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NIST's Fred
Byers experiments with optical disks to see how
much abuse they can take.
(Image By: J.
Adam Fenster) |
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| 03/08/04; Vol. 23
No. 5
Breaking point
By Joab
Jackson
GCN Staff
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.
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To test a CD’s
endurance, NIST’s Byers ages the disk rapidly in a
special environmental chamber.
(Image By: J. Adam
Fenster) |
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| 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.
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| Byers gets an
up-close look at how the surface of a disk is holding up
to the torture tests in a NIST lab. |
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| 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.”
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