A backup mechanism enables incremental backup operations for on-line files
of a computer system having an archive bit attribute associated with each
file. The mechanism comprises an archive bit change number (ABCN)
attribute that is also associated with each file and that is manipulated
by a file system of the computer to reflect a correct value of the archive
bit when the file is modified during a current backup operation. The ABCN
attribute is incremented each time the file is modified to ensure that the
file is accurately copied to secondary backup storage during a subsequent
incremental backup operation. The mechanism thus enables reliable on-line
file modifications which, in turn, substantially increases the efficiency
of the computer system.