One of the most important responsibilities of a system administrator is to maintain backups. A backup is a periodic “snapshot” of files that is stored on a magnetic tape. The files are copied onto the tape, and can be retrieved from the tape at a later date.

Every site has its own backup policy. But since backing up a system can take a long time and can tax the system, they usually aren’t done more often than once a day. Generally, backups are performed overnight, when few users might be affected.

The value of daily backups is that they can prevent catastrophes when someone accidently removes a file. As long as the file existed yesterday, then it can be recovered from the overnight backup. You won’t recover any changes made to the file this morning, but at least you haven’t lost a week’s work.

Daily backup tapes are usually overwritten after a while. At our office, they are recycled every four weeks. However, most administrators set aside a permanent “full” backup tape on a regular basis — for example, every month. This is for situations when you don’t realize until two months later that you removed a file you still need. By finding the latest backup from before you removed the file, you might be able to recover at least some version of it.

To retrieve a file off a backup tape, you need to ask your system administrator. Send the administrator email with the full pathname of the file, and the approximate date you want it from. (For example, if you know you removed a file sometime in early August, you might say “take it off the latest backup you have in July.”)