Re: CD/DVD reliablity
Hi,
> 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.
This touches two topics:
1) The question about backup suitability.
2) The question about our tv news shows and their technical
competence.
Yes. Both media are well suited for a reasonable backup
strategy. They are superior to the old medium-cost tape
technologies which i used in the past century (QIC tapes
DAT tapes, DEC TK).
No comparison with the really low-cost tape devices of
that time. They were just a nightmare.
For contemporary desktop technology CD and DVD face only
hard disk as competition. Anything else is too pricy.
Tape might be worth the money if you have to pay
operator hours according to german industry standards.
If you need affordable carry-away backups, then you will
hardly be able to avoid CD/DVD.
With any backup media you need verification after production
of the backup and precautions against subsequent media
deterioration.
CD and DVD are chemical media. So one has to avoid extreme
environmental storage conditions. On the other hand, magnetical
media are prone to electromagnetic influence.
If you can afford qualified storage of tape media then you
can afford qualified storage of CD/DVD too, i'm sure.
With backups of live systems it is important to replace them
regularly or alternatively to checkread those backups which
are stored for a longer time.
You have to fear two disasters: loss of your live data,
or loss of your backup. One of them you can afford. The
backup strategy has to ensure that you take notice of the
first disaster before the second one can happen.
Having more than one backup on shelf increases your chance
to survive.
With archiving of outhoused data you cannot afford a disaster.
So you need means to early recognize deterioration and enough
redundancy in your archive to refresh the deteriorating data.
My proposal for this problem is to have several identical
copies ot the backup volumes, to permutate some of them in
order to expose different blocks to the same systematic errors,
and to have a mesh of block checksums to identify damaged
blocks.
Some archive formats like RAR have more sophisticated
solutions for the same problem.
However, one has to checkread the backups regularly and
take action if they go bad.
Practical experience:
My oldest CD backup under observation is of february 2003,
i.e nearly four years old. 62 CD-RW, some 2x some 4x.
They still all verify with their MD5 checksums.
My oldest DVD backup is of june 2004, 2.5 years now.
14 DVD+RW. All still verifying with reasonable reading
times.
Failure i experience with freshly burned media.
About 10% of my CD-RW which stored data for several months
fail to take the next burn properly. Most of them can be
revived by a second attempt - but some are unreliable
forever.
DVD+RW seem to stand long sleeping times better.
Now for german television:
A museum person who complains about audio CDs from the
1980s going bad. How's the state of his vinyl LPs from
that time ? Those which have been tortured by a diamond
needle ?
(Next question is wether the company which checked the CD
collection isn't by chance offering expensive rescue
services ? No ?)
A professor telling that one should not use CD for storing
holiday photos. Asked what to do if one wants to store
them somehow anyway - he advises to use 2 (two) CDs.
The news anchor then frightens the viewers (i.e MS-Windows
users) that all their data are at stake. Now. Yesterday.
All doomed.
Cut. (Good so.)
Have a nice day :)
Thomas
Reply to: