Re: [Bug-ddrescue] an issue with reading cd-rom discs on linux

[Felix Ehlermann]

On 17.06.2012 17:18, Cezary Sliwa wrote:
> What is the purpose of such checks? One may have a partial (short) 
> copy of the file to rescue he wants to merge in. Done blocks are OK in 
> such a case. Just take care not to truncate the rescued file or the 
> log. Cezary Sliwa
Dear all,

I think checks like this are a good idea in general.
Imagine resuming dd_rescue after rebooting your machine and the devices 
are for some reason in a different order (e.g. my machine does this when 
i connect additional drives to the adaptec-controller).
Even simply mistyping the command (e.g. sdc vs sdd or sdc2 vs sdc3 are 
just one key 'distance' on the keyboard) could happen.

If the size of the source device does not match the logfile it's very 
unusual (for hard disks at least) - so it's a good thing to give at 
least a warning which needs to be confirmed prior to dd_rescue starting 
to write any data.
Without such a check you might otherwise easily overwrite the 
already-rescued part of the data if the mismatch in size is silently 
ignored.

The log file can be easily edited with an text editor - so I think that 
it's already possible do things like merging a partial copy.
(Just create a new log file with the according size and mark the part 
you don't want to copy accordingly - if the output file is larger 
dd_rescue should not complain nor should it truncate the output file?).
Of course it's less intuitive, but I think it's a more advanced thing to 
do so IMO it's OK to require more advanced steps to accomplish.

The Track-at-Once-behavior might be something to be covered in an FAQ 
anyway.

Kind Regards
Felix