Re: Run ddrescue a second time

[Shahrukh Merchant]

On 2019-12-06 6:03 PM, Steve Westmoreland wrote:
> Hello,
> I know this email address is for bugs, but I could not find a reference in the 
> instructions that I needed.

It is also a de facto forum where more knowledgeable people help out 
others in using ddrescue, as in fact they have helped me in the past. So 
let me try to return the favour.

> I ran ddrescue successfully in my linux box, from my affected 1TB drive to a 
> new 2TB drive. I have put the 2TB drive aside for safekeeping and later restore 
> (hopefully) of files.

Umm, what do you mean "put aside for safekeeping and later restore"? And 
what do you mean by "successfully"? Did you confirm that all sectors 
were restored? Was the recover quick or did ddrescue have to work hard 
on certain areas for a long time? AFAIK, the ddrescue operation *is* the 
restore operation and *now* is the time to understand how successful the 
restore was. If all sectors were restored with no errors, great. If not, 
you may want to do another "incremental" run of ddrescue with the same 
destination (with possibly different option combinations) to try to 
improve the results, or do secondary processing to figure out which 
files were affected, and so on. But "later" may be too late to try to 
get more data off the "affected" drive (by which I assume you mean 
failing drive).

> I would like to run ddrescue a second time from the affected 1TB drive to a new 1TB 
> drive, so that I can look into the new 1TB drive and "poke around".

Why (as opposed to poking around, even if only in read-only mode, on the 
already recovered drive)? Each time you run ddrescue on a failing drive 
you're risking more data loss *unless* you're re-running it on an 
existing destination image with its existing mapfile in order to 
*improve* the results on failed sectors.

> When I attempt to run: "ddrescue /dev/sda /dev/sdb /logfile.log", it tells me 
> the output file (logfile.log) exists and if I overwrite it, existing data will be lost. 
> Understandable.

I'm confused about this part. Ddrescue should use an existing mapfile as 
a starting point to improve a partial rescue. Does it have a way to 
figure out that the mapfile belongs to a different recovery attempt 
(different combination of source and destination)? How does it do that 
(I don't see anything like a drive ID or similar in the mapfile)?

It is perfectly normal to rerun ddrescue multiple times using the SAME 
source, destination and mapfile. What is not normal (and also not likely 
to give any useful outcome) is trying to use one ddrescue job's mapfile 
on a new recovery attempt with a new (virgin) destination drive!

> But can I create a second logfile (mapfile) while maintaining the original? Or 
> is this a foolish endeavor?

I don't know about "foolish"--there are legitimate non-obvious reasons 
for doing almost anything if taken in the right context. :-) But it is 
certainly sub-optimal to do a second independent ddrescue run from 
scratch on a failing drive when you already have a presumably successful 
first run. Unless (a) you messed up the first run somehow and don't 
trust it, or (b) you're very confident in the first recovery and are now 
just playing around with ddrescue and want a failing drive for that 
purpose. But given what you said, it seems you have not really examined 
the output of the first ddrescue to see how well it has recovered the 
failing drive, and that would be my priority, especially if additional 
ddrescue "improvement" runs are indicated.

Shahrukh

> Thank you for your attention to this question.
>
> Steve
>
> [cid:image001.png@01D5AC35.9481DB00]
> Steve Westmoreland
> 503-780-0302