1 Introduction

GNU ddrescue is a data recovery tool. It copies data from one file or block device (hard disc, cdrom, etc) to another, trying hard to rescue data 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, read the log, restart it from a new position, etc.

If you use the logfile feature of ddrescue, the data is rescued very efficiently, (only the needed blocks are read). Also you can interrupt the rescue at any time and resume it later at the same point.

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 damaged areas at the same places on different input files is very low. Using the logfile, only the needed blocks are read from the second and successive copies.

Ddrescue recommends lzip for compression of backups because of its reliability and data recovery capabilities, including error-checked merging of backup copies. Lziprecover makes lzip files resistant to bit-flip, one of the most common forms of data corruption, and its recovery capabilities contribute to make of the lzip format one of the best formats for long-term data archiving. The combination ddrescue + lziprecover is the best option for recovering data from multiple damaged copies. See lziprecover-example, for an example.

Recordable CD and DVD media keep their data only for a finite time (typically for many years). After that time, data loss develops slowly with read errors growing from the outer media region towards the inside. Just make two (or more) copies of every important CD/DVD you burn so that you can later recover them with ddrescue.

Because ddrescue needs to read and write at random places, it only works on seekable (random access) input and output files.

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

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.

This document was generated on August 24, 2013 using texi2html 5.0.