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TANIGUCHI Takaki
2017-02-02 22:24:37 +09:00
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\newcounter{qasection}
\newcounter{qaitem}
\newcommand{\qa}[2]{\stepcounter{qaitem}{\bf Q \theqasection.\theqaitem: #1}
\medskip
#2
\bigskip}
\section{Questions and answers}
\label{qa}
\stepcounter{qasection}
This section answers some general questions on dvdisaster. There is also
a sub section about \tlnk{qa-technical}{technical questions}, and one
discussing \tlnk{qa-error}{error messages}.
\bigskip
\qa{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. }
\qa{What are quality scans and why don't you support more?}
{\label{qa-quality-scans}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.
\smallskip
Since dvdisaster is free software, it can only include code and
information which can be redistributed freely. This is currently
true for C2 \tlnk{howto-scan}{scanning} of CD media, which
is officially standardized
and has free documentation available.
\smallskip
On the other hand, DVD quality scans (``PI/PO scans'') and BD disc
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. }
\qa{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. }
\qa{Augmented images have the error correction data appended at the end of the medium. Isn't that a bad choice?}
{\label{qa-ecc-distribution}No. First a bit of terminology: If we augment 80 bytes of user data with 20 bytes
of error correction (ecc) data, we get an ``ecc block'' comprised of 100 bytes.
Now take the following into consideration about the ecc block:
\begin{enumerate}
\item The position of the error correction data within the ecc block does not matter.
The Reed-Solomon (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 the end of the user data or is interleaved with it has no
influence on the error correcting capability.
\item Properly distributing the ecc block offsets influence of bad media spots.
Optical media have a higher probability of failing in the outer area; but 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. This means that under the
above scenario, 80\% of each ecc block comes from the first 80\% of the medium,
and 20\% from the rest (the outer area). 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.
\smallskip
(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.)
\end{enumerate}}
\qa{What's the difference between image based and file based data recovery?}
{\label{qa-image-level}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 \ref{qa-metadata1} (below) there is a directory ``Pics'' holding
three files ``forest.jpg'', ``rock.jpg'' and ``protect.par''\footnote{No
offense intended against the PAR/PAR2 project. Carsten is just confident
that file based protection does not work on optical media :-)}.
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.
\begin{figure}[h]
\centerline{\includegraphics[width=\textwidth]{figures/metadata1.pdf}}
\caption{Relation between file system and sectors on the medium.}
\label{qa-metadata1}
\end{figure}
\paragraph{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 \ref{qa-metadata2}.
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.
\begin{figure}[h]
\centerline{\includegraphics[width=\textwidth]{figures/metadata2.pdf}}
\caption{Losing the metadata sector produces complete data loss.}
\label{qa-metadata2}
\end{figure}
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''.
\paragraph{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. \ref{qa-metadata3} for the
different range of protection.
\begin{figure}[h]
\centerline{\includegraphics[width=\textwidth]{figures/metadata3.pdf}}
\caption{Image level protection}
\label{qa-metadata3}
\end{figure}
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. }
\newpage
\subsection{Technical questions}
\label{qa-technical}
\stepcounter{qasection}
\setcounter{qaitem}{0}
\qa{Which on-screen translations of the program are available?\label{qa-locale}}
{The current version of dvdisaster contains screen texts in the following languages:
\medskip
\begin{tabular}{lll}
Czech &--& unmaintained and outdated\\
English &--& complete\\
German &--& complete\\
Italian &--& unmaintained and outdated\\
Portuguese &--& unmaintained and outdated\\
Russian &--& unmaintained and outdated\\
Swedish &--& unmaintained and outdated\\
\end{tabular}
\medskip
Translators for other languages are welcome!
\bigskip
dvdisaster will automatically obtain language settings from the operating system.
If the local language is not yet supported, english text will be used. A different
language can be selected using environment variables.
\medskip
Example for the bash shell and german language:
\smallskip
{\tt export LANG=de\_DE.utf8}
\medskip
Please note that we are currently not providing translations of this user manual
and the internet site. See \tlnk{qa-translate}{topic Q 2.5} for more details.
}
\qa{Which media types are supported?}
{\label{qa-technical-media} dvdisaster supports (re-)writeable CD, DVD and BD media.
Media containing multiple sessions or copy protection can {\em not} be used.
\bigskip
Usable media by type:
\medskip
{\bf CD-R, CD-RW}
\begin{itemize}
\item only Data CD are supported.
\end{itemize}
\medskip
{\bf DVD-R, DVD+R}
\begin{itemize}
\item No further limitations are known.
\end{itemize}
\medskip
{\bf DVD-R DL, DVD+R DL (two layers)}
\begin{itemize}
\item The drive must be able to \tlnk{qa-identify}{identify (Q3.5)} the medium
type. Typically this is only the case for drives which
can also write two layered media.
\end{itemize}
\medskip
{\bf DVD-RW, DVD+RW}
\begin{itemize}
\item Some drives report \tlnk{qa-rw}{wrong image sizes (Q3.4)}.
Remedy: Determine the image size from the ISO/UDF file system or the ECC data.
\end{itemize}
\medskip
{\bf DVD-RAM}
\begin{itemize}
\item Usable only when written with ISO/UDF images like DVD-R/-RW;
\item Not supported when used as removeable medium for packet writing.
\item Similar issues with \tlnk{qa-rw}{image size recognition (Q3.4)} as noted above.
\end{itemize}
\medskip
{\bf BD-R, BD-RW}
\begin{itemize}
\item No limitations are known so far, but very few results on the
two-layered type (the 50GiB variety) have been reported yet.
\end{itemize}
\medskip
{\bf Not usable types} (image can not be extracted):
\begin{itemize}
\item BD-ROM (pressed BDs), DVD-ROM (pressed DVDs), CD-Audio and CD-Video.
\end{itemize}}
\qa{Which file systems are supported?}
{dvdisaster works exclusively on the image level which is accessed
sector-wise. That means it does not matter with which file system
the medium has been formatted. See also \tlnk{qa-image-level}{Q1.5}.
\smallskip
Since dvdisaster neither knows nor uses the file system structure,
it can not repair logical errors at the file system level. It can not
recover lost or deleted files. }
\qa{What happened to the Windows and Mac OS ports?}
{\label{qa-discontinued-os}As you may have noticed, the project has progressed very slowly in the
last years. The main developer is currently very short of time,
and some co-developers are also busy with other tasks. So the question
was either to stop development at all, or to continue developing using
as few resources as possible.
The main problem is that we have just one version of dvdisaster which
needs to live up to the different standards of GNU/Linux, Windows and
Mac OS. The path of least resistance is to continue developing dvdisaster
for GNU/Linux, since dvdisaster was originally started on GNU/Linux and is
still primarily developed on it. dvdisaster is a ``native'' GNU/Linux
application, meaning that it only uses tools and interfaces which come
bundled with GNU/Linux, such as the GNU compiler suite and the GTK+ graphical
user interface toolkit.
\smallskip
Producing the Windows version requires a huge effort, as dvdisaster can not be
built with native Windows tools like Visual C++. It requires installing an
Unix-like development environment containing the GNU compiler suite (e.g. from
MingW) and a port of the GTK+ toolkit. These tools need to be obtained directly
from the upstream source (e.g. from www.gtk.org), as no responsible developer
would use pre-made binaries from third-party sites. That guarantees for huge
extra work every time the tool chain needs to be updated.
\smallskip
When the project was started in the year 2004, the Windows 2000 port of GTK+
provided a sufficient compatibility layer to make a decent looking and usable
Windows version. Today, since Windows 8 the operating system has diverged so
significantly from GNU/Linux that it makes GTK+ based applications look poor
in terms of usage and visual appearance. Also, Windows is taking different
ways in dealing with 64-bitness and multithreading over several CPU cores.
This makes it very difficult to write an implementation of the RS03 codec
which works well on both GNU/Linux and Windows. To create a version
which lives up to the current standards of both GNU/Linux and Windows,
dvdisaster would have to be written from scratch with native bindings
to the Windows GUI and process interfaces. The same is true for Mac OS;
necessitating another complete rewrite of the source code. Resources for
that are not available, so the project will just continue developing
the GNU/Linux version.
\smallskip
Please note that this does not make your error correction data immediately
worthless under Windows, as you can continue using the current 0.72.3 binary
for a long time. Also, ecc data created under Windows can be used to recover
media using GNU/Linux, even when booting Linux as a live system from a memory stick.}
\qa{How can I help providing translations?\label{qa-translate}}
{There are several ways of providing translations to the project.
\paragraph{Translating the on-screen texts.} See \tlnk{qa-locale}{topic Q 2.1} for
a list of available translations. You are welcome to take over an unmaintained
translation or to provide one for a new language. Send an email to
{\em carsten@dvdisaster.org} for further instructions.
\paragraph{Translating the user manual and/or the internet site.} The project
is {\em currently not accepting contributions for translations of the internet
site and the user manual}. Coordinating a translation effort is a very time
consuming process which is currently exceeding our resources.
\smallskip
You are welcome, of course, to provide an unoffical translation of the
project documentation on your own webspace and at your own discretion. }
\newpage
\subsection{Error messages}
\label{qa-error}
\stepcounter{qasection}
\setcounter{qaitem}{0}
\qa{``Warning: 2 sectors missing at the end of the disc''}
{This warning appears with CD media written in ``TAO'' (track at once) mode.
Some drives report an image size which is 2 sectors too large for such media,
producing 2 pseudo read errors at the end of the medium which do not mean data
loss in this case.
\smallskip
Since the writing mode can not be determined from the medium, dvdisaster
assumes a ``TAO'' CD if exactly the last two sectors are unreadable, and
trims the image accordingly. It is up to you to decide whether this is okay.
You can advise dvdisaster to treat these sectors as real read errors by using
the {\tt --dao} option or the preferences tab for reading/scanning.
\smallskip
To avoid these problems, consider using the ``DAO / Disc at once''
(sometimes also called ``SAO / Session at once'') mode for writing single session media.}
\qa{Program blocks right after invocation}
{ Using old Linux kernel versions (kernel 2.4.x) the program
occasionally blocks right after the start and before any actions are
carried out. It can not be terminated using Ctrl-C or ``kill -9''.
\smallskip
Eject the medium to make the program terminate. Insert the medium again
and wait until the drive recognizes the medium and spins down. Calling
dvdisaster again should work now. }
\qa{What does ``CRC error, sector: n'' mean?}
{The respective sector could be read, but the checksum of its contents does
not match the value noted in the error correction file. Some possible causes are:
\begin{itemize}
\item The image has been mounted with write permission and was
therefore altered (typical evidence: CRC errors in sector 64 and in sectors 200 to 400).
\item The computer has some hardware problems, especially when communicating with
its mass storage devices.
\end{itemize}
If you suspect technical problems, try creating another version of the
image and error correction files and \tlnk{howto-info}{verify} them again. When errors
disappear or surface at a different locations, your computer may be
suffering from defective memory, broken drive cabling, or wrong CPU/system
frequency settings. }
\qa{Read errors or wrong image size with -RW/+RW/-RAM media\label{qa-rw}}
{Some drives report incorrect image sizes when -RW/+RW/-RAM media are used.
Two common cases are:
\smallskip
\begin{tabular}{lp{134mm}}
{\em Problem 1)} & The drive reports the size of the largest image ever written to the medium, not that of the actual image. \\
{\em Symptoms:} & After erasing a medium it is written with a file sized about 100MiB. But the image read back is several GiB long and contains the remainings of older images. \\[3mm]
{\em Problem 2)} & The drive reports the maximum possible medium capacity (typically 2295104 sectors) instead of the number of actually used sectors. \\
{\em Symptoms:} & When reading beyond a certain point of the medium, only read errors occur, but all files on the medium are still readable and complete. \\
\end{tabular}
\paragraph{Possible remedy:} Activate the option for determining the image size from
the ISO/UDF file system or from the ECC (RS02/RS03) data.
\medskip
If the required ISO/UDF sectors are unreadable and you are using error correction
files to recover damaged media there are two possible workarounds:
\begin{itemize}
\item Execute the ``Verify'' function with only the error correction
file being selected/given. Note down the correct image size from the
output and restrict the reading range accordingly.
\item Simply read in the image with the incorrect (larger) size.
When invoking the ``Fix'' function, answer ``OK'' when you are asked
whether the image should be truncated.
\end{itemize}}
\qa{My self-written media is recognized as ``DVD-ROM'' and rejected.\label{qa-identify}}
{The medium book type has probably been set to ``DVD-ROM''. Typically,
a drive capable of writing the same media format is required for
processing the medium with dvdisaster.
\smallskip
For example, a two-layered DVD+R with a wrong book type may only
be accepted on a writer which can write to such media. Try another
drive for reading images in these cases.}
\qa{No drives appear under FreeBSD or NetBSD.}
{ You need read {\em and} write permission for the respective device
(like /dev/pass1 on FreeBSD or /dev/rcd0d on NetBSD).
Please see the respective hints in the {\tt INSTALL} file which is
included in the source code archive.}
\qa{``Ecc file has been created with version 0.40.7.''}
{Some developer versions of dvdisaster mark their ecc files with a special bit.
This causes dvdisaster versions upto 0.65 to falsely display the above error
message. Please use the developer versions only together with dvdisaster
0.66 or newer versions. }