A data storage system comprising: a storage controller; and a plurality of flash packages; wherein each flash package includes a plurality of flash memory chips and a package controller, the plurality of flash memory chips providing a plurality of real blocks, each of the plurality of real blocks being a delete unit; and wherein the storage controller is configured to manage a plurality of package groups being respective groups of said flash packages and including a plurality of real pages, each package group having a respective RAID configuration; characterized in that the storage controller is configured to perform a page transfer process comprising: - calculating, for each of the plurality of flash packages, an average number of deletions of the plurality of real blocks of the respective flash package; - determining a package group that includes a flash package, which conforms with the condition that the respective calculated average number of deletions of the real blocks exceeds a first predetermined threshold, from the plurality of package groups as a transfer-source package group; - determining a package group that includes a flash package, which conforms with the condition that the respective calculated average number of deletions of the real blocks is not larger than a second predetermined threshold, from the plurality of package groups as a transfer-destination package group, the second predetermined threshold being not larger than the first predetermined threshold; - determining a transfer-source page from the plurality of real pages of the transfer-source package group on the basis of an amount of write data; and - determining a transfer-destination page from the plurality of real pages of the transfer-destination package group. a storage controller; and a plurality of flash packages; wherein each flash package includes a plurality of flash memory chips and a package controller, the plurality of flash memory chips providing a plurality of real blocks, each of the plurality of real blocks being a delete unit; and wherein the storage controller is configured to manage a plurality of package groups being respective groups of said flash packages and including a plurality of real pages, each package group having a respective RAID configuration; characterized in that the storage controller is configured to perform a page transfer process comprising: - calculating, for each of the plurality of flash packages, an average number of deletions of the plurality of real blocks of the respective flash package; - determining a package group that includes a flash package, which conforms with the condition that the respective calculated average number of deletions of the real blocks exceeds a first predetermined threshold, from the plurality of package groups as a transfer-source package group; - determining a package group that includes a flash package, which conforms with the condition that the respective calculated average number of deletions of the real blocks is not larger than a second predetermined threshold, from the plurality of package groups as a transfer-destination package group, the second predetermined threshold being not larger than the first predetermined threshold; - determining a transfer-source page from the plurality of real pages of the transfer-source package group on the basis of an amount of write data; and - determining a transfer-destination page from the plurality of real pages of the transfer-destination package group. - calculating, for each of the plurality of flash packages, an average number of deletions of the plurality of real blocks of the respective flash package; - determining a package group that includes a flash package, which conforms with the condition that the respective calculated average number of deletions of the real blocks exceeds a first predetermined threshold, from the plurality of package groups as a transfer-source package group; - determining a package group that includes a flash package, which conforms with the condition that the respective calculated average number of deletions of the real blocks is not larger than a second predetermined threshold, from the plurality of package groups as a transfer-destination package group, the second predetermined threshold being not larger than the first predetermined threshold; - determining a transfer-source page from the plurality of real pages of the transfer-source package group on the basis of an amount of write data; and - determining a transfer-destination page from the plurality of real pages of the transfer-destination package group.