Starting with version 1.4, as an alternative to using virtual disk images (as described in detail in Chapter 5, Virtual storage), VirtualBox can also present either entire physical hard disks or selected partitions thereof as virtual disks to virtual machines.
With VirtualBox, this type of access is called "raw hard disk access"; it allows 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 vs. 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, i.e. whether the virtual disk contains all data written before a host OS crash. Consult your host OS documentation for details on this.
Warning
Raw hard disk access is for expert users only. Incorrect use or use of an outdated configuration can lead to total loss of data on the physical disk. Most importantly, do not attempt to boot the partition with the currently running host operating system in a guest. This will lead to severe data corruption.
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 (Section 5.3, “The Virtual Media Manager”) or VBoxManage to assign the image to a virtual machine.
9.12.1. Access to entire physical hard disk
While this variant is the simplest to set up, you must be aware that this will give a guest operating system direct and full access to an entire physical disk. If your host operating system is also booted from this disk, please take special care to not access the partition from the guest at all. On the positive side, the physical disk can be repartitioned in arbitrary ways without having to recreate the image file that gives access to the raw disk.
To create an image that represents an entire physical hard disk (which will not contain any actual data, as this will all be stored on the physical disk), on a Linux host, use the command
VBoxManage internalcommands createrawvmdk -filename /path/to/file.vmdk
-rawdisk /dev/sda
This creates the image /path/to/file.vmdk (must be absolute), and all data will be read and written from /dev/sda.
On a Windows host, instead of the above device specification, use e.g. \\.\PhysicalDrive0. On a Mac OS X host, instead of the above device specification use e.g. /dev/disk1. Note that on OS X you can only get access to an entire disk if no volume is mounted from it.
Creating the image requires read/write access for the given device. Read/write access is also later needed when using the image from a virtual machine.
Just like with regular disk images, this does not automatically register the newly created image in the internal registry of hard disks. If you want this done automatically, add -register:
VBoxManage internalcommands createrawvmdk -filename /path/to/file.vmdk
-rawdisk /dev/sda -register
After registering, you can assign the newly created image to a virtual machine with
VBoxManage modifyvm WindowsXP -hda /path/to/file.vmdk
When this is done the selected virtual machine will boot from the specified physical disk.
9.12.2. Access to individual physical hard disk partitions
This "raw partition support" is quite similar to the "full hard disk" access described above. However, in this case, any partitioning information will be stored inside the VMDK image, so you can e.g. install a different boot loader in the virtual hard disk without affecting the host's partitioning information. While the guest will be able to see all partitions that exist on the physical disk, access will be filtered in that reading from partitions for which no access is allowed the partitions will only yield zeroes, and all writes to them are ignored.
To create a special image for raw partition support (which will contain a small amount of data, as already mentioned), on a Linux host, use the command
VBoxManage internalcommands createrawvmdk -filename /path/to/file.vmdk
-rawdisk /dev/sda -partitions 1,5
As you can see, the command is identical to the one for "full hard disk" access, except for the additional -partitions parameter. This example would create the image /path/to/file.vmdk (which, again, must be absolute), and partitions 1 and 5 of /dev/sda would be made accessible to the guest.
VirtualBox uses the same partition numbering as your Linux host. As a result, the numbers given in the above example would refer to the first primary partition and the first logical drive in the extended partition, respectively.
On a Windows host, instead of the above device specification, use e.g. \\.\PhysicalDrive0. On a Mac OS X host, instead of the above device specification use e.g. /dev/disk1. Note that on OS X you can only use partitions which are not mounted (eject the respective volume first). Partition numbers are the same on Linux, Windows and Mac OS X hosts.
The numbers for the list of partitions can be taken from the output of
VBoxManage internalcommands listpartitions -rawdisk /dev/sda
The output lists the partition types and sizes to give the user enough information to identify the partitions necessary for the guest.
Images which give access to individual partitions are specific to a particular host disk setup. You cannot transfer these images to another host; also, whenever the host partitioning changes, the image must be recreated.
Creating the image requires read/write access for the given device. Read/write access is also later needed when using the image from a virtual machine. If this is not feasible, there is a special variant for raw partition access (currently only available on Linux hosts) that avoids having to give the current user access to the entire disk. To set up such an image, use
VBoxManage internalcommands createrawvmdk -filename /path/to/file.vmdk
-rawdisk /dev/sda -partitions 1,5 -relative
When used from a virtual machine, the image will then refer not to the entire disk, but only to the individual partitions (in the example /dev/sda1 and /dev/sda5). As a consequence, read/write access is only required for the affected partitions, not for the entire disk. During creation however, read-only access to the entire disk is required to obtain the partitioning information.
In some configurations it may be necessary to change the MBR code of the created image, e.g. to replace the Linux boot loader that is used on the host by another boot loader. This allows e.g. the guest to boot directly to Windows, while the host boots Linux from the "same" disk. For this purpose the -mbr parameter is provided. It specifies a file name from which to take the MBR code. The partition table is not modified at all, so a MBR file from a system with totally different partitioning can be used. An example of this is
VBoxManage internalcommands createrawvmdk -filename /path/to/file.vmdk
-rawdisk /dev/sda -partitions 1,5 -mbr winxp.mbr
The modified MBR will be stored inside the image, not on the host disk.
For each of the above variants, you can register the resulting image for immediate use in VirtualBox by adding -register to the respective command line. The image will then immediately appear in the list of registered disk images. An example is
VBoxManage internalcommands createrawvmdk -filename /path/to/file.vmdk
-rawdisk /dev/sda -partitions 1,5 -relative -register
which creates an image referring to individual partitions, and registers it when the image is successfully created.
With VirtualBox, this type of access is called "raw hard disk access"; it allows 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 vs. 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, i.e. whether the virtual disk contains all data written before a host OS crash. Consult your host OS documentation for details on this.
Warning
Raw hard disk access is for expert users only. Incorrect use or use of an outdated configuration can lead to total loss of data on the physical disk. Most importantly, do not attempt to boot the partition with the currently running host operating system in a guest. This will lead to severe data corruption.
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 (Section 5.3, “The Virtual Media Manager”) or VBoxManage to assign the image to a virtual machine.
9.12.1. Access to entire physical hard disk
While this variant is the simplest to set up, you must be aware that this will give a guest operating system direct and full access to an entire physical disk. If your host operating system is also booted from this disk, please take special care to not access the partition from the guest at all. On the positive side, the physical disk can be repartitioned in arbitrary ways without having to recreate the image file that gives access to the raw disk.
To create an image that represents an entire physical hard disk (which will not contain any actual data, as this will all be stored on the physical disk), on a Linux host, use the command
VBoxManage internalcommands createrawvmdk -filename /path/to/file.vmdk
-rawdisk /dev/sda
This creates the image /path/to/file.vmdk (must be absolute), and all data will be read and written from /dev/sda.
On a Windows host, instead of the above device specification, use e.g. \\.\PhysicalDrive0. On a Mac OS X host, instead of the above device specification use e.g. /dev/disk1. Note that on OS X you can only get access to an entire disk if no volume is mounted from it.
Creating the image requires read/write access for the given device. Read/write access is also later needed when using the image from a virtual machine.
Just like with regular disk images, this does not automatically register the newly created image in the internal registry of hard disks. If you want this done automatically, add -register:
VBoxManage internalcommands createrawvmdk -filename /path/to/file.vmdk
-rawdisk /dev/sda -register
After registering, you can assign the newly created image to a virtual machine with
VBoxManage modifyvm WindowsXP -hda /path/to/file.vmdk
When this is done the selected virtual machine will boot from the specified physical disk.
9.12.2. Access to individual physical hard disk partitions
This "raw partition support" is quite similar to the "full hard disk" access described above. However, in this case, any partitioning information will be stored inside the VMDK image, so you can e.g. install a different boot loader in the virtual hard disk without affecting the host's partitioning information. While the guest will be able to see all partitions that exist on the physical disk, access will be filtered in that reading from partitions for which no access is allowed the partitions will only yield zeroes, and all writes to them are ignored.
To create a special image for raw partition support (which will contain a small amount of data, as already mentioned), on a Linux host, use the command
VBoxManage internalcommands createrawvmdk -filename /path/to/file.vmdk
-rawdisk /dev/sda -partitions 1,5
As you can see, the command is identical to the one for "full hard disk" access, except for the additional -partitions parameter. This example would create the image /path/to/file.vmdk (which, again, must be absolute), and partitions 1 and 5 of /dev/sda would be made accessible to the guest.
VirtualBox uses the same partition numbering as your Linux host. As a result, the numbers given in the above example would refer to the first primary partition and the first logical drive in the extended partition, respectively.
On a Windows host, instead of the above device specification, use e.g. \\.\PhysicalDrive0. On a Mac OS X host, instead of the above device specification use e.g. /dev/disk1. Note that on OS X you can only use partitions which are not mounted (eject the respective volume first). Partition numbers are the same on Linux, Windows and Mac OS X hosts.
The numbers for the list of partitions can be taken from the output of
VBoxManage internalcommands listpartitions -rawdisk /dev/sda
The output lists the partition types and sizes to give the user enough information to identify the partitions necessary for the guest.
Images which give access to individual partitions are specific to a particular host disk setup. You cannot transfer these images to another host; also, whenever the host partitioning changes, the image must be recreated.
Creating the image requires read/write access for the given device. Read/write access is also later needed when using the image from a virtual machine. If this is not feasible, there is a special variant for raw partition access (currently only available on Linux hosts) that avoids having to give the current user access to the entire disk. To set up such an image, use
VBoxManage internalcommands createrawvmdk -filename /path/to/file.vmdk
-rawdisk /dev/sda -partitions 1,5 -relative
When used from a virtual machine, the image will then refer not to the entire disk, but only to the individual partitions (in the example /dev/sda1 and /dev/sda5). As a consequence, read/write access is only required for the affected partitions, not for the entire disk. During creation however, read-only access to the entire disk is required to obtain the partitioning information.
In some configurations it may be necessary to change the MBR code of the created image, e.g. to replace the Linux boot loader that is used on the host by another boot loader. This allows e.g. the guest to boot directly to Windows, while the host boots Linux from the "same" disk. For this purpose the -mbr parameter is provided. It specifies a file name from which to take the MBR code. The partition table is not modified at all, so a MBR file from a system with totally different partitioning can be used. An example of this is
VBoxManage internalcommands createrawvmdk -filename /path/to/file.vmdk
-rawdisk /dev/sda -partitions 1,5 -mbr winxp.mbr
The modified MBR will be stored inside the image, not on the host disk.
For each of the above variants, you can register the resulting image for immediate use in VirtualBox by adding -register to the respective command line. The image will then immediately appear in the list of registered disk images. An example is
VBoxManage internalcommands createrawvmdk -filename /path/to/file.vmdk
-rawdisk /dev/sda -partitions 1,5 -relative -register
which creates an image referring to individual partitions, and registers it when the image is successfully created.
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