Table of Contents
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:
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 Section 5.2, “Disk image files (VDI, VMDK, VHD, HDD)”.
Alternatively, if you have iSCSI storage servers, you can attach such a server to VirtualBox as well; this is described in Section 5.10, “iSCSI servers”.
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 Section 9.9.1, “Using a raw host hard disk from a guest”.
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.
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.
IDE (ATA) 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.
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.)
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.
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.
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.
Serial ATA (SATA) is a newer standard introduced in 2003. Compared to IDE, it supports both much higher speeds and more devices per controller. Also, with physical hardware, devices can be added and removed while the system is running. The standard interface for SATA controllers is called Advanced Host Controller Interface (AHCI).
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).
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.
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, there is no support for AHCI in Windows before Windows Vista, 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.
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.
To change the IDE compatibility mode settings for the SATA controller, please see Section 8.19, “VBoxManage storagectl”.
SCSI 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.
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.
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.
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.
Serial Attached SCSI (SAS) 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.
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.
As with SATA, the SAS controller will only be seen by operating systems with device support for it. In particular, there is no support for SAS in Windows before Windows Vista, so Windows XP (even SP3) will not see such disks unless you install additional drivers.
The USB mass storage device class is a standard to connect external storage devices like hard disksor flash drives to a host through USB. All major operating systems support these devices for a long time and ship generic drivers making third-party drivers superfluous. In particular legacy operating systems without support for SATA controllers may benefit from USB mass storage devices.
The virtual USB storage controller offered by VirtualBox works different than the other storage controller types: When storage controllers appear as a single PCI device to the guest with multiple disks attached to it, the USB-based storage controller does not appear as virtual storage controller. Each disk attached to the controller appears as a dedicated USB device to the guest.
Booting from drives attached via USB is not supported as the BIOS lacks USB support.
In summary, VirtualBox gives you the following categories of virtual storage slots:
four slots attached to the traditional IDE controller, which are always present (one of which typically is a virtual CD/DVD drive);
30 slots attached to the SATA controller, if enabled and supported by the guest operating system;
15 slots attached to the SCSI controller, if enabled and supported by the guest operating system;
eight slots attached to the SAS controller, if enabled and supported by the guest operating system.
eight slots attached to the virtual USB controller, if enabled and supported by the guest operating system.
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.
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.
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 Section 8.23, “VBoxManage modifyhd” for details.
VirtualBox supports four variants of disk image files:
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.
VirtualBox also fully supports the popular and open VMDK container format that is used by many other virtualization products, in particular, by VMware.
VirtualBox also fully supports the VHD format used by Microsoft.
Image files of Parallels version 2 (HDD format) are also supported. 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.
Irrespective of the disk capacity and format, as briefly mentioned in Section 1.7, “Creating your first virtual machine”, there are two options of how to create a disk image: fixed-size or dynamically allocated.
If you create a fixed-size image, an image file will be created on your host system which has roughly the same size as the virtual disk's capacity. So, for a 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.
For more flexible storage management, use a dynamically allocated image. This will initially be very small and not occupy any space for unused virtual disk sectors, but will grow every time a disk sector is written to for the first time, until the drive reaches the maximum capacity chosen when the drive was created. While this format takes less space initially, the fact that VirtualBox needs to expand the image file consumes additional computing resources, so until the disk file size has stabilized, write operations may be slower than with fixed size disks. However, after a time the rate of growth will slow and the average penalty for write operations will be negligible.
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:
all media currently attached to virtual machines;
"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 Section 10.1, “Where VirtualBox stores its files”.
The known media can be viewed and changed in the Virtual Media Manager, which you can access from the "File" menu in the VirtualBox main window:
The known media are conveniently grouped in three tabs for the three possible formats. These formats are:
Hard disk images, either in VirtualBox's own Virtual Disk Image (VDI) format or in the third-party formats listed in the previous chapter;
CD/DVD images in standard ISO format;
floppy images in standard RAW format.
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.
The Virtual Media Manager allows you to
remove an image from the registry (and optionally delete the image file when doing so);
"release" an image, that is, detach it from a virtual machine if it is currently attached to one as a virtual hard disk.
Starting with version 4.0, to create new disk images, 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.
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.
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 Section 5.6, “Cloning disk images” 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 Section 1.14, “Importing and exporting virtual machines”.
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.
By default, images are in "normal" mode. To mark an existing image
with one of the non-standard modes listed below, use
VBoxManage modifyhd; see Section 8.23, “VBoxManage modifyhd”. Alternatively, use VBoxManage to attach
the image to a VM and use the
argument; see Section 8.18, “VBoxManage storageattach”.
With normal images (the default setting), there are no restrictions on how guests can read from and write to the disk.
When you take a snapshot of your virtual machine as described in Section 1.10, “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.
(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.)
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.
By contrast, write-through hard disks are completely unaffected by snapshots: their state is not saved when a snapshot is taken, and not restored when a snapshot is restored.
Shareable hard disks are a variant of write-through hard disks. In principle they behave exactly the same, i.e. their state is not 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.
This is an expert feature, and misuse can lead to data loss -- regular filesystems are not prepared to handle simultaneous changes by several parties.
Next, immutable images 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.
Creating 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.
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).
As a special exception, immutable images are not 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.
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. 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.
If the automatic discarding of the differencing image on VM
startup does not fit your needs, you can turn it off using the
autoreset parameter of
VBoxManage modifyhd; see Section 8.23, “VBoxManage modifyhd” for details.
An image in multiattach mode 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.
Technically, a "multiattach" image behaves identically to an "immutable" image except the differencing image is not reset every time the machine starts.
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.
Finally, the read-only image is used automatically for CD/DVD images, since CDs/DVDs can never be written to.
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.
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.
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.
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".
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.
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.
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:
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.
Differencing images are used in the following situations:
Snapshots. 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.
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.
If you now restore a snapshot -- that is, if you want to go back to the exact machine state that was stored in the snapshot --, the following happens:
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.
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.
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.
If you later delete 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.
Immutable images. 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.
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 only make copies of virtual disk images using the utility supplied with VirtualBox; see Section 8.24, “VBoxManage clonehd”. 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.
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
/boot/grub/menu.lst). The disk ID looks
The ID for the copied image can be determined with
hdparm -i /dev/sda
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.
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.
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 Section 5.10, “iSCSI servers”).
While buffering is a useful default setting for virtualizating a few machines on a desktop computer, there are some disadvantages to this approach:
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.
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.
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.
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.
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.
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:
VBoxManage storagectl "VM name" --name <controllername> --hostiocache off
See Section 8.19, “VBoxManage storagectl” for details.
For the above reasons also, VirtualBox now uses SATA controllers by default for new virtual machines.
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.
Limits are configured through
VBoxManage. 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:
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
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.
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:
VBoxManage bandwidthctl "VM name" set Limit --limit 10M
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:
Host Drive defines that the guest can read from the medium in the host drive.
Image file (typically an ISO file) gives the guest read-only access to the data in the image.
Empty stands for a drive without an inserted medium.
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).
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.
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.
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:
CD/DVD writing from within the guest, if the host DVD drive is a CD/DVD writer;
playing audio CDs;
playing encrypted DVDs.
There is a "Passthrough" checkbox in the GUI dialog for configuring
the media attached to a storage controller, or you can use the
--passthrough option with
VBoxManage storageattach; see Section 8.18, “VBoxManage storageattach” for details.
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.
On Solaris hosts, pass-through requires running VirtualBox with real root permissions due to security measures enforced by the host.
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".
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.
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 Section 8.18, “VBoxManage storageattach”.
 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.
 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.
 VirtualBox recommends the Intel Matrix Storage drivers which can be downloaded from http://downloadcenter.intel.com/Product_Filter.aspx?ProductID=2101.
 Image resizing was added with VirtualBox 4.0.
 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.
 Support was added with VirtualBox 3.1.
 This restriction is more lenient now than it was before VirtualBox 2.2. Previously, each "normal" disk image could only be attached to one single machine. Now it can be attached to more than one machine so long as only one of these machines is running.
 This behavior also changed with VirtualBox 2.2. Previously, the differencing images were discarded when the machine session ended; now they are discarded every time the machine is powered on.