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<chapter id="storage">

<title>Virtual storage</title>

<para>As the virtual machine will most probably expect to see a hard disk

built into its virtual computer, VirtualBox must be able to present "real"

storage to the guest as a virtual hard disk. There are presently three

methods in which to achieve this:</para>

<orderedlist>

<listitem>

<para>Most commonly, VirtualBox will use large image files on a real

hard disk and present them to a guest as a virtual hard disk. This is

described in <xref linkend="vdidetails" />.</para>

</listitem>

<listitem>

<para>Alternatively, if you have iSCSI storage servers, you can attach

such a server to VirtualBox as well; this is described in <xref

linkend="storage-iscsi" />.</para>

</listitem>

<listitem>

<para>Finally, as an advanced feature, you can allow a virtual

machine to access one of your host disks directly; this advanced feature

is described in <xref linkend="rawdisk" />.</para>

</listitem>

</orderedlist>

<para>Each such virtual storage device (image file, iSCSI target or physical

hard disk) will need to be connected to the virtual hard disk controller

that VirtualBox presents to a virtual machine. This is explained in the next

section.</para>

<sect1 id="harddiskcontrollers">

<title>Hard disk controllers: IDE, SATA (AHCI), SCSI, SAS, USB MSC</title>

<para>In a real PC, hard disks and CD/DVD drives are connected to a device

called hard disk controller which drives hard disk operation and data

transfers. VirtualBox can emulate the five most common types of hard disk

controllers typically found in today's PCs: IDE, SATA (AHCI), SCSI,

SAS and USB based mass storage devices.<footnote>

<para>SATA support was added with VirtualBox 1.6; experimental SCSI

support was added with 2.1 and fully implemented with 2.2. Generally,

storage attachments were made much more flexible with VirtualBox 3.1;

see below. Support for the LSI Logic SAS controller was added with

VirtualBox 3.2; USB mass storage devices are supported since

VirtualBox 5.0.</para>

</footnote><itemizedlist>

<listitem>

<para><emphasis role="bold">IDE (ATA)</emphasis> controllers are a

backwards compatible yet very advanced extension of the disk

controller in the IBM PC/AT (1984). Initially, this interface

worked only with hard disks, but was later extended to also support

CD-ROM drives and other types of removable media. In physical PCs,

this standard uses flat ribbon parallel cables with 40 or 80 wires.

Each such cable can connect two devices to a controller, which have

traditionally been called "master" and "slave". Typical PCs had

two connectors for such cables; as a result, support for up to four

IDE devices was most common.</para>

<para>In VirtualBox, each virtual machine may have one IDE

contoller enabled, which gives you up to four virtual storage

devices that you can attach to the machine. (By default, one of

these four -- the secondary master -- is preconfigured to be the

machine's virtual CD/DVD drive, but this can be changed.<footnote>

<para>The assignment of the machine's CD/DVD drive to the

secondary master was fixed before VirtualBox 3.1; it is now

changeable, and the drive can be at other slots of the IDE

controller, and there can be more than one such drive.</para>

</footnote>)</para>

<para>So even if your guest operating system has no support for SCSI

or SATA devices, it should always be able to see an IDE controller.

</para>

<para>You can also select which exact type of IDE controller

hardware VirtualBox should present to the virtual machine (PIIX3,

PIIX4 or ICH6). This makes no difference in terms of performance,

but if you import a virtual machine from another virtualization

product, the operating system in that machine may expect a

particular controller type and crash if it isn't found.</para>

<para>After you have created a new virtual machine with the "New

Virtual Machine" wizard of the graphical user interface, you will

typically see one IDE controller in the machine's "Storage" settings

where the virtual CD/DVD drive will be attached to one of the four

ports of this controller.</para>

</listitem>

<listitem>

<para><emphasis role="bold">Serial ATA (SATA)</emphasis> is a newer

standard introduced in 2003. Compared to IDE, it supports both much

higher speeds and more devices per controller. Also, with

physical hardware, devices can be added and removed while the system

is running. The standard interface for SATA controllers is called

Advanced Host Controller Interface (<emphasis

role="bold">AHCI</emphasis>).</para>

<para>Like a real SATA controller, VirtualBox's virtual SATA

controller operates faster and also consumes fewer CPU resources than

the virtual IDE controller. Also, this allows you to connect up to

30 virtual hard disks to one machine instead of just three, as with

the VirtualBox IDE controller (with the DVD drive already attached).</para>

<para>For this reason, starting with version 3.2 and depending on

the selected guest operating system, VirtualBox uses SATA as the

default for newly created virtual machines. One virtual SATA

controller is created by default, and the default disk that is

created with a new VM is attached to this controller.<warning>

<para>The entire SATA controller and the virtual disks attached

to it (including those in IDE compatibility mode) will not be

seen by operating systems that do not have device support for

AHCI. In particular, <emphasis role="bold">there is no support

for AHCI in Windows before Windows Vista</emphasis>, so Windows

XP (even SP3) will not see such disks unless you install

additional drivers. It is possible to switch from IDE to SATA

after installation by installing the SATA drivers and changing

the controller type in the VM settings dialog.<footnote>

<para>VirtualBox recommends the Intel Matrix Storage drivers

which can be downloaded from <ulink

url="http://downloadcenter.intel.com/Product_Filter.aspx?ProductID=2101">http://downloadcenter.intel.com/Product_Filter.aspx?ProductID=2101</ulink>.</para>

</footnote></para>

</warning></para>

<para>To add a SATA controller to a machine for which it has not

been enabled by default (either because it was created by an earlier

version of VirtualBox, or because SATA is not supported by default

by the selected guest operating system), go to the "Storage" page of

the machine's settings dialog, click on the "Add Controller" button

under the "Storage Tree" box and then select "Add SATA Controller".

After this, the additional controller will appear 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, please see <xref

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

</listitem>

<listitem>

<para><emphasis role="bold">SCSI</emphasis> is another established

industry standard, standing for "Small Computer System Interface".

SCSI was standardized as early as 1986 as a generic interface for

data transfer between all kinds of devices, including storage

devices. Today SCSI is still used for connecting hard disks and tape

devices, but it has mostly been displaced in commodity hardware. It

is still in common use in high-performance workstations and

servers.</para>

<para>Primarily for compatibility with other virtualization

software, VirtualBox optionally supports LSI Logic and BusLogic SCSI

controllers, to each of which up to 15 virtual hard disks can be

attached.</para>

<para>To enable a SCSI controller, on the "Storage" page of a

virtual machine's settings dialog, click on the "Add Controller"

button under the "Storage Tree" box and then select "Add SCSI

Controller". After this, the additional controller will appear as a

separate PCI device in the virtual machine.<warning>

<para>As with the other controller types, a SCSI controller will

only be seen by operating systems with device support for it.

Windows 2003 and later ships with drivers for the LSI Logic

controller, while Windows NT 4.0 and Windows 2000 ships with

drivers for the BusLogic controller. Windows XP ships with

drivers for neither.</para>

</warning></para>

</listitem>

<listitem>

<para><emphasis role="bold">Serial Attached SCSI (SAS)</emphasis> is

another bus standard which uses the SCSI command set. As opposed to

SCSI, however, with physical devices, serial cables are used instead

of parallel ones, which simplifies physical device connections. In

some ways, therefore, SAS is to SCSI what SATA is to IDE: it allows

for more reliable and faster connections.</para>

<para>To support high-end guests which require SAS controllers,

VirtualBox emulates a LSI Logic SAS controller, which can be enabled

much the same way as a SCSI controller. At this time, up to eight

devices can be connected to the SAS controller.</para>

<warning>

<para>As with SATA, the SAS controller will only be seen by

operating systems with device support for it. In particular,

<emphasis role="bold">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 or flash

drives to a host through USB. All major operating systems support

these devices for a long time and ship with a generic driver which

makes third party drivers unnecessary.

The virtual USB storage controller offered by VirtualBox works

different than the other storage controller types. Unlike the

controllers which appear as a single PCI device to the guest with

multiple disks attached to it, the USB based storage controller does

not appear directly in the guest. Each disk attached to the controller

appears as a different USB device to the guest.</para>

<warning>

<para>As of version 5.0 VirtualBox does not support booting from

drives attached via USB.</para>

</warning>

</listitem>

</itemizedlist></para>

<para>In summary, VirtualBox gives you the following categories of virtual

storage slots:<orderedlist>

<listitem>

<para>four slots attached to the traditional IDE controller, which

are always present (one of which typically is a virtual CD/DVD

drive);</para>

</listitem>

<listitem>

<para>30 slots attached to the SATA controller, if enabled and

supported by the guest operating system;</para>

</listitem>

<listitem>

<para>15 slots attached to the SCSI controller, if enabled and

supported by the guest operating system;</para>

</listitem>

<listitem>

<para>eight slots attached to the SAS controller, if enabled and

supported by the guest operating system.</para>

</listitem>

<listitem>

<para>eight slots attached to the virtual USB controller, if enabled and

supported by the guest operating system.</para>

</listitem>

</orderedlist></para>

<para>Given this large choice of storage controllers, you may ask yourself

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 for VirtualBox for compatibility with existing hardware and other

hypervisors.</para>

</sect1>

<sect1 id="vdidetails">

<title>Disk image files (VDI, VMDK, VHD, HDD)</title>

<para>Disk image files reside on the host system and are seen by the guest

systems as hard disks of a certain geometry. When a guest operating system

reads from or writes to a hard disk, VirtualBox redirects the request to

the image file.</para>

<para>Like a physical disk, a virtual disk has a size (capacity), which

must be specified when the image file is created. As opposed to a physical

disk however, VirtualBox allows you to expand an image file after

creation, even if it has data already; see <xref

linkend="vboxmanage-modifyvdi" /> for details.<footnote>

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

</footnote></para>

<para>VirtualBox supports four variants of disk image files:<itemizedlist>

<listitem>

<para>Normally, VirtualBox uses its own container format for guest

hard disks -- Virtual Disk Image (VDI) files. In particular, this

format will be used when you create a new virtual machine with a new

disk.</para>

</listitem>

<listitem>

<para>VirtualBox also fully supports the popular and open VMDK

container format that is used by many other virtualization products,

in particular, by VMware.<footnote>

<para>Initial support for VMDK was added with VirtualBox 1.4;

since version 2.1, VirtualBox supports VMDK fully, meaning that

you can create snapshots and use all the other advanced features

described above for VDI images with VMDK also.</para>

</footnote></para>

</listitem>

<listitem>

<para>VirtualBox also fully supports the VHD format used by

Microsoft.</para>

</listitem>

<listitem>

<para>Image files of Parallels version 2 (HDD format) are also

supported.<footnote>

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

</footnote> For lack of documentation of the format, newer formats

(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>

<para>Irrespective of the disk capacity and format, as briefly mentioned

in <xref linkend="gui-createvm" />, there are two options of how to create

a disk image: fixed-size or dynamically allocated.</para>

<itemizedlist>

<listitem>

<para>If you create a <emphasis role="bold">fixed-size

image</emphasis>, 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 10G disk, you will have a 10G 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>For more flexible storage management, use a <emphasis

role="bold">dynamically allocated image</emphasis>. This will

initially be very small and not occupy any space for unused virtual

disk sectors, but will grow every time a disk sector is written to for

the first time, until the drive reaches the maximum capacity chosen

when the drive was created. While this format takes less space

initially, the fact that VirtualBox needs to expand the image file

consumes additional computing resources, so until the disk file size has

stabilized, write operations may be slower than with fixed size disks.

However, after a time the rate of growth will slow and the average penalty

for write operations will be negligible.</para>

</listitem>

</itemizedlist>

</sect1>

<sect1 id="vdis">

<title>The Virtual Media Manager</title>

<para>VirtualBox keeps track of all the hard disk, CD/DVD-ROM and floppy

disk images which are in use by virtual machines. These are often referred

to as "known media" and come from two sources:<itemizedlist>

<listitem>

<para>all media currently attached to virtual machines;</para>

</listitem>

<listitem>

<para>"registered" media for compatibility with VirtualBox versions

older than version 4.0. For details about how media registration has

changed with version 4.0, please refer to <xref

linkend="vboxconfigdata" />.</para>

</listitem>

</itemizedlist></para>

<para>The known media can be viewed and changed in the <emphasis

role="bold">Virtual Media Manager</emphasis>, which you can access from

the "File" menu in the VirtualBox main window:</para>

<para><mediaobject>

<imageobject>

<imagedata align="center" fileref="images/virtual-disk-manager.png"

width="12cm" />

</imageobject>

</mediaobject>The known media are conveniently grouped in three tabs for

the three possible formats. These formats are:</para>

<itemizedlist>

<listitem>

<para>Hard disk images, either in VirtualBox's own Virtual Disk Image

(VDI) format or in the third-party formats listed in the previous

chapter;</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>As you can see in the screenshot above, for each image, the Virtual

Media Manager shows you the full path of the image file and other

information, such as the virtual machine the image is currently attached

to, if any.</para>

<para>The Virtual Media Manager allows you to</para>

<itemizedlist>

<listitem>

<para><emphasis role="bold">remove</emphasis> an image from the

registry (and optionally delete the image file when doing so);</para>

</listitem>

<listitem>

<para><emphasis role="bold">"release"</emphasis> an image, that is,

detach it from a virtual machine if it is currently attached to one as

a virtual hard disk.</para>

</listitem>

</itemizedlist>

<para>Starting with version 4.0, to <emphasis role="bold">create new disk

images,</emphasis> please use the "Storage" page in a virtual machine's

settings dialog because disk images are now by default stored in each

machine's own folder.</para>

<para>Hard disk image files can be copied onto other host systems and

imported into virtual machines there, although certain guest systems

(notably Windows 2000 and XP) will require that the new virtual machine be

set up in a similar way to the old one.<note>

<para>Do not simply make copies of virtual disk images. If you import

such a second copy into a virtual machine, VirtualBox will complain

with an error, since VirtualBox assigns a unique identifier (UUID) to

each disk image to make sure it is only used once. See <xref

linkend="cloningvdis" /> for instructions on this matter. Also, if you

want to copy a virtual machine to another system, VirtualBox has an

import/export facility that might be better suited for your needs; see

<xref linkend="ovf" />.</para>

</note></para>

</sect1>

<sect1 id="hdimagewrites">

<title>Special image write modes</title>

<para>For each virtual disk image supported by VirtualBox, you can

determine separately how it should be affected by write operations from a

virtual machine and snapshot operations. This applies to all of the

aforementioned image formats (VDI, VMDK, VHD or HDD) and irrespective of

whether an image is fixed-size or dynamically allocated.</para>

<para>By default, images are in "normal" mode. To mark an existing image

with one of the non-standard modes listed below, use

<computeroutput>VBoxManage modifyhd</computeroutput>; see <xref

linkend="vboxmanage-modifyvdi" />. Alternatively, use VBoxManage to attach

the image to a VM and use the <computeroutput>--mtype</computeroutput>

argument; see <xref linkend="vboxmanage-storageattach" />.</para>

<orderedlist>

<listitem>

<para>With <emphasis role="bold">normal images</emphasis> (the default

setting), there are no restrictions on how guests can read from and

write to the disk.</para>

<para>When you take a snapshot of your virtual machine as described in

<xref linkend="snapshots" />, the state of such a "normal hard disk"

will be recorded together with the snapshot, and when reverting to the

snapshot, its state will be fully reset.</para>

<para>(Technically, strictly speaking, the image file itself is not

"reset". Instead, when a snapshot is taken, VirtualBox "freezes" the

image file and no longer writes to it. For the write operations from

the VM, a second, "differencing" image file is created which receives

only the changes to the original image; see the next section for

details.)</para>

<para>While you can attach the same "normal" image to more than one

virtual machine, only one of these virtual machines attached to the

same image file can be executed simultaneously, as otherwise there

would be conflicts if several machines write to the same image

file.<footnote>

<para>This restriction is more lenient now than it was before

VirtualBox 2.2. Previously, each "normal" disk image could only be

<emphasis>attached</emphasis> to one single machine. Now it can be

attached to more than one machine so long as only one of these

machines is running.</para>

</footnote></para>

</listitem>

<listitem>

<para>By contrast, <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, i.e. their state is <emphasis>not</emphasis> saved when a

snapshot is taken, and not restored when a snapshot is restored. The

difference only shows if you attach such disks to several VMs.

Shareable disks may be attached to several VMs which may run

concurrently. This makes them suitable for use by cluster filesystems

between VMs and similar applications which are explicitly prepared to

access a disk concurrently. Only fixed size images can be used in this

way, and dynamically allocated images are rejected.<warning>

<para>This is an expert feature, and misuse can lead to data loss

-- regular filesystems are not prepared to handle simultaneous

changes by several parties.</para>

</warning></para>

</listitem>

<listitem>

<para>Next, <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><emphasis>Creating</emphasis> an immutable image makes little

sense since it would be initially empty and lose its contents with

every machine restart (unless you really want to have a disk that is

always unformatted when the machine starts up). As a result, normally,

you would first create a "normal" image and then, when you deem its

contents useful, later mark it immutable.</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

whose last snapshot was taken while the machine was running (a

so-called "online" snapshot). As a result, if the machine's current

snapshot is such an "online" snapshot, its immutable images behave

exactly like the "normal" images described previously. To re-enable

the automatic resetting of such images, delete the current snapshot

of the machine.</para>

</note>

<para>Again, technically, VirtualBox never writes to an immutable

image directly at all. All write operations from the machine will be

directed to a differencing image; the next time the VM is powered on,

the differencing image is reset so that every time the VM starts, its

immutable images have exactly the same content.<footnote>

<para>This behavior also changed with VirtualBox 2.2. Previously,

the differencing images were discarded when the machine session

<emphasis>ended</emphasis>; now they are discarded every time the

machine is powered on.</para>

</footnote> The differencing image is only reset when the machine is

powered on from within VirtualBox, not when you reboot by requesting a

reboot from within the machine. This is also why immutable images

behave as described above when snapshots are also present, which use

differencing images as well.</para>

<para>If the automatic discarding of the differencing image on VM

startup does not fit your needs, you can turn it off using the

<computeroutput>autoreset</computeroutput> parameter of

<computeroutput>VBoxManage modifyhd</computeroutput>; see <xref

linkend="vboxmanage-modifyvdi" /> for details.</para>

</listitem>

<listitem>

<para>An image in <emphasis role="bold">multiattach mode</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 reletively small and the shared content is stored

only once at the host.</para>

</listitem>

<listitem>

<para>Finally, the <emphasis role="bold">read-only image</emphasis> is

used automatically for CD/DVD images, since CDs/DVDs can never be

written to.</para>

</listitem>

</orderedlist>

<para>To illustrate the differences between the various types with respect

to snapshots: Assume you have installed your guest operating system in

your VM, and you have taken a snapshot. Imagine you have accidentally

infected your VM with a virus and 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>Still, you might find write-through images useful if you want to

preserve critical data irrespective of snapshots, and since you can attach

more than one image to a VM, you may want to have one immutable for the

operating system and one write-through for your data files.</para>

</sect1>

<sect1 id="diffimages">

<title>Differencing images</title>

<para>The previous section hinted at differencing images and how they are

used with snapshots, immutable images and multiple disk attachments. For

the inquisitive VirtualBox user, this section describes in more detail how

they 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 "child", which holds the differences to its

"parent".</para>

<para>When a differencing image is active, it receives all write

operations from the virtual machine instead of its parent. The

differencing image only contains the sectors of the virtual hard disk that

have changed since the differencing image was created. When the machine

reads a sector from such a virtual hard disk, it looks into the

differencing image first. If the sector is present, it is returned from

there; if not, VirtualBox looks into the parent. In other words, the

parent becomes "read-only"; 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 "grandchild" of the original parent. The first differencing

image then becomes read-only as well, and write operations only go to the

second-level differencing image. When reading from the virtual disk,

VirtualBox needs to look into the second differencing image first, then

into the first if the sector was not found, and then into the original

image.</para>

<para>There can be an unlimited number of differencing images, and each

image can have more than one child. As a result, the differencing images

can form a complex tree with parents, "siblings" and children, depending

on how complex your machine configuration is. Write operations always go

to the one "active" differencing image that is attached to the machine,

and for read operations, VirtualBox may need to look up all the parents in

the chain until the sector in question is found. You can look at such a

tree in the Virtual Media Manager:<mediaobject>

<imageobject>

<imagedata align="center" fileref="images/virtual-disk-manager2.png"

width="12cm" />

</imageobject>

</mediaobject></para>

<para>In all of these situations, from the point of view of the virtual

machine, the virtual hard disk behaves like any other disk. While the

virtual machine is running, there is a slight run-time I/O overhead

because VirtualBox might need to look up sectors several times. This is

not noticeable however since the tables with sector information are always

kept in memory and can be looked up quickly.</para>

<para>Differencing images are used in the following

situations:<orderedlist>

<listitem>

<para><emphasis role="bold">Snapshots.</emphasis> When you create a

snapshot, as explained in the previous section, VirtualBox "freezes"

the images attached to the virtual machine and creates differencing

images for each of them (to be precise: one for each image that is

not in "write-through" mode). From the point of view of the virtual

machine, the virtual disks continue to operate before, but all write

operations go into the differencing images. Each time you create

another snapshot, for each hard disk attachment, another

differencing image is created and attached, forming a chain or

tree.</para>

<para>In the above screenshot, you see that the original disk image

is now attached to a snapshot, representing the state of the disk

when the snapshot was taken.</para>

<para>If you now <emphasis role="bold">restore</emphasis> a snapshot

-- that is, if you want to go back to the exact machine state that

was stored in the snapshot --, the following happens:<orderedlist>

<listitem>

<para>VirtualBox copies the virtual machine settings that were

copied into the snapshot back to the virtual machine. As a

result, if you have made changes to the machine configuration

since taking the snapshot, they are undone.</para>

</listitem>

<listitem>

<para>If the snapshot was taken while the machine was running,

it contains a saved machine state, and that state is restored

as well; after restoring the snapshot, the machine will then

be in "Saved" state and resume execution from there when it is

next started. Otherwise the machine will be in "Powered Off"

state and do a full boot.</para>

</listitem>

<listitem>

<para>For each disk image attached to the machine, the

differencing image holding all the write operations since the

current snapshot was taken is thrown away, and the original

parent image is made active again. (If you restored the "root"

snapshot, then this will be the root disk image for each

attachment; otherwise, some other differencing image descended

from it.) This effectively restores the old machine

state.</para>

</listitem>

</orderedlist></para>

<para>If you later <emphasis role="bold">delete</emphasis> a

snapshot in order to free disk space, for each disk attachment, one

of the differencing images becomes obsolete. In this case, the

differencing image of the disk attachment cannot simply be deleted.

Instead, VirtualBox needs to look at each sector of the differencing

image and needs to copy it back into its parent; this is called

"merging" images and can be a potentially lengthy process, depending

on how large the differencing image is. It can also temporarily need

a considerable amount of extra disk space, before the differencing

image obsoleted by the merge operation is deleted.</para>

</listitem>

<listitem>

<para><emphasis role="bold">Immutable images.</emphasis> When an

image is switched to "immutable" mode, a differencing image is

created as well. As with snapshots, the parent image then becomes

read-only, and the differencing image receives all the write

operations. Every time the virtual machine is started, all the

immutable images which are attached to it have their respective

differencing image thrown away, effectively resetting the virtual

machine's virtual disk with every restart.</para>

</listitem>

</orderedlist></para>

</sect1>

<sect1 id="cloningvdis">

<title>Cloning disk images</title>

<para>You can duplicate hard disk image files on the same host to quickly

produce a second virtual machine with the same operating system setup.

However, you should <emphasis>only</emphasis> make copies of virtual disk

images using the utility supplied with VirtualBox; see <xref

linkend="vboxmanage-clonevdi" />. This is because VirtualBox assigns a

unique identity number (UUID) to each disk image, which is also stored

inside the image, and VirtualBox will refuse to work with two images that

use the same number. If you do accidentally try to reimport a disk image

which you copied normally, you can make a second copy using VirtualBox's

utility and import that instead.</para>

<para>Note that newer Linux distributions identify the boot hard disk from

the ID of the drive. The ID VirtualBox reports for a drive is determined

from the UUID of the virtual disk image. So if you clone a disk image and

try to boot the copied image the guest might not be able to determine its

own boot disk as the UUID changed. In this case you have to adapt the disk

ID in your boot loader script (for example

<computeroutput>/boot/grub/menu.lst</computeroutput>). The disk ID looks

like this:<screen>scsi-SATA_VBOX_HARDDISK_VB5cfdb1e2-c251e503</screen></para>

<para>The ID for the copied image can be determined with <screen>hdparm -i /dev/sda</screen></para>

</sect1>

<sect1 id="iocaching">

<title>Host I/O caching</title>

<para>Starting with version 3.2, VirtualBox can optionally disable the I/O

caching that the host operating system would otherwise perform on disk

image files.</para>

<para>Traditionally, VirtualBox has opened disk image files as normal

files, which results in them being cached by the host operating system

like any other file. The main advantage of this is speed: when the guest

OS writes to disk and the host OS cache uses delayed writing, the write

operation can be reported as completed to the guest OS quickly while the

host OS can perform the operation asynchronously. Also, when you start a

VM a second time and have enough memory available for the OS to use for

caching, large parts of the virtual disk may be in system memory, and the

VM can access the data much faster.</para>

<para>Note that this applies only to image files; buffering never occurred

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 virtualizating a few

machines on a desktop computer, there are some disadvantages to this

approach:<orderedlist>

<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 (power failure, host crash), the likelihood of

data loss increases.</para>

</listitem>

<listitem>

<para>Disk image files tend to be very large. Caching them can

therefore quickly use up the entire host OS cache. Depending on the

efficiency of the host OS caching, this may slow down the host

immensely, especially if several VMs run at the same time. For

example, on Linux hosts, host caching may result in Linux delaying

all writes until the host cache is nearly full and then writing out

all these changes at once, possibly stalling VM execution for

minutes. This can result in I/O errors in the guest as I/O requests

time out there.</para>

</listitem>

<listitem>

<para>Physical memory is often wasted as guest operating systems

typically have their own I/O caches, which may result in the data

being cached twice (in both the guest and the host caches) for

little effect.</para>

</listitem>

</orderedlist></para>

<para>If you decide to disable host I/O caching for the above reasons,

VirtualBox uses its own small cache to buffer writes, but no read caching

since this is typically already performed by the guest OS. In addition,

VirtualBox fully supports asynchronous I/O for its virtual SATA, SCSI and

SAS controllers through multiple I/O threads.</para>

<para>Since asynchronous I/O is not supported by IDE controllers, for

performance reasons, you may want to leave host caching enabled for your

VM's virtual IDE controllers.</para>

<para>For this reason, VirtualBox allows you to configure whether the host

I/O cache is used for each I/O controller separately. Either uncheck the

"Use host I/O cache" box in the "Storage" settings for a given virtual

storage controller, or use the following VBoxManage command to disable the

host I/O cache for a virtual storage controller:<screen>VBoxManage storagectl "VM name" --name &lt;controllername&gt; --hostiocache off</screen></para>

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

<para>For the above reasons also, VirtualBox now uses SATA controllers by

default for new virtual machines.</para>

</sect1>

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

<title>Limiting bandwidth for disk images</title>

<para>Starting with version 4.0, VirtualBox allows for limiting the

maximum bandwidth used for asynchronous I/O. Additionally it supports

sharing limits through bandwidth groups for several images. It is possible

to have more than one such limit.</para>

<para>Limits are configured through

<computeroutput>VBoxManage</computeroutput>. The example below creates a

bandwidth group named "Limit", sets the limit to 20 MB/s and assigns the

group to the attached disks of the VM:<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>

<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

MB/s. However, if one disk doesn't 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 MB/s:<screen>VBoxManage bandwidthctl "VM name" set Limit --limit 10M</screen></para>

</sect1>

<sect1 id="storage-cds">

<title>CD/DVD support</title>

<para>The virtual CD/DVD drive(s) by default support only reading. The

medium configuration is changeable at runtime. You can select between

three options to provide the medium data:<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> (typically an ISO

file) gives the guest read-only access to the data in the

image.</para>

</listitem>

<listitem>

<para><emphasis role="bold">Empty</emphasis> stands for a drive

without an inserted medium.</para>

</listitem>

</itemizedlist></para>

<para>Changing between the above, or changing a medium in the host drive

that is accessed by a machine, or changing an image file will signal a

medium change to the guest operating system, which can then react to the

change (e.g. by starting an installation program).</para>

<para>Medium changes can be prevented by the guest, and VirtualBox

reflects that by locking the host drive if appropriate. You can force a

medium removal in such situations via the VirtualBox GUI or the VBoxManage

command line tool. Effectively this is the equivalent of the emergency

eject which many CD/DVD drives provide, with all associated side effects:

the guest OS can issue error messages, just like on real hardware, and

guest applications may misbehave. Use this with caution.<note>

<para>The identification string of the drive provided to the guest

(which, in the guest, would be displayed by configuration tools such

as the Windows Device Manager) is always "VBOX CD-ROM", irrespective

of the current configuration of the virtual drive. This is to prevent

hardware detection from being triggered in the guest operating system

every time the configuration is changed.</para>

</note></para>

<para>The standard CD/DVD emulation allows for reading standard data CD

and DVD formats only. As an experimental feature, for additional

capabilities, it is possible to give the guest direct access to the CD/DVD

host drive by enabling "passthrough" mode. Depending on the host hardware,

this may enable three things to work, potentially:<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>

<para>There is a "Passthrough" checkbox in the GUI dialog for configuring

the media attached to a storage controller, or you can use the

<computeroutput>--passthrough</computeroutput> option with

<computeroutput>VBoxManage storageattach</computeroutput>; see <xref

linkend="vboxmanage-storageattach" /> for details.</para>

<para>Even if pass-through is enabled, unsafe commands, such as updating

the drive firmware, will be blocked. Video CD formats are never supported,

not even in passthrough mode, and cannot be played from a virtual

machine.</para>

<para>On Solaris hosts, pass-through requires running VirtualBox with real

root permissions due to security measures enforced by the host.</para>

</sect1>

<sect1 id="storage-iscsi">

<title>iSCSI servers</title>

<para>iSCSI stands for "Internet SCSI" and is a standard that allows for

using the SCSI protocol over Internet (TCP/IP) connections. Especially

with the advent of Gigabit Ethernet, it has become affordable to attach

iSCSI storage servers simply as remote hard disks to a computer network.

In iSCSI terminology, the server providing storage resources is called an

"iSCSI target", while the client connecting to the server and accessing

its resources is called "iSCSI initiator".</para>

<para>VirtualBox can transparently present iSCSI remote storage to a

virtual machine as a virtual hard disk. The guest operating system will

not see any difference between a virtual disk image (VDI file) and an

iSCSI target. To achieve this, VirtualBox has an integrated iSCSI

initiator.</para>

<para>VirtualBox's iSCSI support has been developed according to the iSCSI

standard and should work with all standard-conforming iSCSI targets. To

use an iSCSI target with VirtualBox, you must use the command line; see

<xref linkend="vboxmanage-storageattach" />.</para>

</sect1>

</chapter>