Backups serve the purpose of having a copy of one's files always available if, for one reason or another, there should be problems on the original device on which they were saved: don't just think of problems of a physical nature (e.g. the actual breaking of the disk or the entire device), because backups can be useful in many other situations. Just to give a few practical examples, backups can also be useful if an important file is deleted by mistake or, again, if the device on which the data is located is lost or stolen (saved files would not be lost forever as they are safe in a backup copy).

Furthermore, it is of paramount importance that the information is backed up in a different place than the original device.

There are different types of backup:

• Normal Backup. All selected files are copied and the backup files are marked (archive attribute Off). To restore all files, it is sufficient to have the most recent copy of the tape or backup file. Normal backup is usually performed the first time a backup archive is created.
• Full Backup. All selected files are copied while maintaining the archive attribute. The full backup is useful for backing up files between other backup types.
• Daily Backup. All selected files that were changed on the day the daily backup was performed are copied.
• Differential Backup. Files created or changed since the last normal or full backup are copied. Restoring files requires the last normal and differential backup.
• Incremental Backup. Only files created or changed since the last normal or incremental backup are included. Restoring data requires the last normal backup and all incremental backups.

There are then two ways in which information can be backed up:

• Replication of files.

The 'manual' method, i.e. using the file copy/paste functions normally available in the operating system, is strongly discouraged. Much better to rely on a tool that takes care of keeping files synchronised between the main location and the replica.

• Archive

If the type of backup chosen is to pack all the files into a large archive (or backup file), it is necessary to rely on software that takes care of this. For the software to be adequate, it must have certain minimum characteristics:

• create the archive;
• check its consistency;
• display its contents (i.e. the list of files it contains);
• extract files (even individually) from the archive.

Where should backups be stored?

The most common media are obviously the disks of a 'server', removable disks (both USB and optical), and in some cases even tapes. However, interesting things can be done with two relatively recent types of media: NAS and cloud storage.