Information Technology Reference
In-Depth Information
VirtualBox can also provide an LSI Logic or BusLogic SCSI controller, if neces-
sary. Such a controller supports up to 16 devices. It is intended to facilitate use
of legacy operating systems that do not support SATA and need more than the 4
devices provided by the IDE controller. This controller can also be used to attach
more than the 30 disks supported by the SATA controller.
Guest hard disks are generally mapped to files on the host platform that contain
a complete image of the guest disk, including the boot sector and partition table.
The disk images have a fixed geometry based on their total size. Once the disk
image is created, its size cannot be altered. When a guest reads from or writes to
the disk, VirtualBox redirects the I/O to the native file system services on the host.
VirtualBox supports four disk image file formats.
VDI, the native VirtualBox disk format. It is the default when you create a
new virtual machine or disk image.
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VMDK, a popular disk format used by VMware.
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VHD, the format used by Microsoft.
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Parallels version 2 HDD format. VirtualBox does not support newer formats,
but those can be converted to version 2 using tools supplied by Parallels.
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With each of these formats, VirtualBox can create fixed-size or dynamically
expanding disk images. Fixed-size image files are completely allocated at creation
time. This type of image file will take longer to create, because it is dependent on
the write performance of the host file system. Once in use, it will be more efficient,
as the system does not need to get new blocks as the guest writes to new storage
areas. In contrast, dynamically expanding disk images start off small and will
grow as the guest writes to new blocks on the virtual disk. These are faster to cre-
ate, but additional work is required by the host to find new blocks the first time a
guest accesses a particular part of the disk. Host file system caching strategies can
hide most of the difference in performance, especially on a host that is not heav-
ily loaded. For performance-critical applications that perform many disk writes,
fixed-size disk images are recommended. For all other uses, the convenience of
dynamically allocated images makes this approach the preferred method.
VirtualBox maintains a library of disk, CD-ROM, and floppy disk images. Before
a disk or CD-ROM image can be used by a guest, it must be registered in the Virtual
Media Manager. This can be done in the VirtualBox GUI or via the
VBoxManage
openmedium
command. Once an image is registered, it can be assigned to an open
port on any guest. Although a disk image may be connected to more than one
guest, it can be used by only one guest at a time. A guest will fail to start if one of
its disk images is connected to another guest that is currently running.
Using the
VBoxManage
command line, the following example creates a 16 GB
dynamically expanding disk image and attaches it to port 3 of the SATA controller
in the guest named Windows 7.
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