Imported Upstream version 0.72
This commit is contained in:
TANIGUCHI Takaki
186
documentation/en/howtos60.php
Normal file
186
documentation/en/howtos60.php
Normal file
@@ -0,0 +1,186 @@
|
||||
<?php
|
||||
# dvdisaster: English homepage translation
|
||||
# Copyright (C) 2004-2009 Carsten Gnörlich
|
||||
#
|
||||
# UTF-8 trigger: äöüß
|
||||
#
|
||||
# Include our PHP sub routines, then call begin_page()
|
||||
# to start the HTML page, insert the header,
|
||||
# navigation and news if appropriate.
|
||||
|
||||
require("../include/dvdisaster.php");
|
||||
begin_page();
|
||||
?>
|
||||
|
||||
<!--- Insert actual page content below --->
|
||||
|
||||
<h3>The big picture - a comparison of dvdisaster with conventional backup</h3>
|
||||
|
||||
dvdisaster stores data on CD/DVD/BD in a way that the data is fully recoverable even after
|
||||
the medium has developed some read errors. The method employed in dvdisaster uses
|
||||
less storage space (or additional media) than a full backup would do.
|
||||
Before using dvdisaster it is important to understand the similarities and
|
||||
differences between dvdisaster and a conventional (full) backup:<p>
|
||||
|
||||
Let's first consider how a conventional backup scheme works:<p>
|
||||
|
||||
<table width="100%">
|
||||
<tr>
|
||||
<td width="60px"><img src="../images/backup1.png"></td>
|
||||
<td width="60px">Copy<br><img src="../images/right-arrow.png"></td>
|
||||
<td width="60px"><img src="../images/backup2.png"></td>
|
||||
<td> </td>
|
||||
<td>An existing medium (1) is copied onto a backup medium (2).</td>
|
||||
</tr>
|
||||
|
||||
<tr>
|
||||
<td align="center"><img src="../images/down-arrow.png"> </td>
|
||||
<td></td>
|
||||
<td align="center"><img src="../images/down-arrow.png"> </td>
|
||||
<td> </td>
|
||||
</tr>
|
||||
|
||||
<tr>
|
||||
<td width="60px"><img src="../images/bad-cd1.png"></td>
|
||||
<td width="60px"> </td>
|
||||
<td width="60px"><img src="../images/backup2.png"></td>
|
||||
<td></td>
|
||||
<td>If any one of the two media is damaged afterwards, we still have
|
||||
an intact medium left.</td>
|
||||
</tr>
|
||||
</table><p>
|
||||
|
||||
There are actually some cases where it is important to keep a second copy of
|
||||
a CD/DVD/BD: One medium might get lost, burst while spinning in the drive,
|
||||
or it may be destroyed due to mishandling. However such cases of complete
|
||||
data loss are rare as long as media are handled properly.<p>
|
||||
|
||||
It is more likely that the medium starts to gradually lose data
|
||||
after a few years - a nearly unavoidable aging process.
|
||||
When the medium is regularly used (or scanned for defects) the data loss
|
||||
will typically be noticed after 5% to 10% of the medium have already become
|
||||
unreadable. At this point the medium is unusable as a whole,
|
||||
but maybe 90% of it is still readable. <i>On the other hand a full backup copy of the
|
||||
medium is not required; we simply need a method for recovering the
|
||||
missing 10% of data.</i><p>
|
||||
|
||||
This is where dvdisaster comes into play. Consider this:<p>
|
||||
|
||||
<table width="100%">
|
||||
<tr>
|
||||
<td width="60px"><img src="../images/good-cd.png"></td>
|
||||
<td width="60px">Create<br><img src="../images/right-arrow.png"><br>ECC</td>
|
||||
<td width="60px"><img src="../images/ecc.png"></td>
|
||||
<td> </td>
|
||||
<td>
|
||||
This time we do not make a full backup. dvdisaster is used to create error correction data
|
||||
("ECC") which can recover up to 20% of a degraded medium.
|
||||
The value of 20% was chosen to have a safety margin over the expected data loss of 5-10%.
|
||||
</td>
|
||||
</tr>
|
||||
|
||||
<tr>
|
||||
<td align="center"><img src="../images/down-arrow.png"> </td>
|
||||
<td></td>
|
||||
<td align="center"><img src="../images/down-arrow.png"> </td>
|
||||
<td> </td>
|
||||
</tr>
|
||||
|
||||
<tr>
|
||||
<td><img src="../images/bad-cd.png"></td>
|
||||
<td> </td>
|
||||
<td><img src="../images/ecc.png"></td>
|
||||
<td> </td>
|
||||
<td>
|
||||
Wenn the medium fails at a later time,
|
||||
its contents are recovered from its still readable parts and from the
|
||||
error correction data.
|
||||
</td>
|
||||
</tr>
|
||||
|
||||
<tr>
|
||||
<td align="right">80%<img src="../images/rdiag-arrow.png"></td>
|
||||
<td> </td>
|
||||
<td align="left"><img src="../images/ldiag-arrow.png">20%</td>
|
||||
<td> </td>
|
||||
<td>
|
||||
For a successful recovery at least 80% of the data must still be readable from the medium,
|
||||
and the remaining 20% are recalculated from the error correction data.</td>
|
||||
</tr>
|
||||
|
||||
<tr>
|
||||
<td> </td>
|
||||
<td> <img src="../images/good-image.png"></td>
|
||||
<td> </td>
|
||||
<td> </td>
|
||||
<td>The completely recovered data is now available as an ISO image on the hard drive
|
||||
(the medium remains defective as physical data loss is irrevocable).
|
||||
</td>
|
||||
</tr>
|
||||
|
||||
<tr>
|
||||
<td> </td>
|
||||
<td align="center"><img src="../images/down-arrow.png"></td>
|
||||
<td> </td>
|
||||
<td> </td>
|
||||
<td>Write the image to a blank medium using your favourite CD/DVD/BD authoring software.</td>
|
||||
</tr>
|
||||
|
||||
<tr>
|
||||
<td> </td>
|
||||
<td align="center"><img src="../images/good-cd.png"></td>
|
||||
<td> </td>
|
||||
<td> </td>
|
||||
<td>You now have a new error-free medium.</td>
|
||||
</tr>
|
||||
</table><p>
|
||||
|
||||
As you have seen the data recovery took more steps then doing a conventional backup.
|
||||
So let's summarize the pros and cons of dvdisaster compared with conventional backup:<p>
|
||||
|
||||
<table>
|
||||
<tr valign="top"><td>Advantages</td>
|
||||
<td><ul>
|
||||
<li>dvdisaster uses less storage. When using error correction data with a 20%
|
||||
recovery capability, protecting 5 media requires only one additional medium for
|
||||
the ECC data.</li>
|
||||
<li>Since all media will eventually age and start losing data in similar places
|
||||
(typically in the outermost region), doing a 1:1 copy might not help at all.
|
||||
Both copies may turn out defective in the same places after a few years.</li>
|
||||
</ul></td></tr>
|
||||
<tr valign="top"><td>Similarities</td>
|
||||
<td><ul><li>Both backup copies and error correction data must be created
|
||||
before the master disc fails. You can't create them from an already defective
|
||||
medium.</li></ul></td></tr>
|
||||
<tr valign="top"><td>Disadvantages</td>
|
||||
<td><ul><li>If the recovery capability of the error correction data is exceeded
|
||||
(or the medium gets lost), no data can be recovered!
|
||||
Especially take note that error correction data with a repair rate of 20% together
|
||||
with a 75% readable the medium does not result in 95% recovery! In that case,
|
||||
nothing beyond the 75% readable data from the medium can be recovered!</li></ul></td></tr>
|
||||
</table>
|
||||
|
||||
The next three pages provide more related information:<p>
|
||||
|
||||
<ul>
|
||||
<li>The general idea of the <a href="howtos61.php">error correction</a> is explained.<p></li>
|
||||
<li>Jane demonstrates the
|
||||
<a href="howtos62.php">proper usage of dvdisaster</a>. She will create error correction
|
||||
data in advance and is therefore able to recover all data when her media become defective.<p></li>
|
||||
<li>However you should <a href="howtos63.php">not follow the way</a> of Joe.
|
||||
He does not use error correction data and finds out that his defective
|
||||
media are not recoverable even after multiple reading passes. As a consequence he loses
|
||||
data from a defective medium.<p></li>
|
||||
</ul>
|
||||
|
||||
Of course these stories are purely fictional and any similarities with existing
|
||||
persons or situations are purely conincidental.
|
||||
|
||||
|
||||
<!--- do not change below --->
|
||||
|
||||
<?php
|
||||
# end_page() adds the footer line and closes the HTML properly.
|
||||
|
||||
end_page();
|
||||
?>
|
||||
Reference in New Issue
Block a user