[Bug-ddrescue] Suggestion: Image size vs. source size sanity check

[Burkart Lingner]

Hi Antonio!

Let me add something to the recent suggestions to further improve ddrescue.

When your hard disk gives you trouble you often have to run ddrescue 
several times, with a good chance of the computer powered down in 
between runs. On my system the ordering of the /dev/sd* letters varies 
between different boot-ups, so one time I probably didn't verify the 
letter (or made a typo) and thus called ddrescue with the wrong imaging 
source parameter. ddrescue then appended the existing image with data 
from another, larger, hard drive than the one I had originally imaged. I 
noticed the mistake relatively quickly and was able to truncate the 
image file back to its correct pre-mistake size and manually corrected 
the log file. Had the wrong drive been smaller than the correct one, 
ddrescue would have probably started filling the previously bad blocks 
with data from the wrong disk, making it much harder to recreate the 
pre-mistake state.

Obviously, what I just described was clearly a usage error on my part. 
Nevertheless I think ddrescue could be improved to help users not to 
make the same mistake. It could read the size of the image from the log 
file and then, if it doesn't match the size of the source device, show a 
warning like "You said you'd like to image /dev/sda whose size doesn't 
match that of the existing image. Are you sure you want to continue? (y/n)"

It's not improving ddrescue's core functionality and one might say that 
it's not the program's duty to prevent human error. However, if it's 
easy to implement, why not help out error-prone humans... like me :-)

Bye, Burkart