This final section covers backing up and restoring the filesystem containing the operating system itself, including the case of a system disk failure. Recovering from such a disaster has come to be known as "bare metal recovery” in recent years. Unix Backup and Restore includes detailed chapters describing these techniques and procedures for several Unix varieties.
Filesystems containing operating system files such as / and /usr pose few problems when all you need to restore is the occasional accidentally deleted or otherwise lost file. When the file in question is an unmodified system file, you can usually restore it from the operating system installation media, provided you have it and that it is readable under normal system conditions. If either of these conditions is not true, you should do a full backup of all system filesystems from time to time.
Files that you modify in the system partitions should be backed up regularly. In Chapter 14, we looked at a script that saves all modified configuration and other files to a user filesystem, allowing them to be backed up regularly and automatically via the system backup procedures. Alternatively, the script could save them directly to the backup media (even to a diskette if the archive is small enough).
When system filesystems need to be completely restored (usually due to hardware problems), some special considerations come into play. There are often two distinct approaches that can be ...