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    1 \section{Overview}
    2 
    3 \paragraph{The challenge: Optical media eventually fail.} Optical media (CD,DVD,BD) keep their data only for a finite time (typically for many years). 
    4 After that time, data loss develops slowly with read errors growing from the outer 
    5 media region towards the inside. 
    6 
    7 \paragraph{The dvdisaster solution: Archival with data loss protection.}
    8 dvdisaster complements optical media \plnk{qa-technical-media}{supported media} with
    9 error correction data
   10 in a way that they are fully recoverable 
   11 even after some read errors have developed. 
   12 This enables you to rescue the complete data to a new medium.
   13 
   14 Error correction data, in short ``ecc data'',  is either added to the medium 
   15 or kept in separate error correction files. dvdisaster works at the image level 
   16 so that the recovery does not depend on the file system of the medium. 
   17 The maximum error correction capacity is user-selectable.
   18 
   19 \subsection{Common misunderstandings about dvdisaster}
   20 
   21 Before we describe in detail what dvdisaster can do, let's first clarify what it can't:
   22 
   23 \paragraph{dvdisaster can not make defective media readable again.}
   24 Like a conventional backup, error correction data must be created from 
   25 a fully functional optical medium - {\em you can not backup data which has already
   26 been lost}. When the optical medium develops defective sectors at a later time,
   27 those defective sectors are restored by re-calculating them from the ecc data. 
   28 This won't make the defective medium working again, but will produce a new iso 
   29 image which can be written to a new medium. 
   30 
   31 As said before, ecc data can not be created from already defective media.
   32 Although unreadable sectors can not be recovered in that case, dvdisaster
   33 might still be helpful in extracting the remaining readable portions of the medium. 
   34 
   35 \paragraph{It's not a ripping tool.} If you want a tool for copying
   36 protected media, you're looking at the wrong place. Such functions are
   37 outside the scope of dvdisaster's internal design and goals.
   38 Contrary to some myths saying otherwise: dvdisaster contains 
   39 no hidden program fragments or switches for reading protected discs.
   40 Check the source code for yourself if you don't trust me.
   41 
   42  
   43 \subsection{How to use this manual}
   44 
   45 Many users just want to see some examples of solving typical tasks. Flip over
   46 to the \tlnk{howtos}{Typical applications section} in that case. 
   47 
   48 \smallskip
   49 
   50 The remainder of this section gives an example of recovering a defective medium including
   51 screen shots, relates using ecc data to performing quality scans and full backups,
   52 and summarizes the pro and con of dvdisaster.
   53 
   54 \smallskip
   55 
   56 The \tlnk{download}{downloads} section provides a link to the download site,
   57 summarizes the \tlnk{download-requirements}{system requirements},
   58 and clarifies that you can get and use dvdisaster as
   59 \tlnk{download-terms}{free software, at no cost and while keeping
   60 your full privacy}.
   61 
   62 \smallskip
   63 
   64 There is also a chapter containing \tlnk{qa}{general questions and answers},
   65 \tlnk{qa-technical}{technical questions and answers}, 
   66 and explanations of \tlnk{qa-error}{error messages}.
   67 
   68 \smallskip
   69 
   70 The \tlnk{background}{background information} section provides details on
   71 the \tlnk{background-properties}{properties of the error correction},
   72 the difference between \tlnk{background-image-level}{image level and file level data recovery},
   73 the \tlnk{background-methods}{RS01, RS02 and RS03 error correction methods},
   74 the \tlnk{background-linear}{linear}
   75 and \tlnk{background-adaptive}{adaptive} reading strategies,
   76 some \tlnk{background-read-errors}{remarks on how media read errors come into existance},
   77 and finally a few \tlnk{background-eccfile-storage}{hints for storing error correction files}.
   78 
   79 \smallskip
   80 
   81 As not all optical disc burning software may be compatible with dvdisaster,
   82 you might want to check
   83 the \tlnk{burning-compatibility}{compatibility table} and the additional
   84 information provided with it.
   85 
   86 \smallskip
   87 
   88 If you encounter a defect (programming error) or
   89 incompatibility with a certain (drive) hardware and software setup,
   90 please see the \tlnk{reporting-defects}{reporting defects} section.
   91 
   92 %\newpage
   93 
   94 \subsection{Example of the error correction}
   95 
   96 \begin{figure}[h]
   97 \centerline{\includegraphics[width=\textwidth]{screenshots/recover-linear.png}}
   98 \caption{Reading a defective medium.}  
   99 \label{recover-linear}
  100 \end{figure}
  101 
  102 \paragraph{Recovery of aged media.} 
  103 The medium processed here has become discolored and partly unreadable in its outer region.
  104 A reading attempt yields about 23.000 unreadable sectors of 342.000 sectors total;
  105 resulting in about 7,2\% defective sectors. Figure \ref{recover-linear} shows the
  106 dvdisaster window after the reading attempt. The distribution of reading speed and
  107 read errors over the medium is graphically shown.
  108 The still readable sectors are stored in an ISO image called {\em medium.iso}.
  109 
  110 \begin{figure}[t]
  111 \centerline{\includegraphics[width=\textwidth]{screenshots/fix-image.png}}
  112 \caption{Repairing the defective image.}  
  113 \label{fix-image}
  114 \end{figure}
  115 
  116 \paragraph{Repairing the defective image.} 
  117 The image which has been just read is incomplete since about 23.000 sectors could
  118 not be read. These sectors are now reconstructed using the error correction data
  119 created with dvdisaster. During the recovery a maximum of 20 errors per error
  120 correction block is encountered (see figure \ref{fix-image}).
  121 This results in a peak error correction load of
  122 63\%, meaning that this degree of damage is handled well by error correction data
  123 created with default settings. The recovered image can now be written to a new medium.
  124 
  125 \paragraph{Recovery needs error correction data:}
  126 The recovery process described above uses error correction (``ecc'') data.
  127 Think of this data as a special form of backup data (it needs less space
  128 than a normal backup, though). Like an ordinary backup, the ecc data needs
  129 to be created before the medium goes defective.
  130 
  131 So if you have a defective medium but never created ecc data for it, you will
  132 not be able to recover the defective sectors (23.000 in the above example).
  133 The data located at the end of the medium will be lost, while you will probably
  134 be able to extract some files which are located at the beginning of the medium.
  135 \newpage
  136 
  137 \subsection{dvdisaster as a complement to quality scans}
  138 
  139 \tlnk{qa-quality-scans}{Quality scans}, e.g. C2 error or PI/PO scans are a valuable tool
  140 for testing the results of the media writing process.
  141 
  142 \smallskip
  143 
  144 But quality scans are {\bf not} a reliable means of
  145 {\bf predicting the lifetime} of optical media.
  146 Consider we are looking for the right time to copy a worn-out medium onto a new one:
  147 
  148 \begin{itemize}
  149 \item Too early: Copying media because of a bad quality scan is cost-ineffective.
  150   Sometimes such media remain readable much longer than expected.
  151 \item Too late: When the quality scan reveals unreadable sectors some data has already been lost.
  152 \item Right before the medium fails: The ideal case, but how to tell?
  153 \end{itemize}
  154 
  155 However, we could do it the dvdisaster way:
  156 
  157 \begin{itemize}
  158 \item \tlnk{howto-ecc}{Create error correction data} for the medium.
  159 \item \tlnk{howto-scan}{Scan the medium} regularly. Use it until the first read errors occur.
  160 \item \tlnk{howto-recover}{Recover} the read errors using the error correction data.
  161   Write the recovered image to a new medium.
  162 \end{itemize}
  163 
  164 \subsection{Error correction data vs. full backup}
  165 \label{overview-backup}
  166 
  167 \paragraph{A conventional backup strategy}$\!\!\!\!\!$ would be making one or 
  168 more copies of the optical medium. This has a few advantages:
  169 Copying a medium is fast, and having two (or more) working copies
  170 available can be convenient, especially when working at different
  171 locations.
  172 
  173 \smallskip
  174 
  175 The disadvantage of this approach is that it guards only against
  176 incidental damage, but not against general aging. It is not helpful
  177 to have ten copies which all decay in a similar manner. If all 
  178 ten copies are unreadable in the outermost region after a few years,
  179 data loss has occurred even though we were spending 900\% of the
  180 original storage capacity for the backup.
  181 
  182 \paragraph{Ecc data behaves differently}$\!\!\!\!\!$ since it is not a verbatim
  183 copy of the original data. It is a mathematical scheme working like
  184 this: Give me any 80\% of the original data and I will be able to
  185 reconstruct the missing 20\%, regardless of {\em where} the 20\%
  186 are missing (whether at the beginning or at the end, maybe in between - doesn't
  187 matter). Incidentally, there is a strong relationship between 
  188 being able to reconstruct a missing percentage of the original data 
  189 and the size of the ecc data: If the ecc data is 20\% of the size of
  190 the original data, it can roughly recover up to 20\% of missing data;
  191 with ecc data being 30\% of the original size up to 30\% can be recovered
  192 and so on. But this relationship isn't even the greatest advantage
  193 of the ecc data; the ``regardless of where the defects are'' is the big deal.
  194 
  195 \smallskip
  196 
  197 Let's assume we want to have a 100\% protection of a specific 4 GiByte 
  198 DVD. Then we create another DVD containing 4 GiBytes of ecc data. 
  199 At a later date, both DVDs decay and the last 30\% of both become
  200 unreadable. Since we have still 70\% of the original data and of the
  201 ecc data, everything is fine! We can still reconstruct the original 
  202 data from them; using the second DVD for ecc data is much more
  203 efficient than creating a second copy on it. In fact putting another
  204 copy on the second DVD would not have saved us from a 30\% data loss.
  205 
  206 \smallskip
  207 
  208 We can even make some assumptions about our media. Maybe we expect
  209 that even a defective medium will not lose more than 15\% of its data 
  210 (don't take my word on it). And we make sure that ecc data will be saved
  211 on a different type of medium which is considered to have a longer life than optical
  212 media. Then creating ecc data with a recovery rate of 20\% (always
  213 leave a safety margin) should suffice our needs. 
  214 This would yield a reasonable data protection
  215 while spending only an additional 20\% of storage for it. 
  216 
  217 \smallskip
  218 
  219 This is not to say that ecc data is the final answer to all
  220 archiving means, but when used well, it can be much more
  221 efficient and secure than a simple backup strategy. See also the
  222 \tlnk{bigpicture-backup}{``Big Picture'' section} for a continued
  223 ecc data vs. full backup discussion.
  224 
  225 \subsection{Pro and con of dvdisaster}
  226 
  227 To summarize from the previous sub sections:
  228 
  229 \bigskip
  230 {\bf Advantages of using dvdisaster:}
  231 
  232 \begin{itemize}
  233 \item {\bf Protects} against aging and accidental medium damage (within certain limits).
  234 \item \tlnk{howto-scan}{Read error tests} run {\bf faster} than quality scans; up to full reading speed depending on the drive.
  235 \item {\bf Cost-effective:} Media must be replaced with a new copy only when they are really defective.
  236 \item {\bf Space-efficient:} Ecc data requires less space than a full backup under most scenarios.
  237 \end{itemize}
  238 
  239 \bigskip
  240 {\bf Limitations of using dvdisaster:}
  241 
  242 \medskip
  243 
  244 You need a backup and testing strategy and at least 15\% of additional storage.
  245 
  246 \begin{itemize}
  247 \item Error correction data {\bf must be created before the medium fails}, preferably at the same time the medium is written.
  248 \item Error correction data requires {\bf additional storage space} either on the protected medium or by using additional media. Using the standard settings the additional storage space amounts to 15\% of the original data size (approx. 700MiB for a full 4.7GiB DVD).
  249 \item No guaranteed protection against data loss as limits and statistical properties of the
  250   error correction may be exceeded with extremely bad luck.
  251 \end{itemize}
  252 
  253 \bigskip
  254 
  255 See also the \tlnk{background}{collection of background information} to learn
  256 more about the functioning of dvdisaster.