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1 Introduction
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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.

   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.

   If you use the mapfile feature of ddrescue, the data are rescued very
efficiently, (only the blocks needed 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.

   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.

   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 blocks needed are read from the second and successive copies.

   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 a damaged medium, the combination ddrescue + lziprecover is
the best option for recovering data from multiple damaged copies. *Note
lziprecover-example::.

   Because ddrescue needs to read and write at random places, it only works
on seekable (random access) input and output files. Therefore, the only way
of creating a compressed image with ddrescue is to create a normal
(uncompressed) image first, and then compress that image.

   If your system supports it, ddrescue can use direct disc access to read
the input file, bypassing the kernel cache.

   One of the 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).

   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.