A full backup is performed on each file system once weekly. This day is not always the same due to the load balancing that takes place. (If we did all full backups on the same day, we'd never finish!) A full backup is a backup of all files, regardless of status.
An incremental backup is performed daily. During an incremental - all files are checked and backed up *only* if they changed since the last full backup. Unchanged files are not backed up until the next full backup.
Backups only run once during a 24-hour period. If someone creates a file in the morning, and deletes it in the afternoon, there is not much we can do to help. (No one has developed a "Norton Utilities" for Unix yet!) One thing to note: if a file changes very often (log files for example) they may not get backed up. The software checks the status of a file to make sure it hasn't changed during a backup attempt. It tries three times before it gives up and skips the file if it keeps changing.
Due to the volatile nature of incoming mail, we cannot get good backups of the incoming mail spools (/var/mail/$LOGNAME). To make sure your mail gets backed up reliably, be sure to allow your mail program to move mail to the files it prefers under your HOME directory (such as Mail/received for elm, or mail/received-mail for pine, etc.). Since we do not backup incoming mail spools, we cannot restore any mail spool files.
We back up an awful lot of data. We do our best to keep at least two rotations (or two weeks worth of backups).