Figure 32 The BIOS hard drive configuration area

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This example shows two controllers (a primary and a secondary), with each supporting two devices (a master and a slave). This configuration allows you to connect a total of four separate devices to the workstation. Besides the hard drive, IDE controllers also support connecting CD/DVD devices, so you'll need to be careful when evaluating your disk controller situation.

If your motherboard contains only one disk controller and uses it for the hard drive and the CD/DVD device, you won't be able to add a second hard drive on that controller. Usually you can find plug-in disk controller cards to add a second controller to the workstation. You'll need to do that if you want to add another hard drive.

If your motherboard contains two disk controllers, you can purchase a second hard drive and easily connect it to one of the controllers to use for Ubuntu. The Ubuntu installation process detects the empty hard drive and will format it for Ubuntu.

If both of the disk controllers already contain one device, you'll need to use a master/ slave configuration to add the second hard drive. This process requires a controller cable that has three plugs—one for connecting to the motherboard and two connectors to plug devices into.

The devices themselves must also be specially configured for this setup. Each device uses a jumper setting to determine where in the chain it's located. One hard drive must be set as the master hard drive, and one must be set as the slave.

Many advanced server systems use a small computer system interface (SCSI) hard drive controller instead of an IDE controller. SCSI controllers typically allow SGCfGt up to seven devices per controller channel (although some newer ones allow 16

devices). These controllers are popular in servers that support multiple hard drives ^ in a redundant array of inexpensive disk (RAID) configuration. RAID systems use multiple hard drives to emulate a single hard drive for fault-tolerance purposes. ^^k^^r There are several formats of RAID support:

• RAID0: Divides data among multiple hard drives but without redundancy.

• RAID1: Writes all data to two or more drives (called mirroring).

• RAID2: Incorporates error detection codes within stored data.

• RAID3: Stores data across several drives at the byte level, reserving one drive to store a parity bit. The parity bit is used to rebuild an individual drive if it fails.

• RAID4: Stores data across several drives at the block level, reserving one drive to store a parity bit.

• RAID5: Stores data across several drives at the block level; also writes a parity bit for each block on the drives.

RAID1 is common when only two hard drives are present. One hard drive is the primary, and the other is a hot backup. The disk controller performs all disk write requests from the operating system on both hard drives. RAID3 and RAID5 are the most popular methods of RAID disk support and usually require three or more hard drives. Both of these methods can recover data from a complete failure of any one disk in the system.

Despite the "inexpensive" part of its name, RAID technology can be expensive, and it is used primarily for high-availability server environments. The Ubuntu server installation can detect and use most SCSI controllers and RAID configurations.

However, don't confuse a hardware RAID environment with a software RAID. Many Windows servers allow you to emulate a RAID environment using standard disk hardware. This is called a software RAID because there aren't multiple disks, but the operating system acts as if there are. Ubuntu will work with standard disk drives, but it won't emulate a RAID environment using them.

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