source: vbox/trunk/doc/manual/en_US/user_Storage.xml@ 92985

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<chapter id="storage">

  <title>Virtual Storage</title>

  <para>
    As the virtual machine will most probably expect to see a hard disk
    built into its virtual computer, &product-name; must be able to
    present real storage to the guest as a virtual hard disk. There are
    presently three methods by which to achieve this:
  </para>

  <itemizedlist>

    <listitem>
      <para>
        &product-name; can use large image files on a real hard disk and
        present them to a guest as a virtual hard disk. This is the most
        common method, described in <xref linkend="vdidetails" />.
      </para>
    </listitem>

    <listitem>
      <para>
        iSCSI storage servers can be attached to &product-name;. This is
        described in <xref linkend="storage-iscsi" />.
      </para>
    </listitem>

    <listitem>
      <para>
        You can allow a virtual machine to access one of your host disks
        directly. This is an advanced feature, described in
        <xref linkend="rawdisk" />.
      </para>
    </listitem>

  </itemizedlist>

  <para>
    Each such virtual storage device, such as an image file, iSCSI
    target, or physical hard disk, needs to be connected to the virtual
    hard disk controller that &product-name; presents to a virtual
    machine. This is explained in the next section.
  </para>

  <sect1 id="harddiskcontrollers">

    <title>Hard Disk Controllers</title>

    <para>
      In a computing device, hard disks and CD/DVD drives are connected
      to a device called a hard disk controller, which drives hard disk
      operation and data transfers. &product-name; can emulate the most
      common types of hard disk controllers typically found in computing
      devices: IDE, SATA (AHCI), SCSI, SAS, USB-based, NVMe and
      virtio-scsi mass storage devices.
    </para>

    <itemizedlist>

      <listitem>
        <para>
          <emphasis role="bold">IDE (ATA)</emphasis> controllers are a
          backwards-compatible yet very advanced extension of the disk
          controller in the IBM PC/AT (1984). Initially, this interface
          worked only with hard disks, but was later extended to also
          support CD-ROM drives and other types of removable media. In
          physical PCs, this standard uses flat ribbon parallel cables
          with 40 or 80 wires. Each such cable can connect two devices,
          called device 0 and device 1, to a controller. Typical PCs had
          two connectors for such cables. As a result, support for up to
          four IDE devices was most common: primary device 0, primary
          device 1, secondary device 0, and secondary device 1.
        </para>

        <para>
          In &product-name;, each virtual machine may have one IDE
          controller enabled, which gives you up to four virtual storage
          devices that you can attach to the machine. By default, one of
          these virtual storage devices, device 0 on the secondary
          channel, is preconfigured to be the virtual machine's virtual
          CD/DVD drive. However, you can change the default setting.
        </para>

        <para>
          Even if your guest OS has no support for SCSI or SATA devices,
          it should always be able to see an IDE controller.
        </para>

        <para>
          You can also select which exact type of IDE controller
          hardware &product-name; should present to the virtual machine:
          PIIX3, PIIX4, or ICH6. This makes no difference in terms of
          performance, but if you import a virtual machine from another
          virtualization product, the OS in that machine may expect a
          particular controller type and crash if it is not found.
        </para>

        <para>
          After you have created a new virtual machine with the
          <emphasis role="bold">New Virtual Machine</emphasis> wizard of
          the VirtualBox Manager, you will typically see one IDE
          controller in the machine's
          <emphasis role="bold">Storage</emphasis> settings. The virtual
          CD/DVD drive will be attached to one of the four ports of this
          controller.
        </para>
      </listitem>

      <listitem>
        <para>
          <emphasis role="bold">Serial ATA (SATA)</emphasis> is a more
          recent standard than IDE. Compared to IDE, it supports both
          much higher speeds and more devices per controller. Also, with
          physical hardware, devices can be added and removed while the
          system is running. The standard interface for SATA controllers
          is called Advanced Host Controller Interface (AHCI).
        </para>

        <para>
          Like a real SATA controller, &product-name;'s virtual SATA
          controller operates faster and also consumes fewer CPU
          resources than the virtual IDE controller. Also, this enables
          you to connect up to 30 virtual hard disks to one machine
          instead of just three, when compared to the &product-name; IDE
          controller with a DVD drive attached.
        </para>

        <para>
          For this reason, depending on the selected guest OS,
          &product-name; uses SATA as the default for newly created
          virtual machines. One virtual SATA controller is created by
          default, and the default disk that is created with a new VM is
          attached to this controller.
        </para>

        <warning>
          <para>
            The entire SATA controller and the virtual disks attached to
            it, including those in IDE compatibility mode, will not be
            seen by OSes that do not have device support for AHCI. In
            particular, <emphasis>there is no support for AHCI in
            Windows versions before Windows Vista</emphasis>. Legacy
            Windows versions such as Windows XP, even with SP3
            installed, will not see such disks unless you install
            additional drivers. It is possible to switch from IDE to
            SATA after installation by installing the SATA drivers and
            changing the controller type in the VM
            <emphasis role="bold">Settings</emphasis> dialog.
          </para>

          <para>
            &product-name; recommends the Intel Matrix Storage drivers,
            which can be downloaded from
            <ulink url="http://downloadcenter.intel.com/Product_Filter.aspx?ProductID=2101" />.
          </para>
        </warning>

        <para>
          To add a SATA controller to a machine for which it has not
          been enabled by default, either because it was created by an
          earlier version of &product-name;, or because SATA is not
          supported by default by the selected guest OS, do the
          following. Go to the <emphasis role="bold">Storage</emphasis>
          page of the machine's
          <emphasis role="bold">Settings</emphasis> dialog, click
          <emphasis role="bold">Add Controller</emphasis> under the
          Storage Tree box and then select <emphasis role="bold">Add
          SATA Controller</emphasis>. The new controller appears as a
          separate PCI device in the virtual machine, and you can add
          virtual disks to it.
        </para>

        <para>
          To change the IDE compatibility mode settings for the SATA
          controller, see <xref linkend="vboxmanage-storagectl" />.
        </para>
      </listitem>

      <listitem>
        <para>
          <emphasis role="bold">SCSI</emphasis> is another established
          industry standard, standing for Small Computer System
          Interface. SCSI is as a generic interface for data transfer
          between all kinds of devices, including storage devices. SCSI
          is still used for connecting some hard disks and tape devices,
          but it has mostly been displaced in commodity hardware. It is
          still in common use in high-performance workstations and
          servers.
        </para>

        <para>
          Primarily for compatibility with other virtualization
          software, &product-name; optionally supports LSI Logic and
          BusLogic SCSI controllers, to each of which up to fifteen
          virtual hard disks can be attached.
        </para>

        <para>
          To enable a SCSI controller, on the
          <emphasis role="bold">Storage</emphasis> page of a virtual
          machine's <emphasis role="bold">Settings</emphasis> dialog,
          click <emphasis role="bold">Add Controller</emphasis> under
          the Storage Tree box and then select <emphasis role="bold">Add
          SCSI Controller</emphasis>. The new controller appears as a
          separate PCI device in the virtual machine.
        </para>

        <warning>
          <para>
            As with the other controller types, a SCSI controller will
            only be seen by OSes with device support for it. Windows
            2003 and later ships with drivers for the LSI Logic
            controller, while Windows NT 4.0 and Windows 2000 ships with
            drivers for the BusLogic controller. Windows XP ships with
            drivers for neither.
          </para>
        </warning>
      </listitem>

      <listitem>
        <para>
          <emphasis role="bold">Serial Attached SCSI (SAS)</emphasis> is
          another bus standard which uses the SCSI command set. As
          opposed to SCSI physical devices, serial cables are used
          instead of parallel cables. This simplifies physical device
          connections. In some ways, therefore, SAS is to SCSI what SATA
          is to IDE: it enables more reliable and faster connections.
        </para>

        <para>
          To support high-end guests which require SAS controllers,
          &product-name; emulates a LSI Logic SAS controller, which can
          be enabled much the same way as a SCSI controller. At this
          time, up to 255 devices can be connected to the SAS
          controller.
        </para>

        <warning>
          <para>
            As with SATA, the SAS controller will only be seen by OSes
            with device support for it. In particular, <emphasis>there
            is no support for SAS in Windows before Windows
            Vista</emphasis>. So Windows XP, even SP3, will not see such
            disks unless you install additional drivers.
          </para>
        </warning>
      </listitem>

      <listitem>
        <para>
          The <emphasis role="bold">USB mass storage device
          class</emphasis> is a standard to connect external storage
          devices like hard disks or flash drives to a host through USB.
          All major OSes support these devices and ship generic drivers
          making third-party drivers superfluous. In particular, legacy
          OSes without support for SATA controllers may benefit from USB
          mass storage devices.
        </para>

        <para>
          The virtual USB storage controller offered by &product-name;
          works differently to the other storage controller types. While
          most storage controllers appear as a single PCI device to the
          guest with multiple disks attached to it, the USB-based
          storage controller does not appear as virtual storage
          controller. Each disk attached to the controller appears as a
          dedicated USB device to the guest.
        </para>

        <warning>
          <para>
            Booting from drives attached using USB is only supported
            when EFI is used as the BIOS lacks USB support.
          </para>
        </warning>
      </listitem>

      <listitem>
        <para>
          <emphasis role="bold">Non volatile memory express
          (NVMe)</emphasis> is a standard for connecting non volatile
          memory (NVM) directly over PCI Express to lift the bandwidth
          limitation of the previously used SATA protocol for
          solid-state devices. Unlike other standards the command set is
          very simple in order to achieve maximum throughput and is not
          compatible with ATA or SCSI. OSes need to support NVMe devices
          to make use of them. For example, Windows 8.1 added native
          NVMe support. For Windows 7, native support was added with an
          update.
        </para>

        <para>
          The NVMe controller is part of the extension pack.
        </para>

        <warning>
          <para>
            Booting from drives attached using NVMe is only supported
            when EFI is used as the BIOS lacks the appropriate driver.
          </para>
        </warning>
      </listitem>

      <listitem>
        <para>
          <emphasis role="bold">Virtual I/O Device SCSI</emphasis> is a
          standard to connect virtual storage devices like hard disks or
          optical drives to a VM. Recent Linux and Windows versions
          support these devices, but Windows needs additional drivers.
          Currently virtio-scsi controller support is experimental.
        </para>

        <warning>
          <para>
            The virtio-scsi controller will only be seen by OSes with
            device support for it. In particular, <emphasis>there is no
            built-in support in Windows</emphasis>. So Windows will not
            see such disks unless you install additional drivers.
          </para>
        </warning>
      </listitem>

    </itemizedlist>

    <para>
      In summary, &product-name; gives you the following categories of
      virtual storage slots:
    </para>

    <itemizedlist>

      <listitem>
        <para>
          Four slots attached to the traditional IDE controller, which
          are always present. One of these is typically a virtual CD/DVD
          drive.
        </para>
      </listitem>

      <listitem>
        <para>
          30 slots attached to the SATA controller, if enabled and
          supported by the guest OS.
        </para>
      </listitem>

      <listitem>
        <para>
          15 slots attached to the SCSI controller, if enabled and
          supported by the guest OS.
        </para>
      </listitem>

      <listitem>
        <para>
          Up to 255 slots attached to the SAS controller, if enabled and
          supported by the guest OS.
        </para>
      </listitem>

      <listitem>
        <para>
          Eight slots attached to the virtual USB controller, if enabled
          and supported by the guest OS.
        </para>
      </listitem>

      <listitem>
        <para>
          Up to 255 slots attached to the NVMe controller, if enabled
          and supported by the guest OS.
        </para>
      </listitem>

      <listitem>
        <para>
          Up to 256 slots attached to the virtio-scsi controller, if
          enabled and supported by the guest OS.
        </para>
      </listitem>

    </itemizedlist>

    <para>
      Given this large choice of storage controllers, you may not know
      which one to choose. In general, you should avoid IDE unless it is
      the only controller supported by your guest. Whether you use SATA,
      SCSI, or SAS does not make any real difference. The variety of
      controllers is only supplied by &product-name; for compatibility
      with existing hardware and other hypervisors.
    </para>

  </sect1>

  <sect1 id="vdidetails">

    <title>Disk Image Files (VDI, VMDK, VHD, HDD)</title>

    <para>
      Disk image files reside on the host system and are seen by the
      guest systems as hard disks of a certain geometry. When a guest OS
      reads from or writes to a hard disk, &product-name; redirects the
      request to the image file.
    </para>

    <para>
      Like a physical disk, a virtual disk has a size, or capacity,
      which must be specified when the image file is created. As opposed
      to a physical disk however, &product-name; enables you to expand
      an image file after creation, even if it has data already. See
      <xref linkend="vboxmanage-modifymedium" />.
    </para>

    <para>
      &product-name; supports the following types of disk image files:
    </para>

    <itemizedlist>

      <listitem>
        <para>
          <emphasis role="bold">VDI.</emphasis> Normally, &product-name;
          uses its own container format for guest hard disks. This is
          called a Virtual Disk Image (VDI) file. This format is used
          when you create a new virtual machine with a new disk.
        </para>
      </listitem>

      <listitem>
        <para>
          <emphasis role="bold">VMDK.</emphasis> &product-name; also
          fully supports the popular and open VMDK container format that
          is used by many other virtualization products, such as VMware.
        </para>
      </listitem>

      <listitem>
        <para>
          <emphasis role="bold">VHD.</emphasis> &product-name; also
          fully supports the VHD format used by Microsoft.
        </para>
      </listitem>

      <listitem>
        <para>
          <emphasis role="bold">HDD.</emphasis> Image files of Parallels
          version 2 (HDD format) are also supported.
        </para>

        <para>
          Due to lack of documentation of the format, newer versions
          such as 3 and 4 are not supported. You can however convert
          such image files to version 2 format using tools provided by
          Parallels.
        </para>
      </listitem>

    </itemizedlist>

    <para>
      Irrespective of the disk capacity and format, as mentioned in
      <xref linkend="gui-createvm" />, there are two options for
      creating a disk image: fixed-size or dynamically allocated.
    </para>

    <itemizedlist>

      <listitem>
        <para>
          <emphasis role="bold">Fixed-size.</emphasis> If you create a
          fixed-size image, an image file will be created on your host
          system which has roughly the same size as the virtual disk's
          capacity. So, for a 10 GB disk, you will have a 10 GB file.
          Note that the creation of a fixed-size image can take a long
          time depending on the size of the image and the write
          performance of your hard disk.
        </para>
      </listitem>

      <listitem>
        <para>
          <emphasis role="bold">Dynamically allocated.</emphasis> For
          more flexible storage management, use a dynamically allocated
          image. This will initially be very small and not occupy any
          space for unused virtual disk sectors, but will grow every
          time a disk sector is written to for the first time, until the
          drive reaches the maximum capacity chosen when the drive was
          created. While this format takes less space initially, the
          fact that &product-name; needs to expand the image file
          consumes additional computing resources, so until the disk
          file size has stabilized, write operations may be slower than
          with fixed size disks. However, after a time the rate of
          growth will slow and the average penalty for write operations
          will be negligible.
        </para>
      </listitem>

    </itemizedlist>

  </sect1>

  <sect1 id="vdis">

    <title>The Virtual Media Manager</title>

    <para>
      &product-name; keeps track of all the hard disk, CD/DVD-ROM, and
      floppy disk images which are in use by virtual machines. These are
      often referred to as <emphasis>known media</emphasis> and come
      from two sources:
    </para>

    <itemizedlist>

      <listitem>
        <para>
          All media currently attached to virtual machines.
        </para>
      </listitem>

      <listitem>
        <para>
          Registered media, for compatibility with legacy &product-name;
          versions.
        </para>
      </listitem>

    </itemizedlist>

    <para>
      The known media can be viewed and changed using the
      <emphasis role="bold">Virtual Media Manager</emphasis>, which you
      can access from the <emphasis role="bold">File</emphasis> menu in
      the VirtualBox Manager window.
    </para>

    <figure id="fig-virtual-media-manager">
      <title>The Virtual Media Manager</title>
    <mediaobject>
        <imageobject>
          <imagedata align="center" fileref="images/virtual-disk-manager.png"
                     width="12cm" />
        </imageobject>
      </mediaobject>
    </figure>

    <para>
      The known media are conveniently grouped in separate tabs for the
      supported formats. These formats are:
    </para>

    <itemizedlist>

      <listitem>
        <para>
          Hard disk images, either in &product-name;'s own Virtual Disk
          Image (VDI) format, or in the third-party formats listed in
          <xref linkend="vdidetails"/>.
        </para>
      </listitem>

      <listitem>
        <para>
          CD/DVD images in standard ISO format.
        </para>
      </listitem>

      <listitem>
        <para>
          Floppy images in standard RAW format.
        </para>
      </listitem>

    </itemizedlist>

    <para>
      For each image, the Virtual Media Manager shows you the full path
      of the image file and other information, such as the virtual
      machine the image is currently attached to.
    </para>

    <para>
      The Virtual Media Manager enables you to do the following:
    </para>

    <itemizedlist>

      <listitem>
        <para>
          <emphasis role="bold">Add</emphasis> an image to the known
          media.
        </para>
      </listitem>

      <listitem>
        <para>
          <emphasis role="bold">Create</emphasis> a new disk image.
        </para>

        <itemizedlist>

          <listitem>
            <para>
              For virtual hard disks, the <emphasis role="bold">Create
              Virtual Hard Disk</emphasis> wizard is shown.
            </para>
          </listitem>

          <listitem>
            <para>
              For optical disks, the <emphasis role="bold">VISO
              Creator</emphasis> screen is shown. This enables you to
              create a virtual ISO from selected files on the host.
            </para>
          </listitem>

          <listitem>
            <para>
              For floppy disks, the <emphasis role="bold">Floppy Disk
              Creator</emphasis> screen is shown.
            </para>
          </listitem>

        </itemizedlist>
      </listitem>

      <listitem>
        <para>
          <emphasis role="bold">Copy</emphasis> an image to create
          another one.
        </para>

        <para>
          For virtual hard disks, you can specify one of the following
          target types: VDI, VHD, or VMDK.
        </para>
      </listitem>

      <listitem>
        <para>
          <emphasis role="bold">Move</emphasis> an image to another
          location.
        </para>

        <para>
          A file dialog prompts you for the new image file location.
        </para>

        <para>
          When you use the Virtual Media Manager to move a disk image,
          &product-name; updates all related configuration files
          automatically.
        </para>

        <note>
          <para>
            Always use the Virtual Media Manager or the
            <command>VBoxManage modifymedium</command> command to move a
            disk image.
          </para>

          <para>
            If you use a file management feature of the host OS to move
            a disk image to a new location, run the <command>VBoxManage
            modifymedium</command> <option>--setlocation</option>
            command to configure the new path of the disk image on the
            host file system. This command updates the &product-name;
            configuration automatically.
          </para>
        </note>
      </listitem>

      <listitem>
        <para>
          <emphasis role="bold">Remove</emphasis> an image from the
          known media. You can optionally delete the image file when
          removing the image.
        </para>
      </listitem>

      <listitem>
        <para>
          <emphasis role="bold">Release</emphasis> an image to detach it
          from a VM. This action only applies if the image is currently
          attached to a VM as a virtual hard disk.
        </para>
      </listitem>

      <listitem>
        <para>
          <emphasis role="bold">Search</emphasis> for an image by name
          or UUID.
        </para>
      </listitem>

      <listitem>
        <para>
          View and edit the <emphasis role="bold">Properties</emphasis>
          of a disk image.
        </para>

        <para>
          Available properties include the following:
        </para>

        <itemizedlist>

          <listitem>
            <para>
              <emphasis role="bold">Type:</emphasis> Specifies the
              snapshot behavior of the disk. See
              <xref linkend="hdimagewrites"/>.
            </para>
          </listitem>

          <listitem>
            <para>
              <emphasis role="bold">Location:</emphasis> Specifies the
              location of the disk image file on the host system. You
              can use a file dialog to browse for the disk image
              location.
            </para>
          </listitem>

          <listitem>
            <para>
              <emphasis role="bold">Description:</emphasis> Specifies a
              short description of the disk image.
            </para>
          </listitem>

          <listitem>
            <para>
              <emphasis role="bold">Size:</emphasis> Specifies the size
              of the disk image. You can use the slider to increase or
              decrease the disk image size.
            </para>
          </listitem>

          <listitem>
            <para>
              <emphasis role="bold">Information:</emphasis> Specifies
              detailed information about the disk image.
            </para>
          </listitem>

        </itemizedlist>
      </listitem>

      <listitem>
        <para>
          <emphasis role="bold">Refresh</emphasis> the property values
          of the selected disk image.
        </para>
      </listitem>

    </itemizedlist>

    <para>
      To perform these actions, highlight the medium in the Virtual
      Media Manager and then do one of the following:
    </para>

    <itemizedlist>

      <listitem>
        <para>
          Click an icon in the Virtual Media Manager task bar.
        </para>
      </listitem>

      <listitem>
        <para>
          Right-click the medium and select an option.
        </para>
      </listitem>

    </itemizedlist>

    <para>
      Use the <emphasis role="bold">Storage</emphasis> page in a VM's
      <emphasis role="bold">Settings</emphasis> dialog to create a new
      disk image. By default, disk images are stored in the VM's folder.
    </para>

    <para>
      You can copy hard disk image files to other host systems and then
      import them in to VMs from the host system. However, some Windows
      guest OSes may require that you configure the new VM in a similar
      way to the old one.
    </para>

    <note>
      <para>
        Do not simply make copies of virtual disk images. If you import
        such a second copy into a VM, &product-name; issues an error
        because &product-name; assigns a universally unique identifier
        (UUID) to each disk image to ensure that it is only used one
        time. See <xref linkend="cloningvdis" />. Also, if you want to
        copy a VM to another system, use the &product-name; import and
        export features. See <xref linkend="ovf" />.
      </para>
    </note>

  </sect1>

  <sect1 id="hdimagewrites">

    <title>Special Image Write Modes</title>

    <para>
      For each virtual disk image supported by &product-name;, you can
      determine separately how it should be affected by write operations
      from a virtual machine and snapshot operations. This applies to
      all of the aforementioned image formats (VDI, VMDK, VHD, or HDD)
      and irrespective of whether an image is fixed-size or dynamically
      allocated.
    </para>

    <para>
      By default, images are in <emphasis>normal</emphasis> mode. To
      mark an existing image with one of the non-standard modes listed
      below, use <command>VBoxManage modifymedium</command>. See
      <xref linkend="vboxmanage-modifymedium" />. Alternatively, use
      <command>VBoxManage storageattach</command> to attach the image to
      a VM and specify the <option>--mtype</option> argument. See
      <xref linkend="vboxmanage-storageattach" />.
    </para>

    <para>
      The available virtual disk image modes are as follows:
    </para>

    <itemizedlist>

      <listitem>
        <para>
          <emphasis role="bold">Normal images</emphasis> have no
          restrictions on how guests can read from and write to the
          disk. This is the default image mode.
        </para>

        <para>
          When you take a snapshot of your virtual machine as described
          in <xref linkend="snapshots" />, the state of a normal hard
          disk is recorded together with the snapshot, and when
          reverting to the snapshot, its state will be fully reset.
        </para>

        <para>
          The image file itself is not reset. Instead, when a snapshot
          is taken, &product-name; <emphasis>freezes</emphasis> the
          image file and no longer writes to it. For the write
          operations from the VM, a second,
          <emphasis>differencing</emphasis> image file is created which
          receives only the changes to the original image. See
          <xref linkend="diffimages"/>.
        </para>

        <para>
          While you can attach the same normal image to more than one
          virtual machine, only one of these virtual machines attached
          to the same image file can be executed simultaneously, as
          otherwise there would be conflicts if several machines write
          to the same image file.
        </para>
      </listitem>

      <listitem>
        <para>
          <emphasis role="bold">Write-through hard disks</emphasis> are
          completely unaffected by snapshots. Their state is
          <emphasis>not</emphasis> saved when a snapshot is taken, and
          not restored when a snapshot is restored.
        </para>
      </listitem>

      <listitem>
        <para>
          <emphasis role="bold">Shareable hard disks</emphasis> are a
          variant of write-through hard disks. In principle they behave
          exactly the same. Their state is <emphasis>not</emphasis>
          saved when a snapshot is taken, and not restored when a
          snapshot is restored. The difference only shows if you attach
          such disks to several VMs. Shareable disks may be attached to
          several VMs which may run concurrently. This makes them
          suitable for use by cluster filesystems between VMs and
          similar applications which are explicitly prepared to access a
          disk concurrently. Only fixed size images can be used in this
          way, and dynamically allocated images are rejected.
        </para>

        <warning>
          <para>
            This is an expert feature, and misuse can lead to data loss,
            as regular filesystems are not prepared to handle
            simultaneous changes by several parties.
          </para>
        </warning>
      </listitem>

      <listitem>
        <para>
          <emphasis role="bold">Immutable images</emphasis> only
          remember write accesses temporarily while the virtual machine
          is running. All changes are lost when the virtual machine is
          powered on the next time. As a result, as opposed to Normal
          images, the same immutable image can be used with several
          virtual machines without restrictions.
        </para>

        <para>
          Creating an immutable image makes little sense since it would
          be initially empty and lose its contents with every machine
          restart. You would have a disk that is always unformatted when
          the machine starts up. Instead, you can first create a normal
          image and then later mark it as immutable when you decide that
          the contents are useful.
        </para>

        <para>
          If you take a snapshot of a machine with immutable images,
          then on every machine power-up, those images are reset to the
          state of the last (current) snapshot, instead of the state of
          the original immutable image.
        </para>

        <note>
          <para>
            As a special exception, immutable images are
            <emphasis>not</emphasis> reset if they are attached to a
            machine in a saved state or whose last snapshot was taken
            while the machine was running. This is called an
            <emphasis>online snapshot</emphasis>. As a result, if the
            machine's current snapshot is an online snapshot, its
            immutable images behave exactly like the a normal image. To
            reenable the automatic resetting of such images, delete the
            current snapshot of the machine.
          </para>
        </note>

        <para>
          &product-name; never writes to an immutable image directly at
          all. All write operations from the machine are directed to a
          differencing image. The next time the VM is powered on, the
          differencing image is reset so that every time the VM starts,
          its immutable images have exactly the same content.
        </para>

        <para>
          The differencing image is only reset when the machine is
          powered on from within &product-name;, not when you reboot by
          requesting a reboot from within the machine. This is also why
          immutable images behave as described above when snapshots are
          also present, which use differencing images as well.
        </para>

        <para>
          If the automatic discarding of the differencing image on VM
          startup does not fit your needs, you can turn it off using the
          <option>autoreset</option> parameter of <command>VBoxManage
          modifymedium</command>. See
          <xref linkend="vboxmanage-modifymedium"/>.
        </para>
      </listitem>

      <listitem>
        <para>
          <emphasis role="bold">Multiattach mode images</emphasis> can
          be attached to more than one virtual machine at the same time,
          even if these machines are running simultaneously. For each
          virtual machine to which such an image is attached, a
          differencing image is created. As a result, data that is
          written to such a virtual disk by one machine is not seen by
          the other machines to which the image is attached. Each
          machine creates its own write history of the multiattach
          image.
        </para>

        <para>
          Technically, a multiattach image behaves identically to an
          immutable image except the differencing image is not reset
          every time the machine starts.
        </para>

        <para>
          This mode is useful for sharing files which are almost never
          written, for instance picture galleries, where every guest
          changes only a small amount of data and the majority of the
          disk content remains unchanged. The modified blocks are stored
          in differencing images which remain relatively small and the
          shared content is stored only once at the host.
        </para>
      </listitem>

      <listitem>
        <para>
          <emphasis role="bold">Read-only images</emphasis> are used
          automatically for CD/DVD images, since CDs/DVDs can never be
          written to.
        </para>
      </listitem>

    </itemizedlist>

    <para>
      The following scenario illustrates the differences between the
      various image modes, with respect to snapshots.
    </para>

    <para>
      Assume you have installed your guest OS in your VM, and you have
      taken a snapshot. Later, your VM is infected with a virus and you
      would like to go back to the snapshot. With a normal hard disk
      image, you simply restore the snapshot, and the earlier state of
      your hard disk image will be restored as well and your virus
      infection will be undone. With an immutable hard disk, all it
      takes is to shut down and power on your VM, and the virus
      infection will be discarded. With a write-through image however,
      you cannot easily undo the virus infection by means of
      virtualization, but will have to disinfect your virtual machine
      like a real computer.
    </para>

    <para>
      You might find write-through images useful if you want to preserve
      critical data irrespective of snapshots. As you can attach more
      than one image to a VM, you may want to have one immutable image
      for the OS and one write-through image for your data files.
    </para>

  </sect1>

  <sect1 id="diffimages">

    <title>Differencing Images</title>

    <para>
      The previous section mentioned differencing images and how they
      are used with snapshots, immutable images, and multiple disk
      attachments. This section describes in more detail how
      differencing images work.
    </para>

    <para>
      A differencing image is a special disk image that only holds the
      differences to another image. A differencing image by itself is
      useless, it must always refer to another image. The differencing
      image is then typically referred to as a
      <emphasis>child</emphasis>, which holds the differences to its
      <emphasis>parent</emphasis>.
    </para>

    <para>
      When a differencing image is active, it receives all write
      operations from the virtual machine instead of its parent. The
      differencing image only contains the sectors of the virtual hard
      disk that have changed since the differencing image was created.
      When the machine reads a sector from such a virtual hard disk, it
      looks into the differencing image first. If the sector is present,
      it is returned from there. If not, &product-name; looks into the
      parent. In other words, the parent becomes
      <emphasis>read-only</emphasis>. It is never written to again, but
      it is read from if a sector has not changed.
    </para>

    <para>
      Differencing images can be chained. If another differencing image
      is created for a virtual disk that already has a differencing
      image, then it becomes a <emphasis>grandchild</emphasis> of the
      original parent. The first differencing image then becomes
      read-only as well, and write operations only go to the
      second-level differencing image. When reading from the virtual
      disk, &product-name; needs to look into the second differencing
      image first, then into the first if the sector was not found, and
      then into the original image.
    </para>

    <para>
      There can be an unlimited number of differencing images, and each
      image can have more than one child. As a result, the differencing
      images can form a complex tree with parents, siblings, and
      children, depending on how complex your machine configuration is.
      Write operations always go to the one <emphasis>active</emphasis>
      differencing image that is attached to the machine, and for read
      operations, &product-name; may need to look up all the parents in
      the chain until the sector in question is found. You can view such
      a tree in the Virtual Media Manager.
    </para>

    <figure id="fig-diff-images">
      <title>Differencing Images, Shown in Virtual Media Manager</title>
      <mediaobject>
        <imageobject>
          <imagedata align="center" fileref="images/virtual-disk-manager2.png"
                     width="12cm" />
        </imageobject>
      </mediaobject>
    </figure>

    <para>
      In all of these situations, from the point of view of the virtual
      machine, the virtual hard disk behaves like any other disk. While
      the virtual machine is running, there is a slight run-time I/O
      overhead because &product-name; might need to look up sectors
      several times. This is not noticeable however since the tables
      with sector information are always kept in memory and can be
      looked up quickly.
    </para>

    <para>
      Differencing images are used in the following situations:
    </para>

    <itemizedlist>

      <listitem>
        <para>
          <emphasis role="bold">Snapshots.</emphasis> When you create a
          snapshot, as explained in the previous section, &product-name;
          <emphasis>freezes</emphasis> the images attached to the
          virtual machine and creates differencing images for each image
          that is not in <emphasis>write-through</emphasis> mode. From
          the point of view of the virtual machine, the virtual disks
          continue to operate before, but all write operations go into
          the differencing images. Each time you create another
          snapshot, for each hard disk attachment, another differencing
          image is created and attached, forming a chain or tree.
        </para>

        <para>
          In the above screenshot, you see that the original disk image
          is now attached to a snapshot, representing the state of the
          disk when the snapshot was taken.
        </para>

        <para>
          If you <emphasis>restore</emphasis> a snapshot, and want to go
          back to the exact machine state that was stored in the
          snapshot, the following happens:
        </para>

        <itemizedlist>

          <listitem>
            <para>
              &product-name; copies the virtual machine settings that
              were copied into the snapshot back to the virtual machine.
              As a result, if you have made changes to the machine
              configuration since taking the snapshot, they are undone.
            </para>
          </listitem>

          <listitem>
            <para>
              If the snapshot was taken while the machine was running,
              it contains a saved machine state, and that state is
              restored as well. After restoring the snapshot, the
              machine will then be in Saved state and resume execution
              from there when it is next started. Otherwise the machine
              will be in Powered Off state and do a full boot.
            </para>
          </listitem>

          <listitem>
            <para>
              For each disk image attached to the machine, the
              differencing image holding all the write operations since
              the current snapshot was taken is thrown away, and the
              original parent image is made active again. If you
              restored the root snapshot, then this will be the root
              disk image for each attachment. Otherwise, some other
              differencing image descended from it. This effectively
              restores the old machine state.
            </para>
          </listitem>

        </itemizedlist>

        <para>
          If you later <emphasis>delete</emphasis> a snapshot in order
          to free disk space, for each disk attachment, one of the
          differencing images becomes obsolete. In this case, the
          differencing image of the disk attachment cannot simply be
          deleted. Instead, &product-name; needs to look at each sector
          of the differencing image and needs to copy it back into its
          parent. This is called "merging" images and can be a
          potentially lengthy process, depending on how large the
          differencing image is. It can also temporarily need a
          considerable amount of extra disk space, before the
          differencing image obsoleted by the merge operation is
          deleted.
        </para>
      </listitem>

      <listitem>
        <para>
          <emphasis role="bold">Immutable images.</emphasis> When an
          image is switched to immutable mode, a differencing image is
          created as well. As with snapshots, the parent image then
          becomes read-only, and the differencing image receives all the
          write operations. Every time the virtual machine is started,
          all the immutable images which are attached to it have their
          respective differencing image thrown away, effectively
          resetting the virtual machine's virtual disk with every
          restart.
        </para>
      </listitem>

    </itemizedlist>

  </sect1>

  <sect1 id="cloningvdis">

    <title>Cloning Disk Images</title>

    <para>
      You can duplicate hard disk image files on the same host to
      quickly produce a second virtual machine with the same OS setup.
      However, you should <emphasis>only</emphasis> make copies of
      virtual disk images using the utility supplied with
      &product-name;. See <xref linkend="vboxmanage-clonemedium" />.
      This is because &product-name; assigns a UUID to each disk image,
      which is also stored inside the image, and &product-name; will
      refuse to work with two images that use the same number. If you do
      accidentally try to reimport a disk image which you copied
      normally, you can make a second copy using the <command>VBoxManage
      clonevm</command> command and import that instead.
    </para>

    <para>
      Note that Linux distributions identify the boot hard disk from the
      ID of the drive. The ID &product-name; reports for a drive is
      determined from the UUID of the virtual disk image. So if you
      clone a disk image and try to boot the copied image the guest
      might not be able to determine its own boot disk as the UUID
      changed. In this case you have to adapt the disk ID in your boot
      loader script, for example
      <filename>/boot/grub/menu.lst</filename>. The disk ID looks like
      the following:
    </para>

<screen>scsi-SATA_VBOX_HARDDISK_VB5cfdb1e2-c251e503</screen>

    <para>
      The ID for the copied image can be determined as follows:
    </para>

<screen>hdparm -i /dev/sda</screen>

  </sect1>

  <sect1 id="iocaching">

    <title>Host Input/Output Caching</title>

    <para>
      &product-name; can optionally disable the I/O caching that the
      host OS would otherwise perform on disk image files.
    </para>

    <para>
      Traditionally, &product-name; has opened disk image files as
      normal files, which results in them being cached by the host OS
      like any other file. The main advantage of this is speed: when the
      guest OS writes to disk and the host OS cache uses delayed
      writing, the write operation can be reported as completed to the
      guest OS quickly while the host OS can perform the operation
      asynchronously. Also, when you start a VM a second time and have
      enough memory available for the OS to use for caching, large parts
      of the virtual disk may be in system memory, and the VM can access
      the data much faster.
    </para>

    <para>
      Note that this applies only to image files. Buffering does not
      occur for virtual disks residing on remote iSCSI storage, which is
      the more common scenario in enterprise-class setups. See
      <xref linkend="storage-iscsi" />.
    </para>

    <para>
      While buffering is a useful default setting for virtualizing a few
      machines on a desktop computer, there are some disadvantages to
      this approach:
    </para>

    <itemizedlist>

      <listitem>
        <para>
          Delayed writing through the host OS cache is less secure. When
          the guest OS writes data, it considers the data written even
          though it has not yet arrived on a physical disk. If for some
          reason the write does not happen, such as power failure or
          host crash, the likelihood of data loss increases.
        </para>
      </listitem>

      <listitem>
        <para>
          Disk image files tend to be very large. Caching them can
          therefore quickly use up the entire host OS cache. Depending
          on the efficiency of the host OS caching, this may slow down
          the host immensely, especially if several VMs run at the same
          time. For example, on Linux hosts, host caching may result in
          Linux delaying all writes until the host cache is nearly full
          and then writing out all these changes at once, possibly
          stalling VM execution for minutes. This can result in I/O
          errors in the guest as I/O requests time out there.
        </para>
      </listitem>

      <listitem>
        <para>
          Physical memory is often wasted as guest OSes typically have
          their own I/O caches, which may result in the data being
          cached twice, in both the guest and the host caches, for
          little effect.
        </para>
      </listitem>

    </itemizedlist>

    <para>
      If you decide to disable host I/O caching for the above reasons,
      &product-name; uses its own small cache to buffer writes, but no
      read caching since this is typically already performed by the
      guest OS. In addition, &product-name; fully supports asynchronous
      I/O for its virtual SATA, SCSI, and SAS controllers through
      multiple I/O threads.
    </para>

    <para>
      Since asynchronous I/O is not supported by IDE controllers, for
      performance reasons, you may want to leave host caching enabled
      for your VM's virtual IDE controllers.
    </para>

    <para>
      For this reason, &product-name; enables you to configure whether
      the host I/O cache is used for each I/O controller separately.
      Either select the <emphasis role="bold">Use Host I/O
      Cache</emphasis> check box in the
      <emphasis role="bold">Storage</emphasis> settings for a given
      virtual storage controller, or use the following
      <command>VBoxManage</command> command to disable the host I/O
      cache for a virtual storage controller:
    </para>

<screen>VBoxManage storagectl "VM name" --name &lt;controllername&gt; --hostiocache off</screen>

    <para>
      See <xref linkend="vboxmanage-storagectl" />.
    </para>

    <para>
      For the above reasons, &product-name; uses SATA controllers by
      default for new virtual machines.
    </para>

  </sect1>

  <sect1 id="storage-bandwidth-limit">

    <title>Limiting Bandwidth for Disk Images</title>

    <para>
      &product-name; supports limiting of the maximum bandwidth used for
      asynchronous I/O. Additionally it supports sharing limits through
      bandwidth groups for several images. It is possible to have more
      than one such limit.
    </para>

    <para>
      Limits are configured using <command>VBoxManage</command>. The
      example below creates a bandwidth group named Limit, sets the
      limit to 20 MB per second, and assigns the group to the attached
      disks of the VM:
    </para>

<screen>VBoxManage bandwidthctl "VM name" add Limit --type disk --limit 20M
VBoxManage storageattach "VM name" --storagectl "SATA" --port 0 --device 0 --type hdd
                                   --medium disk1.vdi --bandwidthgroup Limit
VBoxManage storageattach "VM name" --storagectl "SATA" --port 1 --device 0 --type hdd
                                   --medium disk2.vdi --bandwidthgroup Limit</screen>

    <para>
      All disks in a group share the bandwidth limit, meaning that in
      the example above the bandwidth of both images combined can never
      exceed 20 MBps. However, if one disk does not require bandwidth
      the other can use the remaining bandwidth of its group.
    </para>

    <para>
      The limits for each group can be changed while the VM is running,
      with changes being picked up immediately. The example below
      changes the limit for the group created in the example above to 10
      MBps:
    </para>

<screen>VBoxManage bandwidthctl "VM name" set Limit --limit 10M</screen>

  </sect1>

  <sect1 id="storage-cds">

    <title>CD/DVD Support</title>

    <para>
      Virtual CD/DVD drives by default support only reading. The medium
      configuration is changeable at runtime. You can select between the
      following options to provide the medium data:
    </para>

    <itemizedlist>

      <listitem>
        <para>
          <emphasis role="bold">Host Drive</emphasis> defines that the
          guest can read from the medium in the host drive.
        </para>
      </listitem>

      <listitem>
        <para>
          <emphasis role="bold">Image file</emphasis> gives the guest
          read-only access to the data in the image. This is typically
          an ISO file.
        </para>
      </listitem>

      <listitem>
        <para>
          <emphasis role="bold">Empty</emphasis> means a drive without
          an inserted medium.
        </para>
      </listitem>

    </itemizedlist>

    <para>
      Changing between the above, or changing a medium in the host drive
      that is accessed by a machine, or changing an image file will
      signal a medium change to the guest OS. The guest OS can then
      react to the change, for example by starting an installation
      program.
    </para>

    <para>
      Medium changes can be prevented by the guest, and &product-name;
      reflects that by locking the host drive if appropriate. You can
      force a medium removal in such situations by using the VirtualBox
      Manager or the <command>VBoxManage</command> command line tool.
      Effectively this is the equivalent of the emergency eject which
      many CD/DVD drives provide, with all associated side effects. The
      guest OS can issue error messages, just like on real hardware, and
      guest applications may misbehave. Use this with caution.
    </para>

    <note>
      <para>
        The identification string of the drive provided to the guest,
        displayed by configuration tools such as the Windows Device
        Manager, is always VBOX CD-ROM, irrespective of the current
        configuration of the virtual drive. This is to prevent hardware
        detection from being triggered in the guest OS every time the
        configuration is changed.
      </para>
    </note>

    <para>
      The standard CD/DVD emulation enables reading of standard data CD
      and DVD formats only. As an experimental feature, for additional
      capabilities, it is possible to give the guest direct access to
      the CD/DVD host drive by enabling <emphasis>passthrough</emphasis>
      mode. Depending on the host hardware, this may potentially enable
      the following things to work:
    </para>

    <itemizedlist>

      <listitem>
        <para>
          CD/DVD writing from within the guest, if the host DVD drive is
          a CD/DVD writer
        </para>
      </listitem>

      <listitem>
        <para>
          Playing audio CDs
        </para>
      </listitem>

      <listitem>
        <para>
          Playing encrypted DVDs
        </para>
      </listitem>

    </itemizedlist>

    <para>
      To enable host drive passthrough you can use the
      <option>--passthrough</option> option of the <command>VBoxManage
      storageattach</command> command. See
      <xref linkend="vboxmanage-storageattach" />.
    </para>

    <para>
      Even if passthrough is enabled, unsafe commands, such as updating
      the drive firmware, will be blocked. Video CD formats are never
      supported, not even in passthrough mode, and cannot be played from
      a virtual machine.
    </para>

    <para>
      On Oracle Solaris hosts, passthrough requires running
      &product-name; with real root permissions due to security measures
      enforced by the host.
    </para>

  </sect1>

  <sect1 id="storage-iscsi">

    <title>iSCSI Servers</title>

    <para>
      iSCSI stands for <emphasis>Internet SCSI</emphasis> and is a
      standard that supports use of the SCSI protocol over Internet
      (TCP/IP) connections. Especially with the advent of Gigabit
      Ethernet, it has become affordable to attach iSCSI storage servers
      simply as remote hard disks to a computer network. In iSCSI
      terminology, the server providing storage resources is called an
      <emphasis>iSCSI target</emphasis>, while the client connecting to
      the server and accessing its resources is called an
      <emphasis>iSCSI initiator</emphasis>.
    </para>

    <para>
      &product-name; can transparently present iSCSI remote storage to a
      virtual machine as a virtual hard disk. The guest OS will not see
      any difference between a virtual disk image (VDI file) and an
      iSCSI target. To achieve this, &product-name; has an integrated
      iSCSI initiator.
    </para>

    <para>
      &product-name;'s iSCSI support has been developed according to the
      iSCSI standard and should work with all standard-conforming iSCSI
      targets. To use an iSCSI target with &product-name;, you must use
      the command line. See <xref linkend="vboxmanage-storageattach" />.
    </para>

  </sect1>

  <sect1 id="vboximg-mount">

    <title>vboximg-mount: A Utility for FUSE Mounting a Virtual Disk Image</title>

    <para>
      <command>vboximg-mount</command> is a command line utility for Mac
      OS and Linux hosts that provides raw access to an &product-name;
      virtual disk image on the host system. Use this utility to mount,
      view, and optionally modify the disk image contents.
    </para>

    <para>
      The utility is based on Filesystem in Userspace (FUSE) technology
      and uses the VirtualBox runtime engine. Ensure that &product-name;
      is running on the host system.
    </para>

    <note>
      <para>
        When using <command>vboximg-mount</command>, ensure that the
        following conditions apply:
      </para>

      <itemizedlist>

        <listitem>
          <para>
            The disk image is not being used by any other systems, such
            as by guest VMs.
          </para>
        </listitem>

        <listitem>
          <para>
            No VMs are running on the host system.
          </para>
        </listitem>

      </itemizedlist>
    </note>

    <para>
      Raw access using FUSE is preferred over direct loopback mounting
      of virtual disk images, because it is snapshot aware. It can
      selectively merge disk differencing images in an exposed virtual
      hard disk, providing historical or up-to-date representations of
      the virtual disk contents.
    </para>

    <para>
      <command>vboximg-mount</command> enables you to view information
      about registered VMs, their attached disk media, and any
      snapshots. Also, you can view partition information for a disk
      image.
    </para>

    <para>
      The <command>vboximg-mount </command>command includes experimental
      read-only access to file systems inside a VM disk image. This
      feature enables you to extract some files from the disk image
      without starting the VM and without requiring third-party file
      system drivers on the host system. FAT, NTFS, ext2, ext3, and ext4
      file systems are supported.
    </para>

    <para>
      Use the <option>--help</option> option to view information about
      the <command>vboximg-mount</command> command usage. The complete
      command reference is described in
      <xref linkend="man_vboximg-mount" />.
    </para>

    <para>
      When <command>vboximg-mount</command> mounts an &product-name;
      disk image, it creates a one level deep file system at a mount
      point that you specify. The file system includes a device node
      that represents the synthesized disk image as a readable or
      readable-writeable bytestream. This bytestream can be mounted
      either by using the host OS or by using other FUSE-based file
      systems.
    </para>

    <sect2 id="vboximg-mount-display">

      <title>Viewing Detailed Information About a Virtual Disk Image</title>

      <para>
        The following examples show how to use the
        <command>vboximg-mount</command> command to view information
        about virtual disk images.
      </para>

      <para>
        The following command outputs detailed information about all
        registered VMs and associated snapshots:
      </para>

<screen>$ vboximg-mount --list --verbose

    ------------------------------------------------------
    VM Name:   "macOS High Sierra 10.13"
    UUID:      3887d96d-831c-4187-a55a-567c504ff0e1
    Location:  /Volumes/work/vm_guests/macOS High Sierra 10.13/macOS High Sierra 10.13.vbox
       -----------------------
       HDD base:   "macOS High Sierra 10.13.vdi"
       UUID:       f9ea7173-6869-4aa9-b487-68023a655980
       Location:   /Volumes/work/vm_guests/macOS High Sierra 10.13/macOS High Sierra 10.13.vdi

         Diff 1:
              UUID:       98c2bac9-cf37-443d-a935-4e879b70166d
              Location:   /Volumes/work/vm_guests/macOS High Sierra 10.13/
              Snapshots/{98c2bac9-cf37-443d-a935-4e879b70166d}.vdi
         Diff 2:
              UUID:       f401f381-7377-40b3-948e-3c61241b1a42
              Location:   /Volumes/work/vm_guests/macOS High Sierra 10.13/
              Snapshots/{f401f381-7377-40b3-948e-3c61241b1a42}.vdi
       -----------------------
       HDD base:   "simple_fixed_disk.vdi"
       UUID:       ffba4d7e-1277-489d-8173-22ca7660773d
       Location:   /Volumes/work/vm_guests/macOS High Sierra 10.13/simple_fixed_disk.vdi

         Diff 1:
              UUID:       aecab681-0d2d-468b-8682-93f79dc97a48
              Location:   /Volumes/work/vm_guests/macOS High Sierra 10.13/
              Snapshots/{aecab681-0d2d-468b-8682-93f79dc97a48}.vdi
         Diff 2:
              UUID:       70d6b34d-8422-47fa-8521-3b6929a1971c
              Location:   /Volumes/work/vm_guests/macOS High Sierra 10.13/
              Snapshots/{70d6b34d-8422-47fa-8521-3b6929a1971c}.vdi
      ------------------------------------------------------
      VM Name:   "debian"
      UUID:      5365ab5f-470d-44c0-9863-dad532ee5905
      Location:  /Volumes/work/vm_guests/debian/debian.vbox
         -----------------------
         HDD base:   "debian.vdi"
         UUID:       96d2e92e-0d4e-46ab-a0f1-008fdbf997e7
         Location:   /Volumes/work/vm_guests/debian/ol7.vdi

            Diff 1:
                UUID:       f9cc866a-9166-42e9-a503-bbfe9b7312e8
                Location:   /Volumes/work/vm_guests/debian/Snapshots/
                {f9cc866a-9166-42e9-a503-bbfe9b7312e8}.vdi</screen>

      <para>
        The following command outputs partition information about the
        specified disk image:
      </para>

<screen>$ vboximg-mount --image=f9ea7173-6869-4aa9-b487-68023a655980 --list

    Virtual disk image:

       Path: /Volumes/work/vm_guests/macOS High Sierra 10.13/macOS High Sierra 10.13.vdi
       UUID: f9ea7173-6869-4aa9-b487-68023a655980

      #     Start  Sectors     Size       Offset  Type
      1        40  409599    199.9M        20480  EFI System
      2    409640  67453071   32.1G    209735680  Hierarchical File System Plus (HFS+)
      3  67862712  1269535   107.8M  34745708544  Apple Boot (Recovery HD)</screen>

    </sect2>

    <sect2 id="vboximg-mount-steps">

      <title>Mounting a Virtual Disk Image</title>

      <para>
        The following steps show how to use the
        <command>vboximg-mount</command> command to mount a partition of
        a virtual disk image on the host OS.
      </para>

      <orderedlist>

        <listitem>
          <para>
            Create a mount point on the host OS. For example:
          </para>

<screen>$ mkdir macos_sysdisk</screen>
        </listitem>

        <listitem>
          <para>
            Show partition information about the virtual disk image.
          </para>

<screen>$ vboximg-mount --image=<replaceable>uuid</replaceable> --list</screen>

          <para>
            where <replaceable>uuid</replaceable> is the UUID of the
            disk image.
          </para>
        </listitem>

        <listitem>
          <para>
            Use <command>vboximg-mount</command> to perform a FUSE mount
            of a partition on the virtual disk image. For example:
          </para>

<screen>$ vboximg-mount --image=<replaceable>uuid</replaceable> -p 2 macos_sysdisk</screen>

          <para>
            where <replaceable>uuid</replaceable> is the UUID for the
            disk image.
          </para>

          <para>
            In this example, partition 2 is mounted on the
            <filename>macos_sysdisk</filename> mount point. The mount
            includes all snapshots for the disk image.
          </para>
        </listitem>

        <listitem>
          <para>
            Use the host OS to mount the <literal>vhdd</literal> device
            node. The FUSE-mounted device node represents the virtual
            disk image.
          </para>

<screen>$ ls macos_sysdisk
   macOS High Sierra 10.13.vdi  vhdd
$ sudo mount macos_sysdisk/vhdd /mnt</screen>
        </listitem>

      </orderedlist>

    </sect2>

  </sect1>

</chapter>