[ale] ddrescue on cdrom

[John Anderson]

On 02/25/2012 09:06 AM, JD wrote:
> On 02/24/2012 05:58 PM, James Sumners wrote:
>> I don't understand this. The CD already includes Reed-Solomon codes for error
>> correction. Sucking up more space on the disc with a second helping of
>> Reed-Solomon codes shouldn't do anything other than reduce the amount of usable
>> disc space. Especially since PARs are usually ~20% of the original data.
>>
>> On Thursday, February 23, 2012, JD wrote:
>>
>>      Try a different drive.
>>
>>      In the future, you might want to include par2 files with the data. This isn't
>>      foolproof, but it has helped me to recover 2 failing DVDs that were not
>>      completely gone.
> I use 10% par2s, not 20%.  All that I know is that when files couldn't be
> properly read from multiple DVD drives, I used dd_rescue to get what I could
> which was also incomplete.  Then used 'par2 r' and was able to get everything
> back, verified.
>
> I have hundreds of DVDs with par2 data as added protection - some are over 10
> years old and are still working.  2 have seen some failures, so I pulled the
> data off, recovered it and wrote it back to new media. Good for 5-10 more years
> the way I see it.
>
> I'll leave the how/why this works to smarter people.
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I hadn't really though about par2 on the same disk. My concept of how to 
use that would be to back up a large chunk of data to multiple disks 
with par data for the whole archive either on separate disks or with 
some par data on each disk. I.E. back up 5000 Mb on 10 cds, with 200 mb 
parity data on each disk, so that multiple cd's can fail with there 
still being a decent chance to recover the data.

I don't know of any existing utilities that do this as a fire and forget 
option however.