The file bit is a small piece of information attached to a computer file that tells a backup program whether or not the file has changed since the last backup. Allow this software to make an incremental backup, that is, only copy the files that have been modified. The feature is only compatible with computers running Microsoft Windows® and may be unreliable when used with various backup programs.
man holding computer
A bit is the simplest unit of information possible in digital computing systems. It has only two possible values: 1 or 0. In the case of an archive bit, a value of 1 or “set” indicates that a file or directory has changed since the last backup, while a value of 0 or ” delete” means there have been no changes. This allows backup software to do something called an incremental backup. Unlike a full backup, an incremental backup only makes copies of the files that have changed, which can save time and disk space.
The file bit is an example of additional information, called a file attribute, that helps a computer’s operating system determine what to do with a particular file or folder. File attributes are implemented by the file system, that is, the general method of organizing data on a disk. Different file systems support separate types of file attributes. On computers running DOS or Microsoft Windows®, each file and folder on a disk contains a file bit, while computers running other operating systems do not support this feature.
A Windows® user can easily check the bit status of a given file by right-clicking on the file and selecting “Properties”. A checked box next to “Archive” or “Ready to Archive” means that the bit has a value of 1 and has been set. Many backup programs also have the ability to reset the bit to a value of 0 after the operation is complete. This prevents the software from making another identical copy of the file if it has not been changed at the time of the next backup. The bit is only reset when a file is modified; simply opening or reading a file has no impact on the backup procedure, although changing other attributes of the file may reset the bit.
When used with multiple backup programs on the same computer, the archive bit may not be reliable. If a program clears the bit, other programs will not back up the file. Misbehaving programs can also make changes to a file without setting the bit, preventing the modified file from being archived. For these reasons, modern backup software can often handle file changes on its own and perform backups without relying on the archive bit.
