Running Linux, 5th Edition - Matthias Kalle Dalheimer [472]
Backups are usually made to tape, floppy, CD-R(W), or DVD-R(W). None of these media is 100% reliable, although tape, CD-R(W), and DVD-R(W) are more dependable than floppies in the long term. These days, with the cost of hard disks plummeting and the capacity increasing, backing up to a hard disk is also an option.
Many tools are available to help you make backups. In the simplest case, you can use a combination of gzip (or bzip2) and tar to back up files from your hard drive to removable media. This is the best method to use when you make only occasional backups—no more often than, say, once a month.
If you have numerous users on your system or you make frequent changes to the system configuration, it makes more sense to employ an incremental backup scheme. Under such a scheme, you would make a "full backup" of the system only about once a month. Then, every week, you would back up only those files that changed in the last week. Likewise, each night, you could back up just those files that changed over the previous 24 hours. There are several tools to aid you in this type of backup.
The idea behind an incremental backup is that it is more efficient to make backups in small steps; you use fewer tapes or CDs, and the weekly and nightly backups are shorter and easier to run. With this method, you have a backup that is at most a day old. If you were to, say, accidentally delete your entire system, you would restore it from backup in the following manner:
Restore from the most recent monthly backup. For instance, if you wiped the system on July 17, you would restore the July 1 full backup. Your system now reflects the state of files when the July 1 backup was made.
Restore from each weekly backup made so far this month. In our case, we could restore from the two weekly backups from July 7 and 14. Restoring each weekly backup updates all the files that changed during that week.
Restore from each daily backup during the last week—that is, since the last weekly backup. In this case, we would restore the daily backups from July 15 and 16. The system now looks as it did when the daily backup was taken on July 16; no more than a day's worth of files have been lost.
Depending on the size of your system, the full monthly backup might require 4 GB or more of backup storage—often not more than one DVD-R(W) or tape, but quite a few Zip disks. However, the weekly and daily backups would generally require much less storage space . Depending on how your system is used, you might decide to make the weekly backup on Sunday night and not bother with daily backups for the weekend.
One important characteristic that backups should (usually) have is the ability to select individual files from the backup for restoration. This way, if you accidentally delete a single file or group of files, you can simply restore those files without having to do a full system restoration. Depending on how you make backups , however, this task will be either very easy or painfully difficult.
It's also highly desirable to keep the backup media physically separate from the computer. If you choose to back up to hard disk, consider an external USB, FireWire, or SCSI drive. USB and FireWire are particularly nice because they can be easily plugged into or removed from the system as needed. If you choose to back up to a second, internal hard disk, it would be wise to at least keep the disk unmounted when it's not in use, so that if you were to accidentally delete one or more filesystems, your backup would be spared. An important decision you need to make is evaluating the relative importance of your data's safety and recoverability versus the cost and convenience of the backup media you choose, as well as how you use it.
In this section, we talk about the use of tar,