dvdisaster Version 0.72.4 / 0.72 (pl4) | To the Internet version

Contents

News
Overview
Howtos
Download
Questions and Answers
· | Technical Questions
· | Error messages
· | Background information
| - | Error correction properties
| - | Image level data recovery
| - | RS01, RS02 and RS03
| - | Linear reading strategy
| - | Adaptive reading strategy
| - | Remarks on read errors
| - | Hints for storing the error correction files
Bug reporting

### The RS01, RS02 and RS03 methods

dvdisaster contains three error correction methods. RS01 and RS02 are the existing and proven methods while RS03 is still under development.

Comparison of the methods. All three methods build on the same Reed-Solomon error correction. They calculate error correction information for ISO images which is used to recover unreadable sectors if the disc becomes damaged afterwards.

The methods differ in the way the error correction information is stored:

- RS01 creates error correction files which are stored separately from the image they belong to. Since data protection at the file level is difficult, error correction files must be stored on media which are protected against data loss by dvdisaster, too.

- To apply the RS02 method an image is first created on hard disc using a CD/DVD writing software. Before the image is written on the medium, dvdisaster is used to augment the image with error correction data. Therefore the data to be protected and the error correction information are located at the same medium. Damaged sectors in the error correction information reduce the data recovery capacity, but do not make recovery impossible - a second medium for keeping or protecting the error correction information is not required.

RS03 is a further development of RS01 and RS02. It can create both error correction files and augmented images:

- RS03 can distribute work over multiple processor cores and is therefore much faster than RS01/RS02 on modern hardware.
- RS03 error correction files are - contrary to RS01 - robust against damage. This should not delude you into careless handling of your error correction files though - the disadvantages of reading at the filesystem level are still valid.
- RS03 augmented images do not require so-called master blocks holding important information. This makes RS03 a bit more robust, but also more restrictive: The augmented image must completely fill the medium now while the size of augmented image can be freely chosen in RS02.

The changes for parallel computation and higher robustness make RS03 a bit less space efficient, e.g. RS03 error correction data has slighly less error correction capacity than its RS01/RS02 counterpart with equal size.

Comparison of error correction storage.

The following table summarizes the differences between error correction files (RS01, RS03) and augmented images (RS02, RS03):

Error correction files | Image augmented with error correction data
any possible redundancy can be chosen | redundancy is limited by free space on medium (= medium capacity - size of data image)
already effective at 15% redundancy since error correction files are required to be free of damage | requires more redundancy (recommended: 20-30%) to compensate defects in the error correction data
medium can be completely filled with data | usable medium capacity is reduced by amount of error correction data
can be created for already existing media | only applicable before writing the new medium since the image must be augmented with error correction information in advance
separately storing the error correction file from user data strengthens data protection | common storage of user data and error correction data may reduce error correction capacity
Mapping between error correction files and media must be kept. Error correction files must be protected against damage. | Easy one-medium solution; error correction information needs not to be cataloged or explicitly protected.
no compatibilty issues with play-back units | media with augmented images may not play correctly on all units

Copyright 2004-2012 Carsten Gnörlich. Verbatim copying and distribution of this entire article is permitted in any medium, provided this notice is preserved.