how good is the data reliability with CD ROM and DVD RAM?

allison allisonportable at
Mon Jul 23 10:07:32 CDT 2018

On 07/23/2018 09:21 AM, Devin Monnens via cctech wrote:
>> I have a lot of backup here stored in CDs, and I have recently bought
>> an SCSI DVDRAM unit to create new backups in caddies DVD-RAMs (of
>> 4.2Gbyte each)
> what is your experience?
>  I recently disposed of a couple hundred DVD and CD backups I'd made. As
> mentioned in a previous comment, it's simply too impractical to store
> terabytes of information in 4.7GB segments, plus they take up a LOT of
> space. HDDs aren't the most reliable, but this is what I use now for that
> reason. I make sure to keep the previous backup in case something happens.
> I'll only use optical backups now with the most important data.
> Backblaze has some interesting stats regarding HDD reliability (they are a
> data center using thousands of drives running constantly):
> As noted previously, beyond storage conditions, disc longevity depends on
> the types of dyes used in the discs. Gold is supposed to be best. Early on,
> they experimented with a wide variety of dye types, and the silver dyes
> were least reliable, oxidizing in only about 10 years.
> The thing is, no media format is going to last forever. The only really
> reliable way of keeping data around is multiple backups and data migration.
> Basically, for your really important stuff, you'll want a couple of
> backups, stored in different geographical locations (one local, one on
> cloud works, too). You'll want to periodically refresh the backups by
> migrating the data onto fresh media.
> In the preservation business, the ideal is to refresh after the cost of
> storage media is 1/2 of the initial investment. So, if you paid $1 a GB for
> the initial storage media, you'll want to migrate once the new format is
> $0.50 a GB, and then again when it is $0.25 and so on. This way, the total
> cost is double what you initially invested.
> Of course, while the cost per GB might drop steadily, the total amount on a
> particular media format will increase as well, such that the $150 HDD you
> bought 5 years ago will have twice the storage for...$150. Definitely open
> to other suggestions.

I remember the first video disks that after 10 years would develop
(video noise from errors).

I rarely use optical disks of any for though I still have CDR as a small
locally handy media.  There are many others over the years.  I generally
keep a few formats as working copies.  Any media for that gets refreshed
as needed and master copies are abundant as a backup.  Its rare they all
at the same time, least I've never seen that.

For example for the CP/M systems 8", 5,25, and 3.5" floppies, refreshed
every so often.
The exception is the hard sector 5.25 stuff.  The pdp11 has RX01/2 and RX33
Both PDP11 and uVAX I have a large number of RD52 (Quantum D540s 31mb)
I use as cold swap backup storage media.  They are very good as the
media is
plated nickel-cobalt not the usual brown rust.  With more than two
decades of
doing that none have thrown an error or failed.  Of the larger RZ56s are
used the same way save for SCSI class.

For the PC, I use older PC with big disks.  They are air-gapped
backups.  There are
several with all the same stuff in case one fails.  The routine is to
install and run
a new data drive for a while then copy the smaller to it and archive the
Over the last few years I've resorted to using  large disks in a USB
case (so called
backup drives) where the case is open-able and I swap drive into them as
copies with a write, verify, remove, and store cycle.  Big drives 300GB
to 1TB are
dirt cheap and I use them like flash sticks. 

 I also use USB flash as they seem solid though somewhat small if you
don't re-write
a lot.   I have a few that are a mere 128K byte that are over a decade
old and still
going.  However I am wary of widows systems as they tend to write a lot
of crap
on them besides the actual file.  Linux is kinder to them.


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