Access controller that controls access to files by using access control list

A storage system coupled to a backup server and providing a first type of file system for managing a first type of access control list (ACL), the storage system comprising:

• a processor;
• and
• a storage resource storing a plurality of files,
• wherein, in response to a restore request designating a backup image from the backup server, the processor restores files of the backup image and a second type of ACL associated with each of the files in the storage resource, and
• wherein the processor locks each one of the restored files, convert the restored second type of ACL associated with the locked file to the first type of ACL, and then unlock the locked file,
• wherein the storage system receives a file access request and uses an ACL to control access with respect to a file designated by the file access request,
• wherein the processor determines whether the first type of ACL can be written to a logical storage space, which comprises files managed by the file system,
• wherein if determined that writing is possible, the processor writes the first type of ACL corresponding with the files to both the logical storage space and the storage resource, and
• wherein if determined that writing is impossible, the processor does not write the first type of ACL corresponding to the files to the logical storage space, but writes the first type of ACL to the storage resource,
• wherein the processor writes the first type of ACL on a storage resource coupled to the storage system, except when a) a target file system, which is a file system of the file designated by the file access request, is not mounted as a read only, b) the target system has not been exhausted, c) a utilization of a logical storage space managed by the target file system is less than a predetermined upper limit value, and d) a file access count of the file designated by the file access request has reached a predetermined count, and
• wherein if the processor makes a determination as to whether the first type of ACL is associated with a file designated by the received file access request by referencing the storage resource, and when the first type of ACL which corresponds with the file is stored in the storage resource, the result of the determination is affirmative.