Adding upstream version 7.0.14-dfsg.upstream/7.0.14-dfsg

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 2112 insertions, 0 deletions

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+<?xml version="1.0" encoding="UTF-8"?>
+<!--
+    Copyright (C) 2006-2023 Oracle and/or its affiliates.
+
+    This file is part of VirtualBox base platform packages, as
+    available from https://www.virtualbox.org.
+
+    This program is free software; you can redistribute it and/or
+    modify it under the terms of the GNU General Public License
+    as published by the Free Software Foundation, in version 3 of the
+    License.
+
+    This program is distributed in the hope that it will be useful, but
+    WITHOUT ANY WARRANTY; without even the implied warranty of
+    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+    General Public License for more details.
+
+    You should have received a copy of the GNU General Public License
+    along with this program; if not, see <https://www.gnu.org/licenses>.
+
+    SPDX-License-Identifier: GPL-3.0-only
+-->
+<!DOCTYPE chapter PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
+"http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd"[
+<!ENTITY % all.entities SYSTEM "all-entities.ent">
+%all.entities;
+]>
+<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 in
+          &vbox-mgr;, 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> window.
+          </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> window, 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> window,
+          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="create-vm-wizard" />, 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="virtual-media-manager">
+
+    <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> tool, which
+      you access by clicking <emphasis role="bold">Media</emphasis> on
+      the global <emphasis role="bold">Tools</emphasis> menu in
+      &vbox-mgr;.
+    </para>
+
+    <figure id="fig-virtual-media-manager">
+      <title>The Virtual Media Manager, Showing Hard Disk Images</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 hard disks, the <emphasis role="bold">Create Virtual
+              Hard Disk</emphasis> wizard is shown. See
+              <xref linkend="create-virtual-hard-disk-image"/>.
+            </para>
+          </listitem>
+
+          <listitem>
+            <para>
+              For optical disks, the <emphasis role="bold">VISO
+              Creator</emphasis> tool is shown. See
+              <xref linkend="create-optical-disk-image"/>.
+            </para>
+          </listitem>
+
+          <listitem>
+            <para>
+              For floppy disks, the <emphasis role="bold">Floppy Disk
+              Creator</emphasis> tool is shown. See
+              <xref linkend="create-floppy-disk-image"/>.
+            </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">Clear</emphasis> all inaccessible disk
+          images from the list. The disk images are released from the
+          VMs they are attached to and removed from the known media.
+        </para>
+
+        <note>
+          <para>
+            This option is for optical disks and floppy disks only.
+          </para>
+        </note>
+      </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 toolbar.
+        </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> window 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>
+
+    <sect2 id="create-virtual-hard-disk-image">
+
+      <title>Creating a Virtual Hard Disk Image</title>
+
+      <para>
+        Use the <emphasis role="bold">Create Virtual Hard
+        Disk</emphasis> wizard to create a hard disk image.
+      </para>
+
+      <orderedlist>
+
+        <listitem>
+          <para>
+            Display the <emphasis role="bold">Hard Disks</emphasis> tab
+            in Virtual Media Manager and click
+            <emphasis role="bold">Create</emphasis>.
+          </para>
+
+          <para>
+            The <emphasis role="bold">Create Virtual Hard
+            Disk</emphasis> wizard is shown.
+          </para>
+
+          <figure id="fig-virtual-hard-disk-wizard">
+            <title>Create Virtual Hard Disk Wizard</title>
+      <mediaobject>
+        <imageobject>
+          <imagedata align="center" fileref="images/virtual-hard-disk-wizard.png"
+                     width="12cm" />
+        </imageobject>
+      </mediaobject>
+          </figure>
+        </listitem>
+
+        <listitem>
+          <para>
+            On the <emphasis role="bold">Virtual Hard Disk File
+            Type</emphasis> page, select a file type for the new virtual
+            hard disk image.
+          </para>
+
+          <para>
+            Click <emphasis role="bold">Next</emphasis>.
+          </para>
+        </listitem>
+
+        <listitem>
+          <para>
+            On the <emphasis role="bold">Storage on Physical Hard
+            Disk</emphasis> page, select whether the size of the virtual
+            hard disk file is dynamically allocated or is of fixed size.
+          </para>
+
+          <para>
+            Click <emphasis role="bold">Next</emphasis>.
+          </para>
+        </listitem>
+
+        <listitem>
+          <para>
+            On the <emphasis role="bold">File Location and
+            Size</emphasis> page, configure the location of the virtual
+            hard disk file and use the slider to set the size limit for
+            the virtual hard disk.
+          </para>
+
+          <para>
+            Click <emphasis role="bold">Finish</emphasis> to create the
+            virtual hard disk file.
+          </para>
+
+          <para>
+            The virtual hard disk image is created in the specified
+            location and added to the <emphasis role="bold">Hard
+            Disks</emphasis> tab in Virtual Media Manager.
+          </para>
+        </listitem>
+
+      </orderedlist>
+
+    </sect2>
+
+    <sect2 id="create-optical-disk-image">
+
+      <title>Creating a Virtual Optical Disk Image</title>
+
+      <para>
+        Use the <emphasis role="bold">VISO Creator</emphasis> tool to
+        create a virtual optical disk image. This enables you to create
+        a virtual ISO from selected files on the host.
+      </para>
+
+      <orderedlist>
+
+        <listitem>
+          <para>
+            Display the <emphasis role="bold">Optical Disks</emphasis>
+            tab in Virtual Media Manager and click
+            <emphasis role="bold">Create</emphasis>.
+          </para>
+
+          <para>
+            The <emphasis role="bold">VISO Creator</emphasis> tool is
+            shown.
+          </para>
+        </listitem>
+
+        <listitem>
+          <para>
+            Create the virtual ISO file.
+          </para>
+
+          <orderedlist>
+
+            <listitem>
+              <para>
+                Configure the name of the ISO file.
+              </para>
+
+              <para>
+                Click <emphasis role="bold">Configuration</emphasis> and
+                enter a name in the <emphasis role="bold">Viso
+                Name</emphasis> field.
+              </para>
+            </listitem>
+
+            <listitem>
+              <para>
+                Add files to your virtual ISO.
+              </para>
+
+              <para>
+                In the <emphasis role="bold">Host File System</emphasis>
+                pane, select files to copy from the host system to the
+                virtual ISO.
+              </para>
+
+              <para>
+                Click <emphasis role="bold">Add Items To
+                VISO</emphasis>. The files are displayed in the
+                <emphasis role="bold">VISO Content</emphasis> pane.
+              </para>
+
+              <para>
+                The following file operations are also available:
+              </para>
+
+              <itemizedlist>
+
+                <listitem>
+                  <para>
+                    To create folders on the virtual ISO, click
+                    <emphasis role="bold">Create New
+                    Directory</emphasis>.
+                  </para>
+                </listitem>
+
+                <listitem>
+                  <para>
+                    To remove files from the virtual ISO, select files
+                    in the <emphasis role="bold">VISO Content</emphasis>
+                    pane and click <emphasis role="bold">Remove Items
+                    From VISO</emphasis>.
+                  </para>
+                </listitem>
+
+                <listitem>
+                  <para>
+                    To remove <emphasis>all</emphasis> files from the
+                    virtual ISO, click <emphasis role="bold">Reset the
+                    VISO Content</emphasis>.
+                  </para>
+                </listitem>
+
+              </itemizedlist>
+            </listitem>
+
+          </orderedlist>
+        </listitem>
+
+        <listitem>
+          <para>
+            Create the virtual ISO image.
+          </para>
+
+          <para>
+            Click <emphasis role="bold">Create</emphasis>.
+          </para>
+
+          <para>
+            A virtual ISO file with the specified name and content is
+            created.
+          </para>
+        </listitem>
+
+      </orderedlist>
+
+    </sect2>
+
+    <sect2 id="create-floppy-disk-image">
+
+      <title>Creating a Virtual Floppy Disk Image</title>
+
+      <para>
+        Use the <emphasis role="bold">Floppy Disk Creator</emphasis>
+        tool to create a floppy disk image.
+      </para>
+
+      <orderedlist>
+
+        <listitem>
+          <para>
+            Display the <emphasis role="bold">Floppy Disks</emphasis>
+            tab in Virtual Media Manager and click
+            <emphasis role="bold">Create</emphasis>.
+          </para>
+
+          <para>
+            The <emphasis role="bold">Floppy Disk Creator</emphasis>
+            tool is shown.
+          </para>
+        </listitem>
+
+        <listitem>
+          <para>
+            Configure the following settings:
+          </para>
+
+          <itemizedlist>
+
+            <listitem>
+              <para>
+                <emphasis role="bold">File Path:</emphasis> The name and
+                location of the floppy disk image.
+              </para>
+            </listitem>
+
+            <listitem>
+              <para>
+                <emphasis role="bold">Size:</emphasis> Select from the
+                list of supported floppy disk sizes.
+              </para>
+            </listitem>
+
+            <listitem>
+              <para>
+                <emphasis role="bold">Format Disk as FAT 12:</emphasis>
+                This is the default format used for most floppy disks.
+                For an unformatted disk, do not select this option.
+              </para>
+            </listitem>
+
+          </itemizedlist>
+        </listitem>
+
+        <listitem>
+          <para>
+            Create the floppy disk image file.
+          </para>
+
+          <para>
+            Click <emphasis role="bold">Create</emphasis>.
+          </para>
+
+          <para>
+            The floppy disk image is created in the specified location
+            and added to the <emphasis role="bold">Floppy
+            Disks</emphasis> tab in Virtual Media Manager.
+          </para>
+        </listitem>
+
+      </orderedlist>
+
+    </sect2>
+
+  </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-manager-2.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>