Re: ddrescue: detect repeated reads of the same data

[Antonio Diaz Diaz]

Hi Matheus,

Matheus Afonso Martins Moreira wrote:
> Allow me to begin with words of gratitude.

You are welcome! I'm glad that ddrescue helped you to recover at least part 
of your data.

> Still, I'd like to suggest a feature based on this experience:
> detection of repeatedly equal non-null reads.

Thank you for your detailed report.

Ddrescue already provides the option '-J, --check-on-error' which after 
every read error, reads again the last good sector found and checks that it 
returns the same data.

This mechanism could be surely modified to check the last good sector after 
two identical non-zero reads, and enabled with an option like 
'--check-on-identical'. I'll take a look at it as soon as I can.

> Even simply halting the rescue process until the user intervened
> would be reasonable though.

I guess this is all ddrescue can do. At least until it acquires human-level 
AI and a robot body. :-)

> In my case, I ended up reading what's likely invalid data from the disk
> and writing it to the ddrescue image. This went on for an undefined
> but hopefully short amount of time. The rescued data count falsely
> increased and the sectors were marked as rescued in the ddrescue's
> map file even though the data probably couldn't be read correctly.
> They won't be retried in the later passes and when trimming and
> scraping the disk.

Maybe a new mode that marks as non-tried the sectors containing known bad 
data could be added to ddrescue. Then a normal rescue run would reread those 
sectors.

> Detecting this condition would increase the confidence that blocks
> marked by ddrescue as rescued were truly rescued.

Indeed.

Best regards,
Antonio.