1 | <?xml version="1.0" encoding="UTF-8"?>
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2 | <!DOCTYPE chapter PUBLIC "-//OASIS//DTD DocBook XML V4.4//EN"
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3 | "http://www.oasis-open.org/docbook/xml/4.4/docbookx.dtd">
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4 | <chapter id="storage">
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5 | <title>Virtual storage</title>
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6 |
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7 | <para>As the virtual machine will most probably expect to see a hard disk
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8 | built into its virtual computer, VirtualBox must be able to present "real"
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9 | storage to the guest as a virtual hard disk. There are presently three
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10 | methods in which to achieve this:</para>
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11 |
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12 | <orderedlist>
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13 | <listitem>
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14 | <para>Most commonly, VirtualBox will use large image files on a real
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15 | hard disk and present them to a guest as a virtual hard disk. This is
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16 | described in <xref linkend="vdidetails" />.</para>
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17 | </listitem>
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18 |
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19 | <listitem>
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20 | <para>Alternatively, if you have iSCSI storage servers, you can attach
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21 | such a server to VirtualBox as well; this is described in <xref
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22 | linkend="storage-iscsi" />.</para>
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23 | </listitem>
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24 |
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25 | <listitem>
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26 | <para>Finally, as an advanced feature, you can allow a virtual
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27 | machine to access one of your host disks directly; this advanced feature
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28 | is described in <xref linkend="rawdisk" />.</para>
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29 | </listitem>
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30 | </orderedlist>
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31 |
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32 | <para>Each such virtual storage device (image file, iSCSI target or physical
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33 | hard disk) will need to be connected to the virtual hard disk controller
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34 | that VirtualBox presents to a virtual machine. This is explained in the next
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35 | section.</para>
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36 |
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37 | <sect1 id="harddiskcontrollers">
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38 | <title>Hard disk controllers: IDE, SATA (AHCI), SCSI, SAS</title>
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39 |
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40 | <para>In a real PC, hard disks and CD/DVD drives are connected to a device
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41 | called hard disk controller which drives hard disk operation and data
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42 | transfers. VirtualBox can emulate the four most common types of hard disk
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43 | controllers typically found in today's PCs: IDE, SATA (AHCI), SCSI and
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44 | SAS.<footnote>
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45 | <para>SATA support was added with VirtualBox 1.6; experimental SCSI
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46 | support was added with 2.1 and fully implemented with 2.2. Generally,
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47 | storage attachments were made much more flexible with VirtualBox 3.1;
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48 | see below. Support for the LSI Logic SAS controller was added with
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49 | VirtualBox 3.2.</para>
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50 | </footnote><itemizedlist>
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51 | <listitem>
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52 | <para><emphasis role="bold">IDE (ATA)</emphasis> controllers are a
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53 | backwards compatible yet very advanced extension of the disk
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54 | controller in the IBM PC/AT (1984). Initially, this interface
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55 | worked only with hard disks, but was later extended to also support
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56 | CD-ROM drives and other types of removable media. In physical PCs,
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57 | this standard uses flat ribbon parallel cables with 40 or 80 wires.
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58 | Each such cable can connect two devices to a controller, which have
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59 | traditionally been called "master" and "slave". Typical PCs had
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60 | two connectors for such cables; as a result, support for up to four
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61 | IDE devices was most common.</para>
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62 |
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63 | <para>In VirtualBox, each virtual machine may have one IDE
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64 | contoller enabled, which gives you up to four virtual storage
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65 | devices that you can attach to the machine. (By default, one of
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66 | these four -- the secondary master -- is preconfigured to be the
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67 | machine's virtual CD/DVD drive, but this can be changed.<footnote>
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68 | <para>The assignment of the machine's CD/DVD drive to the
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69 | secondary master was fixed before VirtualBox 3.1; it is now
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70 | changeable, and the drive can be at other slots of the IDE
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71 | controller, and there can be more than one such drive.</para>
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72 | </footnote>)</para>
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73 |
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74 | <para>So even if your guest operating system has no support for SCSI
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75 | or SATA devices, it should always be able to see an IDE controller.
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76 | </para>
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77 |
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78 | <para>You can also select which exact type of IDE controller
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79 | hardware VirtualBox should present to the virtual machine (PIIX3,
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80 | PIIX4 or ICH6). This makes no difference in terms of performance,
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81 | but if you import a virtual machine from another virtualization
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82 | product, the operating system in that machine may expect a
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83 | particular controller type and crash if it isn't found.</para>
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84 |
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85 | <para>After you have created a new virtual machine with the "New
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86 | Virtual Machine" wizard of the graphical user interface, you will
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87 | typically see one IDE controller in the machine's "Storage" settings
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88 | where the virtual CD/DVD drive will be attached to one of the four
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89 | ports of this controller.</para>
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90 | </listitem>
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91 |
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92 | <listitem>
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93 | <para><emphasis role="bold">Serial ATA (SATA)</emphasis> is a newer
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94 | standard introduced in 2003. Compared to IDE, it supports both much
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95 | higher speeds and more devices per controller. Also, with
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96 | physical hardware, devices can be added and removed while the system
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97 | is running. The standard interface for SATA controllers is called
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98 | Advanced Host Controller Interface (<emphasis
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99 | role="bold">AHCI</emphasis>).</para>
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100 |
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101 | <para>Like a real SATA controller, VirtualBox's virtual SATA
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102 | controller operates faster and also consumes fewer CPU resources than
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103 | the virtual IDE controller. Also, this allows you to connect up to
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104 | 30 virtual hard disks to one machine instead of just three, as with
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105 | the VirtualBox IDE controller (with the DVD drive already attached).</para>
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106 |
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107 | <para>For this reason, starting with version 3.2 and depending on
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108 | the selected guest operating system, VirtualBox uses SATA as the
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109 | default for newly created virtual machines. One virtual SATA
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110 | controller is created by default, and the default disk that is
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111 | created with a new VM is attached to this controller.<warning>
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112 | <para>The entire SATA controller and the virtual disks attached
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113 | to it (including those in IDE compatibility mode) will not be
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114 | seen by operating systems that do not have device support for
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115 | AHCI. In particular, <emphasis role="bold">there is no support
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116 | for AHCI in Windows before Windows Vista</emphasis>, so Windows
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117 | XP (even SP3) will not see such disks unless you install
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118 | additional drivers. It is possible to switch from IDE to SATA
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119 | after installation by installing the SATA drivers and changing
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120 | the controller type in the VM settings dialog.<footnote>
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121 | <para>VirtualBox recommends the Intel Matrix Storage drivers
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122 | which can be downloaded from <ulink
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123 | url="http://downloadcenter.intel.com/Product_Filter.aspx?ProductID=2101">http://downloadcenter.intel.com/Product_Filter.aspx?ProductID=2101</ulink>.</para>
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124 | </footnote></para>
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125 | </warning></para>
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126 |
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127 | <para>To add a SATA controller to a machine for which it has not
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128 | been enabled by default (either because it was created by an earlier
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129 | version of VirtualBox, or because SATA is not supported by default
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130 | by the selected guest operating system), go to the "Storage" page of
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131 | the machine's settings dialog, click on the "Add Controller" button
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132 | under the "Storage Tree" box and then select "Add SATA Controller".
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133 | After this, the additional controller will appear as a separate PCI
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134 | device in the virtual machine, and you can add virtual disks to
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135 | it.</para>
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136 |
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137 | <para>To change the IDE compatibility mode settings for the SATA
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138 | controller, please see <xref
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139 | linkend="vboxmanage-storagectl" />.</para>
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140 | </listitem>
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141 |
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142 | <listitem>
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143 | <para><emphasis role="bold">SCSI</emphasis> is another established
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144 | industry standard, standing for "Small Computer System Interface".
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145 | SCSI was standardized as early as 1986 as a generic interface for
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146 | data transfer between all kinds of devices, including storage
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147 | devices. Today SCSI is still used for connecting hard disks and tape
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148 | devices, but it has mostly been displaced in commodity hardware. It
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149 | is still in common use in high-performance workstations and
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150 | servers.</para>
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151 |
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152 | <para>Primarily for compatibility with other virtualization
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153 | software, VirtualBox optionally supports LSI Logic and BusLogic SCSI
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154 | controllers, to each of which up to 15 virtual hard disks can be
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155 | attached.</para>
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156 |
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157 | <para>To enable a SCSI controller, on the "Storage" page of a
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158 | virtual machine's settings dialog, click on the "Add Controller"
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159 | button under the "Storage Tree" box and then select "Add SCSI
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160 | Controller". After this, the additional controller will appear as a
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161 | separate PCI device in the virtual machine.<warning>
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162 | <para>As with the other controller types, a SCSI controller will
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163 | only be seen by operating systems with device support for it.
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164 | Windows 2003 and later ships with drivers for the LSI Logic
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165 | controller, while Windows NT 4.0 and Windows 2000 ships with
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166 | drivers for the BusLogic controller. Windows XP ships with
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167 | drivers for neither.</para>
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168 | </warning></para>
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169 | </listitem>
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170 |
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171 | <listitem>
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172 | <para><emphasis role="bold">Serial Attached SCSI (SAS)</emphasis> is
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173 | another bus standard which uses the SCSI command set. As opposed to
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174 | SCSI, however, with physical devices, serial cables are used instead
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175 | of parallel ones, which simplifies physical device connections. In
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176 | some ways, therefore, SAS is to SCSI what SATA is to IDE: it allows
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177 | for more reliable and faster connections.</para>
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178 |
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179 | <para>To support high-end guests which require SAS controllers,
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180 | VirtualBox emulates a LSI Logic SAS controller, which can be enabled
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181 | much the same way as a SCSI controller. At this time, up to eight
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182 | devices can be connected to the SAS controller.</para>
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183 |
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184 | <warning>
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185 | <para>As with SATA, the SAS controller will only be seen by
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186 | operating systems with device support for it. In particular,
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187 | <emphasis role="bold">there is no support for SAS in Windows
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188 | before Windows Vista</emphasis>, so Windows XP (even SP3) will not
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189 | see such disks unless you install additional drivers.</para>
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190 | </warning>
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191 | </listitem>
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192 | </itemizedlist></para>
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193 |
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194 | <para>In summary, VirtualBox gives you the following categories of virtual
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195 | storage slots:<orderedlist>
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196 | <listitem>
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197 | <para>four slots attached to the traditional IDE controller, which
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198 | are always present (one of which typically is a virtual CD/DVD
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199 | drive);</para>
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200 | </listitem>
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201 |
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202 | <listitem>
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203 | <para>30 slots attached to the SATA controller, if enabled and
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204 | supported by the guest operating system;</para>
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205 | </listitem>
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206 |
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207 | <listitem>
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208 | <para>15 slots attached to the SCSI controller, if enabled and
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209 | supported by the guest operating system;</para>
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210 | </listitem>
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211 |
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212 | <listitem>
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213 | <para>eight slots attached to the SAS controller, if enabled and
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214 | supported by the guest operating system.</para>
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215 | </listitem>
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216 | </orderedlist></para>
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217 |
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218 | <para>Given this large choice of storage controllers, you may ask yourself
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219 | which one to choose. In general, you should avoid IDE unless it is the
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220 | only controller supported by your guest. Whether you use SATA, SCSI or SAS
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221 | does not make any real difference. The variety of controllers is only
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222 | supplied for VirtualBox for compatibility with existing hardware and other
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223 | hypervisors.</para>
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224 | </sect1>
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225 |
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226 | <sect1 id="vdidetails">
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227 | <title>Disk image files (VDI, VMDK, VHD, HDD)</title>
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228 |
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229 | <para>Disk image files reside on the host system and are seen by the guest
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230 | systems as hard disks of a certain geometry. When a guest operating system
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231 | reads from or writes to a hard disk, VirtualBox redirects the request to
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232 | the image file.</para>
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233 |
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234 | <para>Like a physical disk, a virtual disk has a size (capacity), which
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235 | must be specified when the image file is created. As opposed to a physical
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236 | disk however, VirtualBox allows you to expand an image file after
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237 | creation, even if it has data already; see <xref
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238 | linkend="vboxmanage-modifyvdi" /> for details.<footnote>
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239 | <para>Image resizing was added with VirtualBox 4.0.</para>
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240 | </footnote></para>
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241 |
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242 | <para>VirtualBox supports four variants of disk image files:<itemizedlist>
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243 | <listitem>
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244 | <para>Normally, VirtualBox uses its own container format for guest
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245 | hard disks -- Virtual Disk Image (VDI) files. In particular, this
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246 | format will be used when you create a new virtual machine with a new
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247 | disk.</para>
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248 | </listitem>
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249 |
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250 | <listitem>
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251 | <para>VirtualBox also fully supports the popular and open VMDK
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252 | container format that is used by many other virtualization products,
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253 | in particular, by VMware.<footnote>
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254 | <para>Initial support for VMDK was added with VirtualBox 1.4;
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255 | since version 2.1, VirtualBox supports VMDK fully, meaning that
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256 | you can create snapshots and use all the other advanced features
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257 | described above for VDI images with VMDK also.</para>
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258 | </footnote></para>
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259 | </listitem>
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260 |
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261 | <listitem>
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262 | <para>VirtualBox also fully supports the VHD format used by
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263 | Microsoft.</para>
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264 | </listitem>
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265 |
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266 | <listitem>
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267 | <para>Image files of Parallels version 2 (HDD format) are also
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268 | supported.<footnote>
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269 | <para>Support was added with VirtualBox 3.1.</para>
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270 | </footnote> For lack of documentation of the format, newer formats
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271 | (3 and 4) are not supported. You can however convert such image
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272 | files to version 2 format using tools provided by Parallels.</para>
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273 | </listitem>
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274 | </itemizedlist></para>
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275 |
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276 | <para>Irrespective of the disk capacity and format, as briefly mentioned
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277 | in <xref linkend="gui-createvm" />, there are two options of how to create
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278 | a disk image: fixed-size or dynamically allocated.</para>
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279 |
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280 | <itemizedlist>
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281 | <listitem>
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282 | <para>If you create a <emphasis role="bold">fixed-size
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283 | image</emphasis>, an image file will be created on your host system
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284 | which has roughly the same size as the virtual disk's capacity. So,
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285 | for a 10G disk, you will have a 10G file. Note that the creation of a
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286 | fixed-size image can take a long time depending on the size of the
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287 | image and the write performance of your hard disk.</para>
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288 | </listitem>
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289 |
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290 | <listitem>
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291 | <para>For more flexible storage management, use a <emphasis
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292 | role="bold">dynamically allocated image</emphasis>. This will
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293 | initially be very small and not occupy any space for unused virtual
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294 | disk sectors, but will grow every time a disk sector is written to for
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295 | the first time, until the drive reaches the maximum capacity chosen
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296 | when the drive was created. While this format takes less space
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297 | initially, the fact that VirtualBox needs to expand the image file
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298 | consumes additional computing resources, so until the disk file size has
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299 | stabilized, write operations may be slower than with fixed size disks.
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300 | However, after a time the rate of growth will slow and the average penalty
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301 | for write operations will be negligible.</para>
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302 | </listitem>
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303 | </itemizedlist>
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304 | </sect1>
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305 |
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306 | <sect1 id="vdis">
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307 | <title>The Virtual Media Manager</title>
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308 |
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309 | <para>VirtualBox keeps track of all the hard disk, CD/DVD-ROM and floppy
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310 | disk images which are in use by virtual machines. These are often referred
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311 | to as "known media" and come from two sources:<itemizedlist>
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312 | <listitem>
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313 | <para>all media currently attached to virtual machines;</para>
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314 | </listitem>
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315 |
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316 | <listitem>
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317 | <para>"registered" media for compatibility with VirtualBox versions
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318 | older than version 4.0. For details about how media registration has
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319 | changed with version 4.0, please refer to <xref
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320 | linkend="vboxconfigdata" />.</para>
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321 | </listitem>
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322 | </itemizedlist></para>
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323 |
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324 | <para>The known media can be viewed and changed in the <emphasis
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325 | role="bold">Virtual Media Manager</emphasis>, which you can access from
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326 | the "File" menu in the VirtualBox main window:</para>
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327 |
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328 | <para><mediaobject>
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329 | <imageobject>
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330 | <imagedata align="center" fileref="images/virtual-disk-manager.png"
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331 | width="12cm" />
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332 | </imageobject>
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333 | </mediaobject>The known media are conveniently grouped in three tabs for
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334 | the three possible formats. These formats are:</para>
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335 |
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336 | <itemizedlist>
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337 | <listitem>
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338 | <para>Hard disk images, either in VirtualBox's own Virtual Disk Image
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339 | (VDI) format or in the third-party formats listed in the previous
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340 | chapter;</para>
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341 | </listitem>
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342 |
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343 | <listitem>
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344 | <para>CD/DVD images in standard ISO format;</para>
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345 | </listitem>
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346 |
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347 | <listitem>
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348 | <para>floppy images in standard RAW format.</para>
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349 | </listitem>
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350 | </itemizedlist>
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351 |
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352 | <para>As you can see in the screenshot above, for each image, the Virtual
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353 | Media Manager shows you the full path of the image file and other
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354 | information, such as the virtual machine the image is currently attached
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355 | to, if any.</para>
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356 |
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357 | <para>The Virtual Media Manager allows you to</para>
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358 |
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359 | <itemizedlist>
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360 | <listitem>
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361 | <para><emphasis role="bold">remove</emphasis> an image from the
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362 | registry (and optionally delete the image file when doing so);</para>
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363 | </listitem>
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364 |
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365 | <listitem>
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366 | <para><emphasis role="bold">"release"</emphasis> an image, that is,
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367 | detach it from a virtual machine if it is currently attached to one as
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368 | a virtual hard disk.</para>
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369 | </listitem>
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370 | </itemizedlist>
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371 |
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372 | <para>Starting with version 4.0, to <emphasis role="bold">create new disk
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373 | images,</emphasis> please use the "Storage" page in a virtual machine's
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374 | settings dialog because disk images are now by default stored in each
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375 | machine's own folder.</para>
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376 |
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377 | <para>Hard disk image files can be copied onto other host systems and
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378 | imported into virtual machines there, although certain guest systems
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379 | (notably Windows 2000 and XP) will require that the new virtual machine be
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380 | set up in a similar way to the old one.<note>
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381 | <para>Do not simply make copies of virtual disk images. If you import
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382 | such a second copy into a virtual machine, VirtualBox will complain
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383 | with an error, since VirtualBox assigns a unique identifier (UUID) to
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384 | each disk image to make sure it is only used once. See <xref
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385 | linkend="cloningvdis" /> for instructions on this matter. Also, if you
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386 | want to copy a virtual machine to another system, VirtualBox has an
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387 | import/export facility that might be better suited for your needs; see
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388 | <xref linkend="ovf" />.</para>
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389 | </note></para>
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390 | </sect1>
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391 |
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392 | <sect1 id="hdimagewrites">
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393 | <title>Special image write modes</title>
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394 |
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395 | <para>For each virtual disk image supported by VirtualBox, you can
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396 | determine separately how it should be affected by write operations from a
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397 | virtual machine and snapshot operations. This applies to all of the
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398 | aforementioned image formats (VDI, VMDK, VHD or HDD) and irrespective of
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399 | whether an image is fixed-size or dynamically allocated.</para>
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400 |
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401 | <para>By default, images are in "normal" mode. To mark an existing image
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402 | with one of the non-standard modes listed below, use
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403 | <computeroutput>VBoxManage modifyhd</computeroutput>; see <xref
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404 | linkend="vboxmanage-modifyvdi" />. Alternatively, use VBoxManage to attach
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405 | the image to a VM and use the <computeroutput>--mtype</computeroutput>
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406 | argument; see <xref linkend="vboxmanage-storageattach" />.</para>
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407 |
|
---|
408 | <orderedlist>
|
---|
409 | <listitem>
|
---|
410 | <para>With <emphasis role="bold">normal images</emphasis> (the default
|
---|
411 | setting), there are no restrictions on how guests can read from and
|
---|
412 | write to the disk.</para>
|
---|
413 |
|
---|
414 | <para>When you take a snapshot of your virtual machine as described in
|
---|
415 | <xref linkend="snapshots" />, the state of such a "normal hard disk"
|
---|
416 | will be recorded together with the snapshot, and when reverting to the
|
---|
417 | snapshot, its state will be fully reset.</para>
|
---|
418 |
|
---|
419 | <para>(Technically, strictly speaking, the image file itself is not
|
---|
420 | "reset". Instead, when a snapshot is taken, VirtualBox "freezes" the
|
---|
421 | image file and no longer writes to it. For the write operations from
|
---|
422 | the VM, a second, "differencing" image file is created which receives
|
---|
423 | only the changes to the original image; see the next section for
|
---|
424 | details.)</para>
|
---|
425 |
|
---|
426 | <para>While you can attach the same "normal" image to more than one
|
---|
427 | virtual machine, only one of these virtual machines attached to the
|
---|
428 | same image file can be executed simultaneously, as otherwise there
|
---|
429 | would be conflicts if several machines write to the same image
|
---|
430 | file.<footnote>
|
---|
431 | <para>This restriction is more lenient now than it was before
|
---|
432 | VirtualBox 2.2. Previously, each "normal" disk image could only be
|
---|
433 | <emphasis>attached</emphasis> to one single machine. Now it can be
|
---|
434 | attached to more than one machine so long as only one of these
|
---|
435 | machines is running.</para>
|
---|
436 | </footnote></para>
|
---|
437 | </listitem>
|
---|
438 |
|
---|
439 | <listitem>
|
---|
440 | <para>By contrast, <emphasis role="bold">write-through hard
|
---|
441 | disks</emphasis> are completely unaffected by snapshots: their state
|
---|
442 | is <emphasis>not</emphasis> saved when a snapshot is taken, and not
|
---|
443 | restored when a snapshot is restored.</para>
|
---|
444 | </listitem>
|
---|
445 |
|
---|
446 | <listitem>
|
---|
447 | <para><emphasis role="bold">Shareable hard disks</emphasis> are a
|
---|
448 | variant of write-through hard disks. In principle they behave exactly
|
---|
449 | the same, i.e. their state is <emphasis>not</emphasis> saved when a
|
---|
450 | snapshot is taken, and not restored when a snapshot is restored. The
|
---|
451 | difference only shows if you attach such disks to several VMs.
|
---|
452 | Shareable disks may be attached to several VMs which may run
|
---|
453 | concurrently. This makes them suitable for use by cluster filesystems
|
---|
454 | between VMs and similar applications which are explicitly prepared to
|
---|
455 | access a disk concurrently. Only fixed size images can be used in this
|
---|
456 | way, and dynamically allocated images are rejected.<warning>
|
---|
457 | <para>This is an expert feature, and misuse can lead to data loss
|
---|
458 | -- regular filesystems are not prepared to handle simultaneous
|
---|
459 | changes by several parties.</para>
|
---|
460 | </warning></para>
|
---|
461 | </listitem>
|
---|
462 |
|
---|
463 | <listitem>
|
---|
464 | <para>Next, <emphasis role="bold">immutable images</emphasis> only
|
---|
465 | remember write accesses temporarily while the virtual machine is
|
---|
466 | running; all changes are lost when the virtual machine is powered on
|
---|
467 | the next time. As a result, as opposed to "normal" images, the same
|
---|
468 | immutable image can be used with several virtual machines without
|
---|
469 | restrictions.</para>
|
---|
470 |
|
---|
471 | <para><emphasis>Creating</emphasis> an immutable image makes little
|
---|
472 | sense since it would be initially empty and lose its contents with
|
---|
473 | every machine restart (unless you really want to have a disk that is
|
---|
474 | always unformatted when the machine starts up). As a result, normally,
|
---|
475 | you would first create a "normal" image and then, when you deem its
|
---|
476 | contents useful, later mark it immutable.</para>
|
---|
477 |
|
---|
478 | <para>If you take a snapshot of a machine with immutable images, then
|
---|
479 | on every machine power-up, those images are reset to the state of the
|
---|
480 | last (current) snapshot (instead of the state of the original
|
---|
481 | immutable image).</para>
|
---|
482 |
|
---|
483 | <note>
|
---|
484 | <para>As a special exception, immutable images are
|
---|
485 | <emphasis>not</emphasis> reset if they are attached to a machine
|
---|
486 | whose last snapshot was taken while the machine was running (a
|
---|
487 | so-called "online" snapshot). As a result, if the machine's current
|
---|
488 | snapshot is such an "online" snapshot, its immutable images behave
|
---|
489 | exactly like the "normal" images described previously. To re-enable
|
---|
490 | the automatic resetting of such images, delete the current snapshot
|
---|
491 | of the machine.</para>
|
---|
492 | </note>
|
---|
493 |
|
---|
494 | <para>Again, technically, VirtualBox never writes to an immutable
|
---|
495 | image directly at all. All write operations from the machine will be
|
---|
496 | directed to a differencing image; the next time the VM is powered on,
|
---|
497 | the differencing image is reset so that every time the VM starts, its
|
---|
498 | immutable images have exactly the same content.<footnote>
|
---|
499 | <para>This behavior also changed with VirtualBox 2.2. Previously,
|
---|
500 | the differencing images were discarded when the machine session
|
---|
501 | <emphasis>ended</emphasis>; now they are discarded every time the
|
---|
502 | machine is powered on.</para>
|
---|
503 | </footnote> The differencing image is only reset when the machine is
|
---|
504 | powered on from within VirtualBox, not when you reboot by requesting a
|
---|
505 | reboot from within the machine. This is also why immutable images
|
---|
506 | behave as described above when snapshots are also present, which use
|
---|
507 | differencing images as well.</para>
|
---|
508 |
|
---|
509 | <para>If the automatic discarding of the differencing image on VM
|
---|
510 | startup does not fit your needs, you can turn it off using the
|
---|
511 | <computeroutput>autoreset</computeroutput> parameter of
|
---|
512 | <computeroutput>VBoxManage modifyhd</computeroutput>; see <xref
|
---|
513 | linkend="vboxmanage-modifyvdi" /> for details.</para>
|
---|
514 | </listitem>
|
---|
515 |
|
---|
516 | <listitem>
|
---|
517 | <para>An image in <emphasis role="bold">multiattach mode</emphasis>
|
---|
518 | can be attached to more than one virtual machine at the same time,
|
---|
519 | even if these machines are running simultaneously. For each virtual
|
---|
520 | machine to which such an image is attached, a differencing image is
|
---|
521 | created. As a result, data that is written to such a virtual disk by
|
---|
522 | one machine is not seen by the other machines to which the image is
|
---|
523 | attached; each machine creates its own write history of the
|
---|
524 | multiattach image.</para>
|
---|
525 |
|
---|
526 | <para>Technically, a "multiattach" image behaves identically to an
|
---|
527 | "immutable" image except the differencing image is not reset every
|
---|
528 | time the machine starts.</para>
|
---|
529 | </listitem>
|
---|
530 |
|
---|
531 | <listitem>
|
---|
532 | <para>Finally, the <emphasis role="bold">read-only image</emphasis> is
|
---|
533 | used automatically for CD/DVD images, since CDs/DVDs can never be
|
---|
534 | written to.</para>
|
---|
535 | </listitem>
|
---|
536 | </orderedlist>
|
---|
537 |
|
---|
538 | <para>To illustrate the differences between the various types with respect
|
---|
539 | to snapshots: Assume you have installed your guest operating system in
|
---|
540 | your VM, and you have taken a snapshot. Imagine you have accidentally
|
---|
541 | infected your VM with a virus and would like to go back to the snapshot.
|
---|
542 | With a normal hard disk image, you simply restore the snapshot, and the
|
---|
543 | earlier state of your hard disk image will be restored as well (and your
|
---|
544 | virus infection will be undone). With an immutable hard disk, all it takes
|
---|
545 | is to shut down and power on your VM, and the virus infection will be
|
---|
546 | discarded. With a write-through image however, you cannot easily undo the
|
---|
547 | virus infection by means of virtualization, but will have to disinfect
|
---|
548 | your virtual machine like a real computer.</para>
|
---|
549 |
|
---|
550 | <para>Still, you might find write-through images useful if you want to
|
---|
551 | preserve critical data irrespective of snapshots, and since you can attach
|
---|
552 | more than one image to a VM, you may want to have one immutable for the
|
---|
553 | operating system and one write-through for your data files.</para>
|
---|
554 | </sect1>
|
---|
555 |
|
---|
556 | <sect1 id="diffimages">
|
---|
557 | <title>Differencing images</title>
|
---|
558 |
|
---|
559 | <para>The previous section hinted at differencing images and how they are
|
---|
560 | used with snapshots, immutable images and multiple disk attachments. For
|
---|
561 | the inquisitive VirtualBox user, this section describes in more detail how
|
---|
562 | they work.</para>
|
---|
563 |
|
---|
564 | <para>A differencing image is a special disk image that only holds the
|
---|
565 | differences to another image. A differencing image by itself is useless,
|
---|
566 | it must always refer to another image. The differencing image is then
|
---|
567 | typically referred to as a "child", which holds the differences to its
|
---|
568 | "parent".</para>
|
---|
569 |
|
---|
570 | <para>When a differencing image is active, it receives all write
|
---|
571 | operations from the virtual machine instead of its parent. The
|
---|
572 | differencing image only contains the sectors of the virtual hard disk that
|
---|
573 | have changed since the differencing image was created. When the machine
|
---|
574 | reads a sector from such a virtual hard disk, it looks into the
|
---|
575 | differencing image first. If the sector is present, it is returned from
|
---|
576 | there; if not, VirtualBox looks into the parent. In other words, the
|
---|
577 | parent becomes "read-only"; it is never written to again, but it is read
|
---|
578 | from if a sector has not changed.</para>
|
---|
579 |
|
---|
580 | <para>Differencing images can be chained. If another differencing image is
|
---|
581 | created for a virtual disk that already has a differencing image, then it
|
---|
582 | becomes a "grandchild" of the original parent. The first differencing
|
---|
583 | image then becomes read-only as well, and write operations only go to the
|
---|
584 | second-level differencing image. When reading from the virtual disk,
|
---|
585 | VirtualBox needs to look into the second differencing image first, then
|
---|
586 | into the first if the sector was not found, and then into the original
|
---|
587 | image.</para>
|
---|
588 |
|
---|
589 | <para>There can be an unlimited number of differencing images, and each
|
---|
590 | image can have more than one child. As a result, the differencing images
|
---|
591 | can form a complex tree with parents, "siblings" and children, depending
|
---|
592 | on how complex your machine configuration is. Write operations always go
|
---|
593 | to the one "active" differencing image that is attached to the machine,
|
---|
594 | and for read operations, VirtualBox may need to look up all the parents in
|
---|
595 | the chain until the sector in question is found. You can look at such a
|
---|
596 | tree in the Virtual Media Manager:<mediaobject>
|
---|
597 | <imageobject>
|
---|
598 | <imagedata align="center" fileref="images/virtual-disk-manager2.png"
|
---|
599 | width="12cm" />
|
---|
600 | </imageobject>
|
---|
601 | </mediaobject></para>
|
---|
602 |
|
---|
603 | <para>In all of these situations, from the point of view of the virtual
|
---|
604 | machine, the virtual hard disk behaves like any other disk. While the
|
---|
605 | virtual machine is running, there is a slight run-time I/O overhead
|
---|
606 | because VirtualBox might need to look up sectors several times. This is
|
---|
607 | not noticeable however since the tables with sector information are always
|
---|
608 | kept in memory and can be looked up quickly.</para>
|
---|
609 |
|
---|
610 | <para>Differencing images are used in the following
|
---|
611 | situations:<orderedlist>
|
---|
612 | <listitem>
|
---|
613 | <para><emphasis role="bold">Snapshots.</emphasis> When you create a
|
---|
614 | snapshot, as explained in the previous section, VirtualBox "freezes"
|
---|
615 | the images attached to the virtual machine and creates differencing
|
---|
616 | images for each of them (to be precise: one for each image that is
|
---|
617 | not in "write-through" mode). From the point of view of the virtual
|
---|
618 | machine, the virtual disks continue to operate before, but all write
|
---|
619 | operations go into the differencing images. Each time you create
|
---|
620 | another snapshot, for each hard disk attachment, another
|
---|
621 | differencing image is created and attached, forming a chain or
|
---|
622 | tree.</para>
|
---|
623 |
|
---|
624 | <para>In the above screenshot, you see that the original disk image
|
---|
625 | is now attached to a snapshot, representing the state of the disk
|
---|
626 | when the snapshot was taken.</para>
|
---|
627 |
|
---|
628 | <para>If you now <emphasis role="bold">restore</emphasis> a snapshot
|
---|
629 | -- that is, if you want to go back to the exact machine state that
|
---|
630 | was stored in the snapshot --, the following happens:<orderedlist>
|
---|
631 | <listitem>
|
---|
632 | <para>VirtualBox copies the virtual machine settings that were
|
---|
633 | copied into the snapshot back to the virtual machine. As a
|
---|
634 | result, if you have made changes to the machine configuration
|
---|
635 | since taking the snapshot, they are undone.</para>
|
---|
636 | </listitem>
|
---|
637 |
|
---|
638 | <listitem>
|
---|
639 | <para>If the snapshot was taken while the machine was running,
|
---|
640 | it contains a saved machine state, and that state is restored
|
---|
641 | as well; after restoring the snapshot, the machine will then
|
---|
642 | be in "Saved" state and resume execution from there when it is
|
---|
643 | next started. Otherwise the machine will be in "Powered Off"
|
---|
644 | state and do a full boot.</para>
|
---|
645 | </listitem>
|
---|
646 |
|
---|
647 | <listitem>
|
---|
648 | <para>For each disk image attached to the machine, the
|
---|
649 | differencing image holding all the write operations since the
|
---|
650 | current snapshot was taken is thrown away, and the original
|
---|
651 | parent image is made active again. (If you restored the "root"
|
---|
652 | snapshot, then this will be the root disk image for each
|
---|
653 | attachment; otherwise, some other differencing image descended
|
---|
654 | from it.) This effectively restores the old machine
|
---|
655 | state.</para>
|
---|
656 | </listitem>
|
---|
657 | </orderedlist></para>
|
---|
658 |
|
---|
659 | <para>If you later <emphasis role="bold">delete</emphasis> a
|
---|
660 | snapshot in order to free disk space, for each disk attachment, one
|
---|
661 | of the differencing images becomes obsolete. In this case, the
|
---|
662 | differencing image of the disk attachment cannot simply be deleted.
|
---|
663 | Instead, VirtualBox needs to look at each sector of the differencing
|
---|
664 | image and needs to copy it back into its parent; this is called
|
---|
665 | "merging" images and can be a potentially lengthy process, depending
|
---|
666 | on how large the differencing image is. It can also temporarily need
|
---|
667 | a considerable amount of extra disk space, before the differencing
|
---|
668 | image obsoleted by the merge operation is deleted.</para>
|
---|
669 | </listitem>
|
---|
670 |
|
---|
671 | <listitem>
|
---|
672 | <para><emphasis role="bold">Immutable images.</emphasis> When an
|
---|
673 | image is switched to "immutable" mode, a differencing image is
|
---|
674 | created as well. As with snapshots, the parent image then becomes
|
---|
675 | read-only, and the differencing image receives all the write
|
---|
676 | operations. Every time the virtual machine is started, all the
|
---|
677 | immutable images which are attached to it have their respective
|
---|
678 | differencing image thrown away, effectively resetting the virtual
|
---|
679 | machine's virtual disk with every restart.</para>
|
---|
680 | </listitem>
|
---|
681 | </orderedlist></para>
|
---|
682 | </sect1>
|
---|
683 |
|
---|
684 | <sect1 id="cloningvdis">
|
---|
685 | <title>Cloning disk images</title>
|
---|
686 |
|
---|
687 | <para>You can duplicate hard disk image files on the same host to quickly
|
---|
688 | produce a second virtual machine with the same operating system setup.
|
---|
689 | However, you should <emphasis>only</emphasis> make copies of virtual disk
|
---|
690 | images using the utility supplied with VirtualBox; see <xref
|
---|
691 | linkend="vboxmanage-clonevdi" />. This is because VirtualBox assigns a
|
---|
692 | unique identity number (UUID) to each disk image, which is also stored
|
---|
693 | inside the image, and VirtualBox will refuse to work with two images that
|
---|
694 | use the same number. If you do accidentally try to reimport a disk image
|
---|
695 | which you copied normally, you can make a second copy using VirtualBox's
|
---|
696 | utility and import that instead.</para>
|
---|
697 |
|
---|
698 | <para>Note that newer Linux distributions identify the boot hard disk from
|
---|
699 | the ID of the drive. The ID VirtualBox reports for a drive is determined
|
---|
700 | from the UUID of the virtual disk image. So if you clone a disk image and
|
---|
701 | try to boot the copied image the guest might not be able to determine its
|
---|
702 | own boot disk as the UUID changed. In this case you have to adapt the disk
|
---|
703 | ID in your boot loader script (for example
|
---|
704 | <computeroutput>/boot/grub/menu.lst</computeroutput>). The disk ID looks
|
---|
705 | like this:<screen>scsi-SATA_VBOX_HARDDISK_VB5cfdb1e2-c251e503</screen></para>
|
---|
706 |
|
---|
707 | <para>The ID for the copied image can be determined with <screen>hdparm -i /dev/sda</screen></para>
|
---|
708 | </sect1>
|
---|
709 |
|
---|
710 | <sect1 id="iocaching">
|
---|
711 | <title>Host I/O caching</title>
|
---|
712 |
|
---|
713 | <para>Starting with version 3.2, VirtualBox can optionally disable the I/O
|
---|
714 | caching that the host operating system would otherwise perform on disk
|
---|
715 | image files.</para>
|
---|
716 |
|
---|
717 | <para>Traditionally, VirtualBox has opened disk image files as normal
|
---|
718 | files, which results in them being cached by the host operating system
|
---|
719 | like any other file. The main advantage of this is speed: when the guest
|
---|
720 | OS writes to disk and the host OS cache uses delayed writing, the write
|
---|
721 | operation can be reported as completed to the guest OS quickly while the
|
---|
722 | host OS can perform the operation asynchronously. Also, when you start a
|
---|
723 | VM a second time and have enough memory available for the OS to use for
|
---|
724 | caching, large parts of the virtual disk may be in system memory, and the
|
---|
725 | VM can access the data much faster.</para>
|
---|
726 |
|
---|
727 | <para>Note that this applies only to image files; buffering never occurred
|
---|
728 | for virtual disks residing on remote iSCSI storage, which is the more common
|
---|
729 | scenario in enterprise-class setups (see <xref
|
---|
730 | linkend="storage-iscsi" />).</para>
|
---|
731 |
|
---|
732 | <para>While buffering is a useful default setting for virtualizating a few
|
---|
733 | machines on a desktop computer, there are some disadvantages to this
|
---|
734 | approach:<orderedlist>
|
---|
735 | <listitem>
|
---|
736 | <para>Delayed writing through the host OS cache is less secure. When
|
---|
737 | the guest OS writes data, it considers the data written even though
|
---|
738 | it has not yet arrived on a physical disk. If for some reason the
|
---|
739 | write does not happen (power failure, host crash), the likelihood of
|
---|
740 | data loss increases.</para>
|
---|
741 | </listitem>
|
---|
742 |
|
---|
743 | <listitem>
|
---|
744 | <para>Disk image files tend to be very large. Caching them can
|
---|
745 | therefore quickly use up the entire host OS cache. Depending on the
|
---|
746 | efficiency of the host OS caching, this may slow down the host
|
---|
747 | immensely, especially if several VMs run at the same time. For
|
---|
748 | example, on Linux hosts, host caching may result in Linux delaying
|
---|
749 | all writes until the host cache is nearly full and then writing out
|
---|
750 | all these changes at once, possibly stalling VM execution for
|
---|
751 | minutes. This can result in I/O errors in the guest as I/O requests
|
---|
752 | time out there.</para>
|
---|
753 | </listitem>
|
---|
754 |
|
---|
755 | <listitem>
|
---|
756 | <para>Physical memory is often wasted as guest operating systems
|
---|
757 | typically have their own I/O caches, which may result in the data
|
---|
758 | being cached twice (in both the guest and the host caches) for
|
---|
759 | little effect.</para>
|
---|
760 | </listitem>
|
---|
761 | </orderedlist></para>
|
---|
762 |
|
---|
763 | <para>If you decide to disable host I/O caching for the above reasons,
|
---|
764 | VirtualBox uses its own small cache to buffer writes, but no read caching
|
---|
765 | since this is typically already performed by the guest OS. In addition,
|
---|
766 | VirtualBox fully supports asynchronous I/O for its virtual SATA, SCSI and
|
---|
767 | SAS controllers through multiple I/O threads.</para>
|
---|
768 |
|
---|
769 | <para>Since asynchronous I/O is not supported by IDE controllers, for
|
---|
770 | performance reasons, you may want to leave host caching enabled for your
|
---|
771 | VM's virtual IDE controllers.</para>
|
---|
772 |
|
---|
773 | <para>For this reason, VirtualBox allows you to configure whether the host
|
---|
774 | I/O cache is used for each I/O controller separately. Either uncheck the
|
---|
775 | "Use host I/O cache" box in the "Storage" settings for a given virtual
|
---|
776 | storage controller, or use the following VBoxManage command to disable the
|
---|
777 | host I/O cache for a virtual storage controller:<screen>VBoxManage storagectl <vm> --name <controllername> --hostiocache off</screen></para>
|
---|
778 |
|
---|
779 | <para>See <xref linkend="vboxmanage-storagectl" /> for details.</para>
|
---|
780 |
|
---|
781 | <para>For the above reasons also, VirtualBox now uses SATA controllers by
|
---|
782 | default for new virtual machines.</para>
|
---|
783 | </sect1>
|
---|
784 |
|
---|
785 | <sect1 id="storage-bandwidth-limit">
|
---|
786 | <title>Limiting bandwidth for disk images</title>
|
---|
787 |
|
---|
788 | <para>Starting with version 4.0, VirtualBox allows for limiting the
|
---|
789 | maximum bandwidth used for asynchronous I/O. Additionally it supports
|
---|
790 | sharing limits through bandwidth groups for several images. It is possible
|
---|
791 | to have more than one such limit.</para>
|
---|
792 |
|
---|
793 | <para>Limits are configured through
|
---|
794 | <computeroutput>VBoxManage</computeroutput>. The example below creates a
|
---|
795 | bandwidth group named "Limit", sets the limit to 20 MB/s and assigns the
|
---|
796 | group to the attached disks of the VM:<screen>VBoxManage bandwidthctl "VM name" add Limit --type disk --limit 20M
|
---|
797 | VBoxManage storageattach "VM name" --controller "SATA" --port 0 --device 0 --type hdd
|
---|
798 | --medium disk1.vdi --bandwidthgroup Limit
|
---|
799 | VBoxManage storageattach "VM name" --controller "SATA" --port 1 --device 0 --type hdd
|
---|
800 | --medium disk2.vdi --bandwidthgroup Limit</screen></para>
|
---|
801 |
|
---|
802 | <para>All disks in a group share the bandwidth limit, meaning that in the
|
---|
803 | example above the bandwidth of both images combined can never exceed 20
|
---|
804 | MB/s. However, if one disk doesn't require bandwidth the other can use the
|
---|
805 | remaining bandwidth of its group.</para>
|
---|
806 |
|
---|
807 | <para>The limits for each group can be changed while the VM is running,
|
---|
808 | with changes being picked up immediately. The example below changes the
|
---|
809 | limit for the group created in the example above to 10 MB/s:<screen>VBoxManage bandwidthctl "VM name" set Limit --limit 10M</screen></para>
|
---|
810 | </sect1>
|
---|
811 |
|
---|
812 | <sect1 id="storage-cds">
|
---|
813 | <title>CD/DVD support</title>
|
---|
814 |
|
---|
815 | <para>The virtual CD/DVD drive(s) by default support only reading. The
|
---|
816 | medium configuration is changeable at runtime. You can select between
|
---|
817 | three options to provide the medium data:<itemizedlist>
|
---|
818 | <listitem>
|
---|
819 | <para><emphasis role="bold">Host Drive</emphasis> defines that the
|
---|
820 | guest can read from the medium in the host drive.</para>
|
---|
821 | </listitem>
|
---|
822 |
|
---|
823 | <listitem>
|
---|
824 | <para><emphasis role="bold">Image file</emphasis> (typically an ISO
|
---|
825 | file) gives the guest read-only access to the data in the
|
---|
826 | image.</para>
|
---|
827 | </listitem>
|
---|
828 |
|
---|
829 | <listitem>
|
---|
830 | <para><emphasis role="bold">Empty</emphasis> stands for a drive
|
---|
831 | without an inserted medium.</para>
|
---|
832 | </listitem>
|
---|
833 | </itemizedlist></para>
|
---|
834 |
|
---|
835 | <para>Changing between the above, or changing a medium in the host drive
|
---|
836 | that is accessed by a machine, or changing an image file will signal a
|
---|
837 | medium change to the guest operating system, which can then react to the
|
---|
838 | change (e.g. by starting an installation program).</para>
|
---|
839 |
|
---|
840 | <para>Medium changes can be prevented by the guest, and VirtualBox
|
---|
841 | reflects that by locking the host drive if appropriate. You can force a
|
---|
842 | medium removal in such situations via the VirtualBox GUI or the VBoxManage
|
---|
843 | command line tool. Effectively this is the equivalent of the emergency
|
---|
844 | eject which many CD/DVD drives provide, with all associated side effects:
|
---|
845 | the guest OS can issue error messages, just like on real hardware, and
|
---|
846 | guest applications may misbehave. Use this with caution.<note>
|
---|
847 | <para>The identification string of the drive provided to the guest
|
---|
848 | (which, in the guest, would be displayed by configuration tools such
|
---|
849 | as the Windows Device Manager) is always "VBOX CD-ROM", irrespective
|
---|
850 | of the current configuration of the virtual drive. This is to prevent
|
---|
851 | hardware detection from being triggered in the guest operating system
|
---|
852 | every time the configuration is changed.</para>
|
---|
853 | </note></para>
|
---|
854 |
|
---|
855 | <para>The standard CD/DVD emulation allows for reading standard data CD
|
---|
856 | and DVD formats only. As an experimental feature, for additional
|
---|
857 | capabilities, it is possible to give the guest direct access to the CD/DVD
|
---|
858 | host drive by enabling "passthrough" mode. Depending on the host hardware,
|
---|
859 | this may enable three things to work, potentially:<itemizedlist>
|
---|
860 | <listitem>
|
---|
861 | <para>CD/DVD writing from within the guest, if the host DVD drive is
|
---|
862 | a CD/DVD writer;</para>
|
---|
863 | </listitem>
|
---|
864 |
|
---|
865 | <listitem>
|
---|
866 | <para>playing audio CDs;</para>
|
---|
867 | </listitem>
|
---|
868 |
|
---|
869 | <listitem>
|
---|
870 | <para>playing encrypted DVDs.</para>
|
---|
871 | </listitem>
|
---|
872 | </itemizedlist></para>
|
---|
873 |
|
---|
874 | <para>There is a "Passthrough" checkbox in the GUI dialog for configuring
|
---|
875 | the media attached to a storage controller, or you can use the
|
---|
876 | <computeroutput>--passthrough</computeroutput> option with
|
---|
877 | <computeroutput>VBoxManage storageattach</computeroutput>; see <xref
|
---|
878 | linkend="vboxmanage-storageattach" /> for details.</para>
|
---|
879 |
|
---|
880 | <para>Even if pass-through is enabled, unsafe commands, such as updating
|
---|
881 | the drive firmware, will be blocked. Video CD formats are never supported,
|
---|
882 | not even in passthrough mode, and cannot be played from a virtual
|
---|
883 | machine.</para>
|
---|
884 |
|
---|
885 | <para>On Solaris hosts, pass-through requires running VirtualBox with real
|
---|
886 | root permissions due to security measures enforced by the host.</para>
|
---|
887 | </sect1>
|
---|
888 |
|
---|
889 | <sect1 id="storage-iscsi">
|
---|
890 | <title>iSCSI servers</title>
|
---|
891 |
|
---|
892 | <para>iSCSI stands for "Internet SCSI" and is a standard that allows for
|
---|
893 | using the SCSI protocol over Internet (TCP/IP) connections. Especially
|
---|
894 | with the advent of Gigabit Ethernet, it has become affordable to attach
|
---|
895 | iSCSI storage servers simply as remote hard disks to a computer network.
|
---|
896 | In iSCSI terminology, the server providing storage resources is called an
|
---|
897 | "iSCSI target", while the client connecting to the server and accessing
|
---|
898 | its resources is called "iSCSI initiator".</para>
|
---|
899 |
|
---|
900 | <para>VirtualBox can transparently present iSCSI remote storage to a
|
---|
901 | virtual machine as a virtual hard disk. The guest operating system will
|
---|
902 | not see any difference between a virtual disk image (VDI file) and an
|
---|
903 | iSCSI target. To achieve this, VirtualBox has an integrated iSCSI
|
---|
904 | initiator.</para>
|
---|
905 |
|
---|
906 | <para>VirtualBox's iSCSI support has been developed according to the iSCSI
|
---|
907 | standard and should work with all standard-conforming iSCSI targets. To
|
---|
908 | use an iSCSI target with VirtualBox, you must use the command line; see
|
---|
909 | <xref linkend="vboxmanage-storageattach" />.</para>
|
---|
910 | </sect1>
|
---|
911 | </chapter>
|
---|