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<span id="Generate-mode-1"></span><h2 class="chapter">13 Generate mode</h2>
<span id="index-generate-Mode"></span>
<p>When ddrescue is invoked with the ‘<samp>--generate-mode</samp>’ option it
operates in "generate mode", which is different from the default "rescue
mode". That is, if you use the ‘<samp>--generate-mode</samp>’ option, ddrescue
does not rescue anything. It only tries to generate a <var>mapfile</var> for
later use.
</p>
<p>So you didn’t read the manual and started ddrescue without a
<var>mapfile</var>. 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.
</p>
<p>Or maybe you started copying a drive with <code>dd conv=noerror,sync</code><!-- /@w --> 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 ‘<samp>sync</samp>’ conversion argument.
</p>
<p>Don’t despair (yet). Ddrescue can in some cases generate an approximate
<var>mapfile</var>, from <var>infile</var> and the (partial) copy in
<var>outfile</var>, that is almost as good as an exact <var>mapfile</var>. It
makes this by simply assuming that sectors containing all zeros were not
rescued.
</p>
<p>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 <var>mapfile</var>, 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.
</p>
<p>For example, if you first tried one of these commands:
</p><div class="example">
<pre class="example">ddrescue infile outfile
or
dd if=infile of=outfile conv=noerror,sync
</pre></div>
<p>then you can generate an approximate mapfile with this command:
</p><div class="example">
<pre class="example">ddrescue --generate-mode infile outfile mapfile
</pre></div>
<p>Note that you must keep the original offset between
‘<samp>--input-position</samp>’ and ‘<samp>--output-position</samp>’ of the original
rescue run.
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