The archive bit is a file attribute that tells backup software if a file has changed since the last backup. It allows for incremental backups, but can be unreliable with multiple backup programs. The bit can be checked in Windows® properties and reset by backup software, but can be affected by rogue programs. Modern backup software can track changes without relying on the archive bit.
The archive 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. It allows such software to perform an incremental backup, i.e. copy only the files that have changed. The feature is only supported on computers running Microsoft Windows® and can be unreliable when used with multiple backup programs.
A bit is the simplest possible unit of information in digital computing systems. It has only two possible values: 1 or 0. In the case of an archive bit, a 1 or “set” value indicates that a file or directory has changed since the last backup, while a 0 or “clear” value means no changes they happened. This allows backup software to do something called an incremental backup. Unlike a full backup, an incremental backup only makes copies of files that have changed, saving you time and disk space.
The archive bit is an example of an additional piece of information, called a file attribute, that helps a computer’s operating system figure out what to do with a particular file or folder. File attributes are implemented by the file system, which 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®, every file and folder on a disk contains an archive bit, while computers running other operating systems do not support the feature.
A Windows® user can easily check the archive bit status for a particular file by right clicking on the file and selecting “Properties”. A checked box next to “Archive” or “Ready to archive” indicates that the bit has the value 1 and has been set. Many backup programs also have the ability to reset the bit to a value of 0 upon completion of the operation. This prevents the software from making another identical copy of the file if it hasn’t changed at the time of the next backup. The bit is reset only when a file is modified; simply opening or reading a file has no impact on the backup process, although changing other file attributes can reset the bit.
When used with multiple backup programs on the same computer, the archive bit can be unreliable. If one program clears the bit, other programs won’t back up the file. Rogue programs can also make changes to a file without setting the bit, thus preventing the modified file from being checked in. For these reasons, modern backup software can often track file changes on its own and perform backups without relying on the archive bit.
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