Pros and Cons of Optical Media

Optical media include Compact Disc Recordable (CD-R), CD-Rewriteable (CD-RW), and assorted recording Digital Versatile Disc (DVD) variants (DVD-R, DVD+R, DVD-RAM, DVD-RW, and DVD+RW). These media all use optical technologies—a laser burns data onto a light-sensitive substrate. The data can be read back by using a laser tuned to a lower power setting.

Note Magneto-optical disks, which use a hybrid magnetic and optical recording technology, are also available. From a software perspective, these disks work like other removable disk technologies, such as Zip disks, so the comments in the preceding section, "Pros and Cons of Removable Disks" apply to them.

Fundamentally, optical media are sequential access during writing but random access when being read. As described in Chapter 2, you write to these disks by creating a filesystem and then using software such as cdrecord to write that filesystem all at once. You can sometimes append to an existing filesystem. Software surrounding the Universal Disk Format (UDF) has made optical media access more closely resemble access to other disks on other platforms, but Linux's UDF support is still rudimentary, and doesn't yet take advantage of most of these features.

Optical media are very durable when properly stored—shelf life estimates range from 10 to 100 years. The media themselves have no moving parts, so they aren't likely to be affected by a fall from table height, unless they're scratched in the process. These media are common enough that software and hardware to read them are likely to be common for decades. They're also compact and so can be easily transported off-site, or you can store many media on a small bookshelf.

At the low end, CD-RW drives can be purchased for well under $100. (CD-R drives are no longer common.) CD-R media cost $0.30-$1.00 and can be used in CD-RW drives, but they can't be reused. Reusable CD-RW media cost $0.50-$1.50. Both CD-R and CD-RW media can store up to 700MB per disc, for a cost of well under $1/GB if you shop carefully. Recordable DVD drives cost $300-$4,000, while media cost $3-$25. These media store 4.7-9.4GB, so their cost hovers around $1/GB. New blue-light laser technologies are on the verge of being introduced. These technologies will increase recordable DVD capacities to 23GB, but introductory prices will be several thousand dollars for the drive alone. Of course, these prices are likely to drop quickly in the next couple of years. In other words, optical media prices are potentially lower than those of hard disks, considering media alone, but you can't back up all the data from a modern hard disk onto a single recordable DVD, much less a CD-R or CD-RW. This limited capacity is the optical media's Achilles' heel from a system backup perspective.

Nonetheless, optical media can be useful for backups. You can back up individual projects, configuration files, and so on. If you don't mind swapping discs during the backup, you may be able to back up a small system onto just a handful of optical discs. If you plan your partitions appropriately, you may be able to fit one partition per optical disc.

Tip If you run a multiboot system and you've installed the core components of each OS on 700MB or smaller partitions, you can record these boot partitions directly to CD-R or CD-RW discs and then restore them using dd. The destination partition must be precisely the same size as the original partition for this procedure to work, though. If you compress the backup, you may be able to fit a 1,4GB partition onto a single CD-R or CD-RW disc.

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