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<span id="Introduction"></span><div class="header">
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<span id="Introduction-1"></span><h2 class="chapter">1 Introduction</h2>
<span id="index-introduction"></span>
<p>GNU ddrescue is a data recovery tool. It copies data from one file or
block device (hard disc, cdrom, etc) to another, trying to rescue the
good parts first in case of read errors.
</p>
<p>The basic operation of ddrescue is fully automatic. That is, you don’t
have to wait for an error, stop the program, restart it from a new
position, etc.
</p>
<p>If you use the mapfile feature of ddrescue, the data is rescued very
efficiently, (only the needed blocks are read). Also you may interrupt
the rescue at any time and resume it later at the same point. The
mapfile is an essential part of ddrescue’s effectiveness. Use it unless
you know what you are doing.
</p>
<p>Ddrescue does not write zeros to the output when it finds bad sectors in
the input, and does not truncate the output file if not asked to. So,
every time you run it on the same output file, it tries to fill in the
gaps without wiping out the data already rescued.
</p>
<p>Automatic merging of backups: If you have two or more damaged copies of
a file, cdrom, etc, and run ddrescue on all of them, one at a time, with
the same output file, you will probably obtain a complete and error-free
file. This is so because the probability of having the same area damaged
in all copies is low (if the errors are randomly located). Using the
mapfile, only the needed blocks are read from the second and successive
copies.
</p>
<p>Ddrescue recommends lzip for compression of backups because the lzip
format is designed for long-term archiving and provides data recovery
capabilities which nicely complement those of ddrescue. (Ddrescue fills
unreadable sectors with data from other copies, while lziprecover
corrects corrupt sectors with data from other copies). If the cause of
file corruption is damaged media, the combination ddrescue + lziprecover
is the best option for recovering data from multiple damaged copies.
See <a href="Optical-media.html#lziprecover_002dexample">lziprecover-example</a>, for an example.
</p>
<p>Because ddrescue needs to read and write at random places, it only works
on seekable (random access) input and output files.
</p>
<p>If your system supports it, ddrescue can use direct disc access to read
the input file, bypassing the kernel cache.
</p>
<p>Ddrescue also features a "fill mode" able to selectively overwrite parts
of the output file, which has a number of interesting uses like wiping
data, marking bad areas or even, in some cases, "repair" damaged
sectors.
</p>
<p>One of the great strengths of ddrescue is that it is interface-agnostic,
and so can be used for any kind of device supported by your kernel (ATA,
SATA, SCSI, old MFM drives, floppy discs, or even flash media cards like
SD).
</p>
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