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<span id="Direct-disc-access"></span><div class="header">
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<span id="Direct-disc-access-1"></span><h2 class="chapter">11 Direct disc access</h2>
<span id="index-direct-disc-access"></span>
<span id="index-raw-devices"></span>
<p>If you notice that the positions and sizes in <var>mapfile</var> are always
multiples of the sector size, maybe your kernel is caching the disc
accesses and grouping them. In this case you may want to use direct disc
access for <var>infile</var>, or read from a raw device, to bypass the kernel
cache and rescue more of your data.
</p>
<p>NOTE! Sector size must be correctly set with the ‘<samp>--sector-size</samp>’
option for direct disc access to work.
</p>
<p>NOTE: Direct disc access can copy arbitrary domains by reading whole
sectors and then writing only the requested part. This is the only case
where ddrescue will try to read data outside of the rescue domain.
</p>
<p>Try the ‘<samp>--idirect</samp>’ option first. If direct disc access is not
available in your system, try raw devices. Read your system
documentation to find how to bind a raw device to a regular block
device. Some OSs provide raw access through especial device names, like
/dev/rdisk.
</p>
<p>Ddrescue aligns its I/O buffer to the sector size so that it can be used
for direct disc access or to read from raw devices. For efficiency
reasons, also aligns it to the memory page size if page size is a
multiple of sector size. On some systems, ddrescue can’t determine the
size of a raw device, so an explicit ‘<samp>--size</samp>’ or
‘<samp>--complete-only</samp>’ option may be needed.
</p>
<p>Using direct disc access, or reading from a raw device, may be slower or
faster than normal cached reading depending on your OS and hardware. In
case it is slower you may want to make a first pass using normal cached
reads and use direct disc access, or a raw device, only to recover the
good sectors inside the failed blocks.
</p>
<br>
<p>Example 1: using direct disc access.
</p>
<div class="example">
<pre class="example">ddrescue -f -n /dev/sdb1 /dev/sdc1 mapfile
ddrescue -d -f -r3 /dev/sdb1 /dev/sdc1 mapfile
e2fsck -v -f /dev/sdc1
mount -t ext2 -o ro /dev/sdc1 /mnt
</pre></div>
<br>
<p>Example 2: using a raw device.
</p>
<div class="example">
<pre class="example">raw /dev/raw/raw1 /dev/sdb1
ddrescue -f -n /dev/sdb1 /dev/sdc1 mapfile
ddrescue -C -f -r3 /dev/raw/raw1 /dev/sdc1 mapfile
raw /dev/raw/raw1 0 0
e2fsck -v -f /dev/sdc1
mount -t ext2 -o ro /dev/sdc1 /mnt
</pre></div>
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