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<?xml version='1.0' encoding='UTF-8'?>
<!DOCTYPE topic PUBLIC "-//OASIS//DTD DITA Topic//EN" "topic.dtd">
<topic xml:lang="en-us" id="rawdisk">
<title>Using a Raw Host Hard Disk From a Guest</title>
<body>
<p>As an alternative to using virtual disk images as described in <xref href="storage.dita">Virtual Storage</xref>,
<ph conkeyref="vbox-conkeyref-phrases/product-name"/> can also present either entire physical hard disks or
selected partitions as virtual disks to virtual machines. </p>
<p>With <ph conkeyref="vbox-conkeyref-phrases/product-name"/>, this type of access is called <i>raw hard disk
access</i>. It enables a guest operating system to access its virtual hard disk without going through the host
OS file system. The actual performance difference for image files compared to raw disk varies greatly depending on
the overhead of the host file system, whether dynamically growing images are used, and on host OS caching
strategies. The caching indirectly also affects other aspects such as failure behavior. For example, whether the
virtual disk contains all data written before a host OS crash. Consult your host OS documentation for details on
this. </p>
<note type="caution">
<p>Raw hard disk access is for expert users only. Incorrect use or use of an outdated configuration can lead to <b
outputclass="bold">total loss of data</b> on the physical disk. Most importantly, <i>do not</i> attempt to
boot the partition with the currently running host operating system in a guest. This will lead to severe data
corruption. </p>
</note>
<p>Raw hard disk access, both for entire disks and individual partitions, is implemented as part of the VMDK image
format support. As a result, you will need to create a special VMDK image file which defines where the data will
be stored. After creating such a special VMDK image, you can use it like a regular virtual disk image. For
example, you can use the Virtual Media Manager, see <xref href="virtual-media-manager.dita">The Virtual Media
Manager</xref>, or <userinput>VBoxManage</userinput> to assign the image to a virtual machine. </p>
</body>
</topic>
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