It is slower than the image copy method, but can be implemented using small backup systems which have less capacity that the file server disk. The ability to selectively backup and restore specific files is the major advantage of this system. The following two methods are used to determine which files to backup on the server. Both methods are used by the system manager to backup the server. This method backs up only those files which have been changed since the last backup occurred.
Most operating systems implement a changed or archive bit associated with files and directories.
When the file is modified, the archive attribute bit is set, indicating the file contents has changed. When the file is backed up, the backup software often resets this archive bit, indicating the file has been archived. Failure to reset this attribute bit would result in the file being unneedlessly being archived again at the next incremental. On a stable LAN, only changing user files will have the archive bit set. Using an incremental system permits the system manager to use less tapes, as only changed information is being written to the backup system, and this will always be less than a full dump.