File: ddrescue.info,  Node: Generate mode,  Next: Ddrescuelog,  Prev: Fill mode,  Up: Top

14 Generate mode
****************

When ddrescue is invoked with the option '--generate-mode' it operates in
"generate mode", which is different from the default "rescue mode". That
is, in "generate mode" ddrescue does not rescue anything. It only tries to
generate a MAPFILE for later use.

   So you didn't read the manual and started ddrescue without a MAPFILE.
Now, two days later, your computer crashed and you can't know how much data
ddrescue managed to save. And even worse, you can't resume the rescue; you
have to restart it from the very beginning.

   Or maybe you started copying a drive with 'dd conv=noerror,sync' and are
now in the same situation described above. In this case, note that you
can't use a copy made by dd unless it was invoked with the 'sync'
conversion argument.

   Don't despair (yet). Ddrescue can in some cases generate an approximate
MAPFILE, from INFILE and the (partial) copy in OUTFILE, that is almost as
good as an exact MAPFILE. It makes this by simply assuming that sectors
containing all zeros were not rescued.

   However, if the destination of the copy was a drive or a partition, (or
an existing regular file and truncation was not requested), most probably
you will need to restart ddrescue from the very beginning. (This time with a
MAPFILE, of course). The reason is that old data may be present in the
drive that have not been overwritten yet, and may be thus non-tried but
non-zero.

   For example, if you first tried one of these commands:
     ddrescue infile outfile
     or
     dd if=infile of=outfile conv=noerror,sync

   then you can generate an approximate mapfile with this command:
     ddrescue --generate-mode infile outfile mapfile

Note that you must keep the original offset between '--input-position' and
'--output-position' of the original rescue run.