source: vbox/trunk/doc/manual/en_US/user_Storage.xml@ 74981

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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, VirtualBox 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>
        VirtualBox 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 VirtualBox. 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 VirtualBox presents to a virtual machine.
    This is explained in the next section.
  </para>

  <sect1 id="harddiskcontrollers">

    <title>Hard Disk Controllers: IDE, SATA (AHCI), SCSI, SAS, USB MSD, NVMe</title>

    <para>
      In a real PC, hard disks and CD/DVD drives are connected to a
      device called hard disk controller which drives hard disk
      operation and data transfers. VirtualBox can emulate the five most
      common types of hard disk controllers typically found in today's
      PCs: IDE, SATA (AHCI), SCSI, SAS, USB-based, and NVMe mass storage
      devices.

      <footnote>

        <para>
          SATA support was added with VirtualBox 1.6; experimental SCSI
          support was added with 2.1 and fully implemented with 2.2.
          Generally, storage attachments were made much more flexible
          with VirtualBox 3.1. Support for the LSI Logic SAS controller
          was added with VirtualBox 3.2. USB mass storage devices are
          supported since VirtualBox 5.0. NVMe controller support was
          added with VirtualBox 5.1.
        </para>

      </footnote>
    </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
          to a controller, which have traditionally been called
          <emphasis>master</emphasis> and <emphasis>slave</emphasis>.
          Typical PCs had two connectors for such cables. As a result,
          support for up to four IDE devices was most common.
        </para>

        <para>
          In VirtualBox, 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 four, the secondary master, is preconfigured to be the
          machine's virtual CD/DVD drive, but this can be changed.

          <footnote>

            <para>
              The assignment of the machine's CD/DVD drive to the
              secondary master was fixed before VirtualBox 3.1. It is
              now changeable, and the drive can be at other slots of the
              IDE controller, and there can be more than one such drive.
            </para>

          </footnote>

          )
        </para>

        <para>
          Even if your guest operating system 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 VirtualBox 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 operating system 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 New
          Virtual Machine wizard of the graphical user interface, you
          will typically see one IDE controller in the machine's Storage
          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 newer
          standard introduced in 2003. 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, VirtualBox's virtual SATA
          controller operates faster and also consumes fewer CPU
          resources than the virtual IDE controller. Also, this allows
          you to connect up to 30 virtual hard disks to one machine
          instead of just three, when compared to the VirtualBox IDE
          controller with a DVD drive attached.
        </para>

        <para>
          For this reason, starting with version 3.2 and depending on
          the selected guest operating system, VirtualBox 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 operating systems that do not have device support
            for AHCI. In particular, <emphasis>there is no support for
            AHCI in Windows before Windows Vista</emphasis>, so Windows
            XP (even SP3) 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 settings dialog.

            <footnote>

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

            </footnote>
          </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 VirtualBox, or because SATA is not
          supported by default by the selected guest operating system,
          do the following. Go to the Storage page of the machine's
          settings 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 was standardized as early as 1986 as a generic
          interface for data transfer between all kinds of devices,
          including storage devices. Today SCSI is still used for
          connecting 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, VirtualBox optionally supports LSI Logic and
          BusLogic SCSI controllers, to each of which up to 15 virtual
          hard disks can be attached.
        </para>

        <para>
          To enable a SCSI controller, on the Storage page of a virtual
          machine's settings 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 operating systems 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, however, with physical devices, serial cables
          are used instead of parallel ones, which simplifies physical
          device connections. In some ways, therefore, SAS is to SCSI
          what SATA is to IDE: it allows for more reliable and faster
          connections.
        </para>

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

        <warning>
          <para>
            As with SATA, the SAS controller will only be seen by
            operating systems 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 operating systems support these devices for a long
          time and ship generic drivers making third-party drivers
          superfluous. In particular, legacy operating systems without
          support for SATA controllers may benefit from USB mass storage
          devices.
        </para>

        <para>
          The virtual USB storage controller offered by VirtualBox 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 via USB is 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 very recent standard which emerged in
          2011 connecting non volatile memory (NVM) directly over PCI
          express to lift the bandwidth limitation of the previously
          used SATA protocol for SSDs. Unlike other standards the
          command set is very simple to achieve maximum throughput and
          is not compatible with ATA or SCSI. Operating systems 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.

          <footnote>

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

          </footnote>
        </para>

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

    </itemizedlist>

    <para>
      In summary, VirtualBox 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 operating system.
        </para>
      </listitem>

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

      <listitem>
        <para>
          Eight slots attached to the SAS controller, if enabled and
          supported by the guest operating system.
        </para>
      </listitem>

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

      <listitem>
        <para>
          Up to 255 slots attached to the NVMe controller, if enabled
          and supported by the guest operating system.
        </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 VirtualBox 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
      operating system reads from or writes to a hard disk, VirtualBox
      redirects the request to the image file.
    </para>

    <para>
      Like a physical disk, a virtual disk has a size (capacity), which
      must be specified when the image file is created. As opposed to a
      physical disk however, VirtualBox allows you to expand an image
      file after creation, even if it has data already. See
      <xref
    linkend="vboxmanage-modifyvdi" />.

      <footnote>

        <para>
          Image resizing was added with VirtualBox 4.0.
        </para>

      </footnote>
    </para>

    <para>
      VirtualBox supports the following types of disk image files:
    </para>

    <itemizedlist>

      <listitem>
        <para>
          <emphasis role="bold">VDI.</emphasis> Normally, VirtualBox
          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> VirtualBox also fully
          supports the popular and open VMDK container format that is
          used by many other virtualization products, in particular, by
          VMware.

          <footnote>

            <para>
              Initial support for VMDK was added with VirtualBox 1.4.
              Since version 2.1, VirtualBox supports VMDK fully, meaning
              that you can create snapshots and use all the other
              advanced features described above for VDI images with VMDK
              also.
            </para>

          </footnote>
        </para>
      </listitem>

      <listitem>
        <para>
          <emphasis role="bold">VHD.</emphasis> VirtualBox 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.

          <footnote>

            <para>
              Support was added with VirtualBox 3.1.
            </para>

          </footnote>

          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 VirtualBox 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>
      VirtualBox 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 VirtualBox versions
          older than version 4.0. For details about how media
          registration has changed with version 4.0, see
          <xref
          linkend="vboxconfigdata" />.
        </para>
      </listitem>

    </itemizedlist>

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

    <mediaobject>
      <imageobject>
        <imagedata align="center" fileref="images/virtual-disk-manager.png"
                     width="12cm" />
      </imageobject>
    </mediaobject>

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

    <itemizedlist>

      <listitem>
        <para>
          Hard disk images, either in VirtualBox'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, if any.
    </para>

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

    <itemizedlist>

      <listitem>
        <para>
          <emphasis role="bold">Remove</emphasis> an image from the
          registry. You can optionally delete the image file when doing
          so.
        </para>
      </listitem>

      <listitem>
        <para>
          <emphasis role="bold">Release</emphasis> an image. Detach it
          from a virtual machine, if it is currently attached to one as
          a virtual hard disk.
        </para>
      </listitem>

      <listitem>
        <para>
          <emphasis role="bold">Copy</emphasis> a virtual hard disk, to
          create another one. The target type can be different.
          Available options are: VDI, VHD, or VMDK.
        </para>
      </listitem>

      <listitem>
        <para>
          <emphasis role="bold">Modify</emphasis> the attributes of the
          disk image file. Available options are: Normal, Immutable,
          Writethrough, Shareable, Multi-attach.
        </para>
      </listitem>

      <listitem>
        <para>
          <emphasis role="bold">Refresh</emphasis> the values for the
          displayed attributes of the currently-selected disk image.
        </para>
      </listitem>

    </itemizedlist>

    <para>
      These commands are accessible once a medium has been selected
      either by selecting from the options shown at the top of the
      window, or by right-clicking the medium and selecting from the
      options shown on the drop-down menu.
    </para>

    <para>
      Starting with version 4.0, to create a new disk image, you use the
      Storage page in a virtual machine's settings dialog. This is
      because disk images are now by default stored in each machine's
      own folder.
    </para>

    <para>
      Hard disk image files can be copied to other host systems and
      imported into virtual machines there, although certain guest
      systems, notably Windows 2000 and XP, will require that the new
      virtual machine be set up 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 virtual machine, VirtualBox will
        complain with an error, since VirtualBox assigns a unique
        identifier (UUID) to each disk image to make sure it is only
        used once. See <xref
        linkend="cloningvdis" />. Also, if
        you want to copy a virtual machine to another system, VirtualBox
        has an import/export facility that might be better suited for
        your needs. See <xref linkend="ovf" />.
      </para>
    </note>

  </sect1>

  <sect1 id="hdimagewrites">

    <title>Special Image Write Modes</title>

    <para>
      For each virtual disk image supported by VirtualBox, 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 <computeroutput>VBoxManage modifyhd</computeroutput>.
      See <xref
    linkend="vboxmanage-modifyvdi" />. Alternatively,
      use VBoxManage to attach the image to a VM and use the
      <computeroutput>--mtype</computeroutput> 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, VirtualBox "freezes" 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.

          <footnote>

            <para>
              This restriction is more lenient now than it was before
              VirtualBox 2.2. Previously, each normal disk image could
              only be <emphasis>attached</emphasis> to one single
              machine. Now it can be attached to more than one machine
              so long as only one of these machines is running.
            </para>

          </footnote>
        </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.

          <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>
        </para>
      </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>
          VirtualBox 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.

          <footnote>

            <para>
              This behavior also changed with VirtualBox 2.2.
              Previously, the differencing images were discarded when
              the machine session <emphasis>ended</emphasis>; now they
              are discarded every time the machine is powered on.
            </para>

          </footnote>

          The differencing image is only reset when the machine is
          powered on from within VirtualBox, 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
          <computeroutput>autoreset</computeroutput> parameter of
          <computeroutput>VBoxManage modifyhd</computeroutput>. See
          <xref
        linkend="vboxmanage-modifyvdi"/>.
        </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 operating system 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 operating system 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, VirtualBox 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, VirtualBox 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, VirtualBox 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>

    <mediaobject>
      <imageobject>
        <imagedata align="center" fileref="images/virtual-disk-manager2.png"
                     width="12cm" />
      </imageobject>
    </mediaobject>

    <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 VirtualBox 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, VirtualBox
          "freezes" the images attached to the virtual machine and
          creates differencing images for each image that is not in
          "write-through" 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>

        <orderedlist>

          <listitem>
            <para>
              VirtualBox 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>

        </orderedlist>

        <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, VirtualBox 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 operating
      system setup. However, you should <emphasis>only</emphasis> make
      copies of virtual disk images using the utility supplied with
      VirtualBox. See <xref
    linkend="vboxmanage-clonevdi" />. This
      is because VirtualBox assigns a unique identity number (UUID) to
      each disk image, which is also stored inside the image, and
      VirtualBox 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 VirtualBox's
      utility and import that instead.
    </para>

    <para>
      Note that newer Linux distributions identify the boot hard disk
      from the ID of the drive. The ID VirtualBox 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
      <computeroutput>/boot/grub/menu.lst</computeroutput>. The disk ID
      looks like this:
    </para>

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

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

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

  </sect1>

  <sect1 id="iocaching">

    <title>Host I/O Caching</title>

    <para>
      Starting with version 3.2, VirtualBox can optionally disable the
      I/O caching that the host operating system would otherwise perform
      on disk image files.
    </para>

    <para>
      Traditionally, VirtualBox has opened disk image files as normal
      files, which results in them being cached by the host operating
      system 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 operating systems
          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,
      VirtualBox uses its own small cache to buffer writes, but no read
      caching since this is typically already performed by the guest OS.
      In addition, VirtualBox 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, VirtualBox allows 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 Storage settings for a given virtual storage
      controller, or use the following VBoxManage 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, VirtualBox now uses SATA controllers by
      default for new virtual machines.
    </para>

  </sect1>

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

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

    <para>
      Starting with version 4.0, VirtualBox allows for limiting 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
      <computeroutput>VBoxManage</computeroutput>. 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 operating system. 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 VirtualBox
      reflects that by locking the host drive if appropriate. You can
      force a medium removal in such situations via the VirtualBox GUI
      or the VBoxManage 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 operating system
        every time the configuration is changed.
      </para>
    </note>

    <para>
      The standard CD/DVD emulation allows for reading 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>
      There is a Passthrough check box in the GUI dialog for configuring
      the media attached to a storage controller, or you can use the
      <computeroutput>--passthrough</computeroutput> option with
      <computeroutput>VBoxManage storageattach</computeroutput>. 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 Solaris hosts, passthrough requires running VirtualBox 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 "Internet SCSI" and is a standard that allows for
      using 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>
      VirtualBox can transparently present iSCSI remote storage to a
      virtual machine as a virtual hard disk. The guest operating system
      will not see any difference between a virtual disk image (VDI
      file) and an iSCSI target. To achieve this, VirtualBox has an
      integrated iSCSI initiator.
    </para>

    <para>
      VirtualBox'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 VirtualBox, you must use the
      command line. See <xref linkend="vboxmanage-storageattach" />.
    </para>

  </sect1>

</chapter>