A system including a plurality of servers each executing an application, a plurality of Wide Area File System (WAFS) clients which are correspondingly connected to the servers, a WAFS server which is connected to each WAFS client, a storage system or network attached storage (NAS), connected to the WAFS server, which stores data in a plurality of volumes or filesystem forming a consistency group, and a backup server for controlling backup of the consistency group. Each WAFS client includes a cache for temporarily storing data, and a cache manager for managing the cache. The WAFS server includes a backup agent that responds to the backup server, and a consistency manager that maintains consistency among the caches. The system is implemented to bring to a state of consistency the caches in the WAFS clients and the corresponding volumes in the WAFS server.