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5 Virtual storage
CD-ROM/DVD drive, this leaves you with up to three virtual hard disks that you
can attach to a virtual machine’s IDE controller.
So even if your guest operating system has no support for SCSI or SATA devices,
it should always be able to see the default IDE controller that is enabled by
default. Of the four slots attached to it, one is normally used when you create
a virtual machine with the “New Virtual Machine” wizard of the graphical user
interface.
• SCSI is another established industry standard, standing for “Small Computer
System Interface”. This was established 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 option-
ally supports LsiLogic and BusLogic SCSI controllers, to which up to 16 virtual
hard disks can be attached.
To enable the SCSI controller, on the “Hard Disks” page of a virtual machine’s
settings dialog, check the “Enable Additional Controller” box and select one of
the two SCSI modes from the list below. After this, the additional controller will
appear as a separate PCI device in the virtual machine.
Warning: There are limitations with the default SCSI drivers shipped with
some operating systems: the standard Windows XP driver for the LsiLogic
controller does not detect a hard disk attached to the controller’s first port,
and the BusLogic controller does not work with Windows NT4 guests.
• Finally, Serial ATA (SATA) is a newer standard introduced in 2003. Compared
to IDE, it supports both much higher speeds and more devices per hard disk con-
troller. Also, with real 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).
For compatibility reasons, AHCI controllers by default operate the disks attached
to it in a so-called IDE compatibility mode, unless SATA support is explicitly
requested. “IDE compatibility mode” means that the BIOS can operate these
drives. Disks assigned to those slots will operate in full-speed AHCI mode once
the guest operating system has loaded its AHCI device driver.
Like a real SATA controller, VirtualBox’s virtual SATA controller operates faster
and also consumes less 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 IDE. Of these, the first four (numbered 0-3 in the graphical user
interface) are operated in IDE compatibility mode by default.
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