PC Hardware in a Nutshell, 3rd Edition

File Attributes and the Archive Bit

If you have a tape drive large enough to back up your entire hard disk and the time necessary to use only complete backups, the status of any particular file doesn’t matter. Every file gets backed up every time, whether it was created that day or has been sitting unchanged for a year. But if you need to use some combination of complete and partial backups, the status of each file becomes critical. If a file is unchanged since the last complete backup, you want to ignore it when doing partial backups. If the file was created or changed since the last complete backup, it needs to be copied to the partial backup tape.

Windows maintains a file attribute for each file called the archive bit. When a file is created or changed, Windows toggles the archive bit on, indicating that that file is a candidate for backup. Backup software can manipulate the archive bit, either turning it off after it backs up the file, or leaving it on so that file will again be backed up the next time you do a partial backup.

The archive bit exists to provide a certain indication that a file requires archiving. Early Windows versions stored one timestamp for a file. In theory, that timestamp was changed when the file was created or modified. In practice, it was possible for an application to modify a file without changing the timestamp, which meant that a backup application that depended on the timestamp could fail to back up a file that had changed contents, which meant that ...