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|  | ### General questions and answers 1.1 How is "dvdisaster" pronounced? 1.2 What are quality scans and why don't you support more? 1.3 Is dvdisaster compatible with future releases? 1.4 Augmented images have the error correction data appended at the end of the medium. Isn't that a bad choice? 1.5 What's the difference between image based and file based data recovery? 1.1 How is "dvdisaster" pronounced? Since the word stems from the english language, simply spell "dv" before saying "disaster". Perhaps "dee-vee-disaster" is a suitable phonetic circumscription. ↑ 1.2 What are quality scans and why don't you support more? Optical media have a built-in error correction which is similar to the method used in dvdisaster. Some drives can report the number of errors corrected while reading a medium. This provides a rough estimate of the writing and media qualities. Since dvdisaster is free software, it can only include code and information which can be redistributed freely. This is currently true for C2 scanning of CD media, which is officially standardized and has free documentation available. On the other hand, DVD quality scans ("PI/PO scans") are not standardized. Those drive vendors who support it are using proprietary programming interfaces. The respective specifications seem not to be available for use in free software. So we must patiently wait until manufacturers understand that having free software available for a drive will sell more drives. ↑ 1.3 Is dvdisaster compatible with future releases? Yes, dvdisaster files are intended for an archival time of many years. When upgrading to a newer version of dvdisaster you can continue using images and error correction data created from previous versions. There is no need to recreate them again. ↑ 1.4 Augmented images have the error correction data appended at the end of the medium. Isn't that a bad choice? No. First a bit of terminology: If we augment 80 bytes of user data with 20 bytes of error correction data, we get an "ecc block" comprised of 100 bytes. Now take the following into consideration about the ecc block: The position of the error correction data within the ecc block does not matter. The RS decoder does not differentiate between user data and error correction data. In the view of the RS decoder our ecc block is a sequence of 100 bytes from which an arbitrary subset of 20 bytes can be recovered. It can recover the first 20 bytes, the last 20 bytes, or any combination from within as long as the remaining 80 bytes are still intact. From this it follows that the position of the ecc data within the ecc block does not matter; whether it is appended at one end of the user data or is interleaved with it has no influence on the error correcting capability. Properly distributing the ecc block offsets influence of bad media spots. Optical media have a higher probability of failing in the outer area; for technical reasons this is the only place where the error correction data can be stored. However this effect is offset by distributing the ecc block content over the medium. Let's assume that our medium is filled 80% with user data, leaving the remaining 20% free for error correction data. Now consider the 100 byte ecc block again. We need to pick 80 bytes from the user data for it and require 20 additional byte positions in the error correction data area. Even under these constraints it is possible to evenly distribute the 100 bytes over the medium, from the inside to the outside, each having a maximum distance to its neighbors. Together with point (1), this negates the influence of bad spots on the medium. Symmetry implies that for each error correction byte stored in the (bad) outer region there will be a user data byte located in the (good) inner medium region. (If you do not already see the point, imagine putting the ecc data into the inner medium region and the user data in the outer region. Consider point (1) again to see that nothing changes with respect to the error correction.) ↑ 1.5 What's the difference between image based and file based data recovery? Optical media are comprised of 2048 byte-wide sectors. Most of those sectors are used to store file data, but some of them hold so-called "meta data", e.g. information on directory folders. In figure 1.5.1 (below) there is a directory "Pics" holding three files "forest.jpg", "rock.jpg" and "protect.par"1). Note how these files are mapped onto physical sectors (green/blue squares) on the medium, and that an additional meta data sector (red square) is needed for storing the "Pics" directory structure.
Shortcoming of file based recovery on optical media. Now let's assume that we are working with file based error correction. The file "protect.par" holds error correction information which can be used to recover unreadable sectors within the files "forest.jpg" and "rock.jpg". This will only work as long as we need to recover sectors which are part of a file. But if meta data sectors become unreadable, the file based protection will collapse. Consider figure 1.5.2. When the red directory sector becomes unreadable, not only the directory "Pics" but also all files under "Pics" become inaccessible. This is due to the logical structure of the ISO/UDF file system, as there is no way to tell how the green and blue sectors relate to files anymore when the directory is lost. So we have a complete data loss although all sectors comprising the files are still physically readable.
Please note that moving "protect.par" to a separate medium does not rectify the problem - the directory block is still not recoverable as it is not protected by the error correction data in "protect.par". Advantages of image level recovery on optical media. dvdisaster applies an image level approach to error recovery. The medium is read and processed as an ISO image. The ISO image contains a sequence of all sectors found on the medium, including those which are meta data for the file system. Since the dvdisaster error correction data protects all sectors in the ISO image, file contents as well as meta data sectors (e.g. directories) can be restored. See fig. 1.5.3 for the different range of protection.
In addition, neither reading the damaged ISO image nor applying the error correction requires any information from the file system contained on the medium. As long as the drive is still able to recognize the medium, dvdisaster will be able to recover the still readable sectors from it. Therefore there are no "single sectors of failure" as in the file based approach.
1) No offense intended against the PAR/PAR2 project. Carsten is just confident that file based protection does not work on optical media :-) ↑

Copyright 2004-2014 Carsten Gnörlich. Verbatim copying and distribution of this entire article is permitted in any medium, provided this notice is preserved.