Are CDs or DVDs Good For Backups?

In a post title Create a Data Backup Solution that Works for You I talked about how I backup my data on various media, including DVD discs. There has been much debate in the past on the quality of backups on DVDs, or CDs for that matter, and whether the data will still be available in a year from now. One thing to keep in mind with regards to DVDs is that all DVDs are not created equally.

In order to fully understand how a DVD can keep your data safe, it is important to look at how they are constructed.

A DVD is made from clear plastic from some kind of polycarbonate. The top side may have a label, while the bottom side is reflective. The one side is reflective because of a metal foil that is glued to one side of the polycarbonate. In an expensive DVD, a protective paper or plastic label will cover the foil. On lower cost brands, the metal foil may be the top portion of the DVD. While this foil can come in many colours, silver (aluminum) and gold are the most common.

The data that you store on a DVD is placed on a dye that is between the upper surface of the polycarbonate and the reflective metal foil. As data is stored on the disc, the laser goes through the polycarbonate and melts (burns) a small pit into the dye. As the beam moves along the disc, the laser beam turns on and off extremely rapidly which then results in melted and unmelted areas. These areas represent the ones and zeroes of the data.

The longevity of a DVD is determined by two things: the metal foil and the type of dye used. Discs that use aluminum can be affected by oxidization, but that is very rare. The discs that use gold will not oxidize so it will not impact data.

So in terms of longevity, the dye used has the largest impact. There are several dyes that are used, each one with a different chemical composition, which will affect both their colour and shelf-life. A list of dyes include:

Name	Colour	Shelf Life
Cyanine	Blue, Blue-Green	10 years
Phthalocyanine	Light Aqua	100+ years
Metallized	Dark Blue	50-100 years

If you are looking for a well-known, and trusted DVD to backup your data, then you should look for Taiyo Yuden CDs and DVDs. I backup my data to discs produced by them. Other brands such as TDK and Verbatim are said to also carry Taiyo Yuden. You may be able to find the MAM-A brand from Mitsui which can also keep your data safe. It has been stated that the MAM-A shelf-life is about 300 years. Although, you probably will only need the disc for a few years until you move to another media.

It is important that you don't backup your data on bulk, cheap DVDs. These discs are not made from good quality and you may end up with much of your data missing or unreadable. You should, instead, find good brands that are mentioned above, and while they may be more expensive, you will not lose your data.