5 Virtual storage 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: 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. Warning: 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 SP2) will not see such disks unless you install additional drivers. In summary, VirtualBox gives you the following categories of virtual storage slots: 1. four slots attached to the traditional IDE controller, which are always present (one of which typically is a virtual CD/DVD drive) 2. 30 slots attached to the SATA controller, if enabled and provided that your guest operating system can see it these slots can either be a) in IDE compatibility mode (by default, slots 0-3) or b) in SATA mode 3. 15 slots attached to the SCSI controller, if enabled and supported by the guest operating system 4. eight slots attached to the SAS 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. 5.2 Disk image files (VDI, VMDK, VHD, HDD) 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. 73