Re: CD/DVD reliablity [was Re: Verifying backup CD/DVD results]

[Dave Platt <dplatt@radagast.org>]

> The more general question is 
> 
> Are CD / DVD media suitable for backup, at all? 
> 
> On German TV there has been a warning recently that a CD (DVD) might get
> unrecoverable errors even with one year.
> Are there any brands which gives waranties and how much are they worth?
> 
> What have you heard / experienced ?

I believe that the survivability of a CD-R depends significantly on
the type and quality of the disc itself.  Some seem to be excellent, 
others are garbage.

I offer several points of information and experience:

-  Some of the major CD-R manufacturers have, for years now, published
   lifetime estimates for their products.  These estimates are based
   on accelerated-aging (e.g. storage at artificially high temperatures,
   humidities, and light levels).  Several manufacturers' estimates state
   that discs make using phthalocyanine dye (the transparent or very
   light-colored type) should have a storage lifetime of 70 - 100 years
   or more.

-  There's always the possibility that aging mechanisms will appear which
   weren't adequately simulated by the acceleraged-aging tests.  For all
   we can prove today, every CD-R disc ever made will spontaneously
   burst into flames the moment it reaches 30 years of age... doesn't
   seem likely, but who knows for sure?  :-)

-  Discs made with cyanine dye (a deeper blue color) may be less stable
   in storage... cyanine is apparently more sensitive to heat and UV than
   phthalocyanine.  There have been anecdotal stores around of some
   cyanine discs going bad within a few days when left on a car's dashboard
   in direct sunlight... but who knows if these stories are true, or (if
   so) whether the problem was just a few bad discs?

-  Many discs these days are optimized for high storage capacity (tight
   track pitch, tight track-wobble and thus land/pit spacing) and for
   high-speed burning with a short laser exposure (thin, highly sensitive
   dye layer).  I believe that these optimizations tend to result in a disc
   which has lower overall signal reflectivity and more difficult tracking
   requirements, and thus less margin against degradation.

-  I've had some disc blanks which seemed to burn OK, but which became
   unreadable almost immediately thereafter.  On the other hand, some
   other discs from the same batch (burned over a decade ago) are still
   playable as audio CDs without obvious error or tracking problems, and
   still others from the same batch (burned as ISO 9660 filesystems) read
   back without any errors.

Given the way the market works, a lot of the discs available "off the shelf"
are both inexpensive, and cheap - that is, their quality is not the best.
If you want archival-quality data backup, buy high-quality media, not the
cheapest stuff you can find at the local bargain store.  You might want
to stick with traditional 74-minute discs (if you can find them) since their
track pitch and rate are right in the middle of the Red Book specification
range... the 80-minute / 700 MB type push right up against the edge of
the Red Book spec and are inherently harder to track and read.

I've had very good results using Taiyo Yuden media... they're the original
inventor of much of the technology and seem to put out a quality product.
Mitsui (MAM, Mitsui Audio Media) also has a very good reputation.  I've
heard good things about HHB, but haven't used their product myself.

I try to avoid the products from the second- and third-tier media companies
such as CMC and Ritek, except for "throwaway" applications.

I used to like the TDK "Certified Plus" metal-stabilized-cyanine media, 
but a few years ago they seem to have discontinued it and started OEMing 
cyanine discs from Ritek... nowhere near as good, in my opinion.

So, the good news is that you can buy CD-R media which should have a good,
long lifetime when burned well and stored properly - its lifetime is
probably going to be a good deal better than you'd get using a magnetic
tape.

The bad news is that you'll have to hunt around a bit to find the good
blanks, and will end up paying quite a bit more for them than you'd
pay for the sort of road-gravel media you'll find marketed under the
big consumer-oriented brand names at your local superstore.  If you buy
archival-quality CD-R blanks in 100-disc spindles from a reputable
professional-media supplier, they'll probably cost you a dollar or more
each.

I can't say much about DVD-R media reliability, as I've done very little
burning and almost no research.  It's my impression that DVD-R burning
is still rather chancier than CD-R burning, with media lifetime and
media/drive compatibility problems still being very much an issue.