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<span id="Basic-concepts"></span><div class="header">
<p>
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<span id="Basic-concepts-1"></span><h2 class="chapter">2 Basic concepts</h2>
<span id="index-basic-concepts"></span>
<dl compact="compact">
<dt>Block</dt>
<dd><p>Any amount of data. A block is described by its starting position and
its size. The starting position (or beginning position) is the lowest
position in the block. The end of the block is its starting position
plus its size.
</p>
</dd>
<dt>Cluster</dt>
<dd><p>Group of consecutive sectors read or written in one go.
</p>
</dd>
<dt>Device</dt>
<dd><p>Piece of hardware containing data. Hard disc drives, cdrom drives, USB
pendrives, are devices. /dev/hda, /dev/sdb, are device names.
</p>
</dd>
<dt>File</dt>
<dd><p>Files are named units of data which are stored by the operating system
for you to retrieve later by name. Devices and partitions are accessed
by means of their associated file names.
</p>
</dd>
<dt>Partition</dt>
<dd><p>Every part in which a device is divided. A partition normally contains a
file system. /dev/hda1, /dev/sdb3, are partition names.
</p>
</dd>
<dt>Recoverable formats</dt>
<dd><p>As ddrescue uses standard library functions to read data from the device
being rescued, only mountable device formats can be rescued with
ddrescue. CD-ROMs and DVDs can be rescued, "compact disc digital audio"
CDs can’t, "video CDs"[1] maybe.<br> [1]
http://en.wikipedia.org/wiki/Video_CD
</p>
</dd>
<dt>Rescue domain</dt>
<dd><p>Block or set of blocks to be acted upon (rescued, listed, etc). You may
define it with the options ‘<samp>--input-position</samp>’, ‘<samp>--size</samp>’ and
‘<samp>--domain-mapfile</samp>’. The rescue domain defaults to the whole input
file or mapfile. If ddrescue can’t determine the size of the input file,
the rescue domain defaults to the maximum size of a block (at least
2^63 <span class="nolinebreak">-</span> 1<!-- /@w --> bytes, or 8 EiB minus 1 byte).
</p>
<p>Ddrescue will never try to read any data outside of the rescue domain
except when unaligned direct disc access is requested (see <a href="Direct-disc-access.html">Direct disc access</a>). If it does, please, report it as a bug.
</p>
<p>The amount of data rescued, number of bad areas, etc, shown by ddrescue
may vary or even become zero if you limit the rescue domain. Don’t
worry, they have not disappeared; they are simply out of the specified
rescue domain.
</p>
</dd>
<dt>Sector</dt>
<dd><p>Hardware block. Smallest accessible amount of data on a device.
</p>
</dd>
</dl>
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