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Monday, September 13, 2010

GNU ddrescue Manual

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GNU ddrescue Manual

This manual is for GNU ddrescue (version 1.13, 27 August 2010).

Introduction: Purpose and features of GNU ddrescue
Basic concepts: Blocks, clusters, devices, files, sectors, etc
Algorithm: How ddrescue recovers the data
Invoking ddrescue: Command line interface
Logfile Structure: Detailed format of the logfile
Examples: A small tutorial with examples
Direct Disc Access: Bypassing the kernel cache
Fill Mode: Selectively overwriting the output file
Generate Mode: Generating an approximate logfile
Problems: Reporting bugs
Concept Index: Index of concepts

This manual is free documentation: you have unlimited permission to copy, distribute and modify it.

Go there...
http://www.gnu.org/software/ddrescue/manual/ddrescue_manual.html#Introduction

Ddrescue - Data recovery tool

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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, run it in reverse mode, 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. The combination ddrescue + lziprecover is the best option for recovering data from multiple damaged copies.

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.

The logfile is periodically saved to disc. So in case of a crash you can resume the rescue with little recopying.

Also, the same logfile can be used for multiple commands that copy different areas of the file, and for multiple recovery attempts over different subsets.

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.

Documentation

The manual is available in the info system of the GNU Operating System. Use info to access the top level info page. Use info ddrescue to access the ddrescue section directly.

An online manual for ddrescue can be found here.

Download

The latest released version of GNU ddrescue can be found in the subdirectory /gnu/ddrescue/ on your favorite GNU mirror. For other ways to obtain ddrescue, please read How to get GNU Software. The latest released version will be the most recent version available at http://ftp.gnu.org/gnu/ddrescue/.

Old versions and testing versions can be found at http://download.savannah.gnu.org/releases/ddrescue/.

Go there...
http://www.gnu.org/software/ddrescue/ddrescue.html

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