287 lines
12 KiB
HTML
287 lines
12 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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<html>
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<head>
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<meta http-equiv="content-type" content="text/html; charset=UTF-8">
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<title>The big picture</title>
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<link rel="stylesheet" type="text/css" href="../include/dvdisaster.css">
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</head>
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<body>
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<!-- Title header -->
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<table width="100%" cellpadding="0" border="0">
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<tr>
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<td align="left">
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<span class="fxxl"><b>dvdisaster</b></span>
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<i>Version 0.72.4 / 0.72 (pl4)</i>
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</td>
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<td align="right">
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<span class="fxxl"> </span><a href="http://dvdisaster.net/en/">To the Internet version</a>
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<a href="../cs/howtos60.html"></a>
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<a href="../de/howtos60.html"></a>
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<a href="../ru/howtos60.html"></a>
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</td>
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</tr>
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</table>
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<!-- Language chooser or separator -->
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<table width="100%" cellpadding="0" border="0">
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<tr>
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<td colspan="2" class="hsep"></td>
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</tr>
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<tr><td colspan="2" style="height: 10px">
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</td></tr>
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</table>
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<!-- Main body (Navigation, actual page content, optional news column) -->
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<table width="100%" cellspacing=0>
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<tr>
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<!-- Navigation -->
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<td style="background-color:#f0f0f0; vertical-align:top; width:20%;">
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<table width="100%" cellpadding="10">
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<tr>
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<td>
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<span class="fs"><b>Contents</b></span>
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<table width="100%" cellpadding="0" cellspacing="0">
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<tr>
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<td class="hsep"></td>
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</tr>
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</table><p>
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<table width="100%">
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<tr><td colspan=3><span class="fs"><a href="news.html">News</a></span></td></tr>
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<tr><td></td><td></td><td style="height:10px;"></td></tr>
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<tr><td colspan=3><span class="fs"><a href="index.html">Overview</a></span></td></tr>
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<tr><td></td><td></td><td style="height:10px;"></td></tr>
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<tr><td colspan=3><span class="fs"><a href="howtos.html">Howtos</a></span></td></tr>
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<tr><td style="width:1%; vertical-align:top;">·</td><td colspan=2><span class="fs">The big picture</span></td></tr>
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<tr><td></td><td style="vertical-align:top; font-size:small;">-</td><td><span class="fs"><a href="howtos61.html">error correction</a></span></td></tr>
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<tr><td></td><td style="vertical-align:top; font-size:small;">-</td><td><span class="fs"><a href="howtos62.html">correct usage</a></span></td></tr>
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<tr><td></td><td style="vertical-align:top; font-size:small;">-</td><td><span class="fs"><a href="howtos63.html">incorrect usage</a></span></td></tr>
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<tr><td style="width:1%; vertical-align:top;">·</td><td colspan=2><span class="fs"><a href="howtos10.html">Scanning media for errors</a></span></td></tr>
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<tr><td style="width:1%; vertical-align:top;">·</td><td colspan=2><span class="fs"><a href="howtos20.html">Putting error correction data on a separate file</a></span></td></tr>
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<tr><td style="width:1%; vertical-align:top;">·</td><td colspan=2><span class="fs"><a href="howtos30.html">Putting error correction data on the medium</a></span></td></tr>
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<tr><td style="width:1%; vertical-align:top;">·</td><td colspan=2><span class="fs"><a href="howtos40.html">Recovering media images</a></span></td></tr>
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<tr><td style="width:1%; vertical-align:top;">·</td><td colspan=2><span class="fs"><a href="howtos50.html">Getting information about images and error correction data</a></span></td></tr>
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<tr><td style="width:1%; vertical-align:top;">·</td><td colspan=2><span class="fs"><a href="howtos90.html">Testing image compatibility</a></span></td></tr>
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<tr><td style="width:1%; vertical-align:top;">·</td><td colspan=2><span class="fs"><a href="howtosa0.html">Dialogs and buttons</a></span></td></tr>
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<tr><td></td><td></td><td style="height:10px;"></td></tr>
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<tr><td colspan=3><span class="fs"><a href="download.html">Download</a></span></td></tr>
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<tr><td></td><td></td><td style="height:10px;"></td></tr>
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<tr><td colspan=3><span class="fs"><a href="qa.html">Questions and Answers</a></span></td></tr>
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<tr><td></td><td></td><td style="height:10px;"></td></tr>
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<tr><td colspan=3><span class="fs"><a href="feedback.html">Bug reporting</a></span></td></tr>
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</table>
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</td>
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</tr>
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</table>
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</td>
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<td></td>
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<td style="vertical-align:top; width:77%;" rowspan="2">
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<!-- Insert actual page content below -->
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<h3 class="top">The big picture - a comparison of dvdisaster with conventional backup</h3>
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dvdisaster stores data on CD/DVD/BD in a way that the data is fully recoverable even after
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the medium has developed some read errors. The method employed in dvdisaster uses
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less storage space (or additional media) than a full backup would do.
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Before using dvdisaster it is important to understand the similarities and
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differences between dvdisaster and a conventional (full) backup:<p>
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Let's first consider how a conventional backup scheme works:<p>
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<table width="100%">
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<tr>
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<td class="w65x"><img src="../images/backup1.png" alt="Icon: Original medium"></td>
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<td class="w65x">Copy<br><img src="../images/right-arrow.png" alt="Icon: Arrow right"></td>
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<td class="w65x"><img src="../images/backup2.png" alt="Icon: Backup medium"></td>
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<td> </td>
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<td>An existing medium (1) is copied onto a backup medium (2).</td>
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</tr>
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<tr>
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<td align="center"><img src="../images/down-arrow.png" alt="Icon: Arrow down"> </td>
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<td></td>
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<td align="center"><img src="../images/down-arrow.png" alt="Icon: Arrow down"> </td>
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<td> </td>
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</tr>
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<tr>
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<td class="w65x"><img src="../images/bad-cd1.png" alt="Icon: Damaged medium"></td>
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<td class="w65x"> </td>
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<td class="w65x"><img src="../images/backup2.png" alt="Icon: Backup medium"></td>
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<td></td>
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<td>If any one of the two media is damaged afterwards, we still have
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an intact medium left.</td>
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</tr>
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</table><p>
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There are actually some cases where it is important to keep a second copy of
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a CD/DVD/BD: One medium might get lost, burst while spinning in the drive,
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or it may be destroyed due to mishandling. However such cases of complete
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data loss are rare as long as media are handled properly.<p>
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It is more likely that the medium starts to gradually lose data
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after a few years - a nearly unavoidable aging process.
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When the medium is regularly used (or scanned for defects) the data loss
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will typically be noticed after 5% to 10% of the medium have already become
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unreadable. At this point the medium is unusable as a whole,
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but maybe 90% of it is still readable. <i>On the other hand a full backup copy of the
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medium is not required; we simply need a method for recovering the
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missing 10% of data.</i><p>
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This is where dvdisaster comes into play. Consider this:<p>
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<table width="100%">
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<tr>
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<td class="w65x"><img src="../images/good-cd.png" alt="Icon: Good medium (without read errors)"></td>
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<td class="w65x">Create<br><img src="../images/right-arrow.png" alt="Icon: Arrow right"><br>ECC</td>
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<td class="w65x"><img src="../images/ecc.png" alt="Icon: Separate file with error correction data"></td>
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<td> </td>
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<td>
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This time we do not make a full backup. dvdisaster is used to create error correction data
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("ECC") which can recover up to 20% of a degraded medium.
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The value of 20% was chosen to have a safety margin over the expected data loss of 5-10%.
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</td>
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</tr>
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<tr>
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<td align="center"><img src="../images/down-arrow.png" alt="Icon: Arrow down"> </td>
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<td></td>
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<td align="center"><img src="../images/down-arrow.png" alt="Icon: Arrow down"> </td>
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<td> </td>
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</tr>
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<tr>
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<td><img src="../images/bad-cd.png" alt="Icon: Damaged medium (partially unreadable)"></td>
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<td> </td>
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<td><img src="../images/ecc.png" alt="Icon: Separate file with error correction data"></td>
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<td> </td>
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<td>
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Wenn the medium fails at a later time,
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its contents are recovered from its still readable parts and from the
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error correction data.
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</td>
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</tr>
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<tr>
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<td align="right" class="w65x">80%<img src="../images/rdiag-arrow.png" alt="Icon: Diagonal arrow right"></td>
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<td> </td>
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<td align="left" class="w65x"><img src="../images/ldiag-arrow.png" alt="Icon: Diagonal arrow left">20%</td>
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<td> </td>
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<td>
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For a successful recovery at least 80% of the data must still be readable from the medium,
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and the remaining 20% are recalculated from the error correction data.</td>
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</tr>
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<tr>
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<td> </td>
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<td> <img src="../images/good-image.png" alt="Icon: Complete image"></td>
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<td> </td>
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<td> </td>
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<td>The completely recovered data is now available as an ISO image on the hard drive
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(the medium remains defective as physical data loss is irrevocable).
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</td>
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</tr>
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<tr>
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<td> </td>
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<td align="center"><img src="../images/down-arrow.png" alt="Icon: Arrow down"></td>
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<td> </td>
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<td> </td>
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<td>Write the image to a blank medium using your favourite CD/DVD/BD authoring software.</td>
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</tr>
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<tr>
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<td> </td>
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<td align="center"><img src="../images/good-cd.png" alt="Icon: Good medium (without read errors)"></td>
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<td> </td>
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<td> </td>
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<td>You now have a new error-free medium.</td>
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</tr>
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</table><p>
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As you have seen the data recovery took more steps then doing a conventional backup.
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So let's summarize the pros and cons of dvdisaster compared with conventional backup:<p>
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<table>
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<tr valign="top"><td>Advantages</td>
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<td><ul>
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<li>dvdisaster uses less storage. When using error correction data with a 20%
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recovery capability, protecting 5 media requires only one additional medium for
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the ECC data.</li>
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<li>Since all media will eventually age and start losing data in similar places
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(typically in the outermost region), doing a 1:1 copy might not help at all.
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Both copies may turn out defective in the same places after a few years.</li>
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</ul></td></tr>
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<tr valign="top"><td>Similarities</td>
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<td><ul><li>Both backup copies and error correction data must be created
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before the master disc fails. You can't create them from an already defective
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medium.</li></ul></td></tr>
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<tr valign="top"><td>Disadvantages</td>
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<td><ul><li>If the recovery capability of the error correction data is exceeded
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(or the medium gets lost), no data can be recovered!
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Especially take note that error correction data with a repair rate of 20% together
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with a 75% readable the medium does not result in 95% recovery! In that case,
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nothing beyond the 75% readable data from the medium can be recovered!</li></ul></td></tr>
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</table>
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The next three pages provide more related information:<p>
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<ul>
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<li>The general idea of the <a href="howtos61.html">error correction</a> is explained.<p></li>
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<li>Jane demonstrates the
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<a href="howtos62.html">proper usage of dvdisaster</a>. She will create error correction
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data in advance and is therefore able to recover all data when her media become defective.<p></li>
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<li>However you should <a href="howtos63.html">not follow the way</a> of Joe.
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He does not use error correction data and finds out that his defective
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media are not recoverable even after multiple reading passes. As a consequence he loses
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data from a defective medium.<p></li>
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</ul>
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Of course these stories are purely fictional and any similarities with existing
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persons or situations are purely conincidental.
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<!-- do not change below -->
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</td> <!-- end of page contents -->
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<td align="center">
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<span class="fs">
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<i> Copyright 2004-2012 Carsten Gnörlich.<br>
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Verbatim copying and distribution of this entire article is permitted in any medium, provided this notice is preserved.
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</i>
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</span>
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</td>
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</table>
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</html>
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