Breaking point

NIST's Fred Byers experiments with optical disks to see how much abuse they can take.

J. Adam Fenster

To test a CD's endurance, NIST's Byers ages the disk rapidly in a special environmental chamber.

J. Adam Fenster

Byers gets an up-close look at how the surface of a disk is holding up to the torture tests in a NIST lab.

Optical disks might not last as long as you would expect'or hope

Don't be mistaken: Optical disks won't last forever.

Many users think the disks are indestructible, but they are wrong, said Fred Byers, an IT specialist at the National Institute of Standards and Technology.

As the government saves more and more records in electronic format, long-term archiving becomes a puzzle. Paper has lasted for centuries. Can optical disks really last for the 100-year life span envisioned for them?

The good news from NIST is that certain types of optical disks might last that long or even double that'but only if handled with care.

But disk life expectancy depends on many factors, some controllable by users, others not, Byers noted in a disk care guide, NIST Special Publication 500-252.
The National Archives and Records Administration requested the report, asking NIST to come up with care and handling instructions for optical media. The goal was to imitate a report about magnetic tape care created by the now-defunct National Media Laboratory. Byers said he spent about a year testing disks and working on the guide.

Although many agencies have no archiving policy, they nonetheless are saving a lot of data on optical disks. 'There are a lot of questions about the implications of that,' Byers said.

Recordable CDs and DVDs can be as reliable as magnetic tape for backup, he said, and they read much faster because they use random access, whereas users must search files stored on tape sequentially.

The useful life span of optical disks varies with temperature, humidity and day-to-day use. Data degradation at first can go unnoticed because of the error-correcting abilities of disk readers.

NIST has found that recordable disks seem to last much longer than rewritable disks, Byers said, and even longer than manufactured disks such as CDs for installing commercial software.

General industry guidelines now estimate office-burned copies of CDs and DVDs could remain readable for 100 to 200 years.

A disk burner records information by laser-heating a dye inside the disk. Over time the dye fades, rendering the information difficult to read. The disk's reflective layer, which sends information back to a photosensor during reading, is also subject to degradation.
The reflective layer is usually of thin gold, silver or silver alloy. Gold does not corrode, though gold-coated disks are expensive. Silver corrodes when exposed to air pollutants such as sulfur. Most silver-coated disks use alloys to inhibit corrosion.

Rewritable CDs and DVDs have a shorter life span of about 25 years, so Byers said he does not recommend them for archiving.

A rewritable disk's metal-alloy data layer is less stable than that in write-once disks. And rewritable disks are affected by light, so they also have a limited number of reads'a number that's still uncertain.

Surprisingly, premanufactured CD-ROMs and DVD-ROMs may not last as long as recordable disks, Byers said. Surprisingly, premanufactured CD-ROMs and DVD-ROMs may not last as long as recordable disks, Byers said.

Most industry estimates place the life span of professionally produced optical disks somewhere between 20 and 100 years. But the weakness of the ROM disks stems largely from their aluminum reflective layer, Byers said.

When exposed to humidity and atmospheric oxygen through scratches, cracks or delaminated areas in the label, the aluminum breaks down.

Byers warned that all the life spans are rough estimates. A hurried burn or a change in manufacturing technique could cause premature failure.

He advised agencies to consider the longevity of their current storage media and always be developing migration strategies to a new one.

'These days, the rate of change in technology is so much faster,' Byers said. 'I'm convinced there will be a new storage technology within 10 years.'

About the Author

Joab Jackson is the senior technology editor for Government Computer News.


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