Information Technology Reference
In-Depth Information
If sufficient free memory is not available at the time, a guest machine may fail
to start with an out-of-memory error, even if it will consume only a small amount
of the memory allocated for its use.
Most guest systems run only a few applications or services. Because the host
system performs most of the real operations on behalf of the guest, smaller mem-
ory allocations for the guest can still produce excellent results. Running a guest
desktop with 512 MB or less may work well as long as enough memory is allocated
for the guest system to boot.
Because a guest operating system needs persistent storage, VirtualBox can make sev-
eral different types of host storage available to the guest in the form of virtual disks:
■
A file that contains a disk image
■
A real disk device
■
An iSCSI target
■
A CD-ROM/DVD or file containing an ISO image
■
A file containing a floppy disk image
All of these storage options must be presented to the guest OS using a virtual disk.
To facilitate this sharing, the VirtualBox virtual platform can have up to three
disk controllers. Each of these controllers can have attached devices that provide
access to the host storage.
Each virtual motherboard has a dual-channel IDE controller. Each of the two
channels has two devices: a master and a slave. By convention, the master device
on the first IDE channel is the boot disk and the master device on the second IDE
channel is a CD-ROM/DVD. Many IDE chipsets are available, and VirtualBox can
emulate an Intel PIIX3, PIIX4, or ICH6. There is no difference in the performance
between these options, but if an operating system is expecting a particular IDE
controller and sees a different one, it may not operate properly. This situation
happens most often when importing a virtual machine from another virtualiza-
tion product. To prevent this problem, set the IDE controller type to match that
of the other virtualization product.
A virtual motherboard may also have a Serial ATA (SATA) controller. Such a
controller can support up to 30 disk devices. By default, the first 4 devices operate
in legacy IDE mode, meaning that the BIOS can use them just like any other IDE
device. Once the guest operating system is up and running and has loaded the
SATA drivers for these devices, they can then be accessed in SATA mode. In addi-
tion to supporting a larger number of devices, SATA is a more efficient interface
both on the guest and in the emulation layer. SATA devices are preferred if the
operating system supports them.
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