO requests on the first one or more logical units of the plurality of logical units; replacing, in the storage processor enclosure, the first storage processor with a third storage processor comprising a third cache; causing, by the storage system, the second storage processor to stop performing the IO requests on the first one or more logical units of the plurality of logical units and the third storage processor to perform the IO requests on the first one or more logical units of the plurality of logical units; mirroring, by the second storage processor and the third storage processor, the second cache and the third cache via the first cache mirroring connection; causing, by the storage system, the second storage processor to stop performing the IO requests on the second one or more logical units of the plurality of logical units and the third storage processor to perform the IO requests on the second one or more logical units of the plurality of logical units; replacing, in the storage processor enclosure, the second storage processor with a fourth storage processor comprising a fourth cache; causing, by the storage system, the third storage processor to stop performing the IO requests on the second one or more logical units of the plurality of logical units and the fourth storage processor to perform the IO requests on the second one or more logical units of the plurality of logical units; and mirroring, by the third storage processor and the fourth storage processor, the third cache and the fourth cache via the first cache mirroring connection.