In some sense, backups are most easily handled locally. If you install a tape drive or similar backup hardware in a computer, you can use software like tar , cpio , or dump to back up a Linux system without creating particularly special configurations. Attempting to perform a backup over a network adds complexity to the task, because both computers need network configurations, and the backup software must be network-aware . (You can sometimes work around lack of network support in backup software by using additional tools, though.) Restoring over a network can be even more difficult, because you may need to perform the restore operation on a computer that's missing its normal package of network tools. For these reasons, it's often desirable to implement a backup policy on a small network using local backup hardware. This solution can be costly, though, particularly when the network (or at least the computers that must be backed up) grows substantially. Tape backup drives suitable for backing up workstations or small servers typically cost $100 to $1,000, and media add to those costs. Network backup drives have higher capacities and are more costly, but when considered on a per-computer basis, the cost may be much lower. Thus, one reason to perform network backups is to save money compared to individualized backups.
Backup Hardware Options
Tape drives are the most common form of backup hardware. As a general rule, the lower-cost units, such as Travan drives, use more expensive media, so by the time you buy tapes, especially for a large server or network, costs don't vary as much as the initial purchase prices might lead you to think. Small networks can often make do with a mid-range or high-end single-drive tape unit for $1,000 or so, which can typically back up 5 to 20 GB of data. Such drives generally use the Digital Audio Tape (DAT) or Digital Linear Tape (DLT) formats. Larger networks require more expensive network backup units that use DLT or various more exotic tape formats. Another option for large networks is a tape changer, which is a single unit that can automatically swap tapes in and out, much like a multi-disc audio CD player. Such drives present themselves to the host computer as an extra-large single-tape drive.
Tape isn't the only backup medium, though. One particularly noteworthy alternative is optical recording media, such as CD Recordable (CD-R), CD Rewriteable (CD-RW), and several recordable DVD technologies. CD-R and CD-RW drives are limited in capacity (650 MB with standard discs), and so aren't very useful for performing true network backups. These media may be useful, however, for maintaining backups of fairly static information, such as a standard installation of a client OS. Recordable DVDs have higher capacities (a few gigabytes), and so are better suited to backing up complete systems, but they're still weak as network backup media. Optical media have unusually long shelf lives (estimates for CD-R range from 10 to 100 years ), so they're very well suited to archival backup duties . Using them in Linux is more complicated than is using a tape drive, because optical media need special utilities like cdrecord to be written. They can be particularly easy to restore, though, because a standard CD-ROM or DVD-ROM drive can read the media.
This chapter is written with the assumption that you'll be using a tape unit for your network backups. You may want to supplement such backups with backups of static information on optical media, though. This can make full emergency restores easier; you can restore an initial configuration using the optical media, then use that system to restore additional data from tape.
Network backups also offer certain convenience factors. By controlling a backup from a central location, you can eliminate backups from the tasks that individual users need to perform. The average office worker, even if given a tape backup unit, is likely to forget to perform routine backups. A centralized backup system, though, may be able to perform backups automatically and transparently to the users. (This isn't always true, though, as described shortly in the section "Types of Network Backup Solutions.")