Tuesday, July 01, 2003

Understanding DVD Storage

I'm looking for ways to archive entire 9.1 GB hard drives to DVD media. I've seen advertisements for 9.4 GB media. First, I learned the "GB" in "9.4 GB" doesn't really mean the "gigabyte" we grew up knowing. We learned in math or science class that a kilobyte wasn't 1000 bytes. It was (note the "was" -- I'll explain below) 2^10, or 1024^1, or 1024 bytes. A megabyte was 2^20, or 1024^2, or 1,048,576 bytes. A gigabyte was 2^30, or 1024^3, or 1,073,741,824 bytes.

According to the NIST Reference on Constants, Units, and Uncertainty, these definitions have changed:

  • kilobyte = 1000 bytes

  • megabyte = 1,000,000 bytes

  • gigabyte = 1,000,000,000 bytes

We have new terminology for the "prefixes of old":

  • kibibyte (kiB) = 1024 bytes

  • mebibyte (MiB) = 1,048,576 bytes

  • gibibyte (GiB) = 1,073,741,824 bytes

For example, discs advertised to be 4.7 GB are actually 4.7 billion bytes, or 4.37 "old GB." (Hard drive manufacturers pull the same trick, with "9.1 GB drives" reporting around 8.68 "old GB.") So, a 9.4 GB DVD really holds 8.75 "old GB" of data, which is bigger than a "9.1 GB" hard drive that really holds 8.68 "old GB".

I guess it's easiest to accept that any modern usage of the terms KB, MB, and GB denotes powers of 10 and not powers of 2, so a "new GB" is a billion bytes -- end of story. Here's a nice summary.

Back to DVDs! Unfortunately, 9.4 GB DVD media are dual-sided, single-layer. That means they must be manually flipped over, because they're essentially two DVD-5 discs glued together. Here's a diagram, courtesy of this site:

What about movie DVDs, which are reported to hold "8.5 GB" (really 8.5 billion bytes or 7.95 "old GB")? Most movie DVDs meet the DVD-9 specification, which is a single-sided, dual-layer disc:

Recognize that you have to be in the DVD manufacturing business to create DVD-9 discs, as consumer-grade DVD burners can't write dual-layer, single-sided media (and it's not for sale to most of us). So, until that changes, I'm restricted to reading and writing in single-sided, 4.37 GB chunks. Some DVD burners, like these from LaCie or Panasonic, advertise writing 9.4 GB media, but that's still to double-sided discs. Keep an eye on the rec.video.dvd.tech list or DVDRHelp.

While researching for ways to archive a 730 MB hard drive, I learned 800 MB CD-Rs exist, but I guess your burner needs to recognize it.