Книга: Fedora™ Unleashed, 2008 edition

The Disk as a Storage Device

The Disk as a Storage Device

Because data storage devices are central to the file system, it is important to understand the workings of the most common data storage device: the hard disk drive. Although they work with a different medium, the basic storage functions of floppy disks and removable disk drives are similar to those of the hard disk.

Mechanically, the hard drive is a metal box that encloses disks, also known as platters, which have a magnetic coating on each side. Multiple disks are typically connected to the same spindle and rotated by a motor. The read and write heads for each side of the disk are moved by a second motor to position them over the area of the disk where the data you are looking for is stored. Each platter is organized into cylinders (the default size is 512 bytes) and sectors, and each platter has a head. Each drive has some electronics on a controller card that, along with the disk controller card on the motherboard of the computer, are capable of placing the heads at the correct space to retrieve the data.

The three components, cylinders, heads, and sectors (CHS), are referred to as the drive geometry and are used to identify specific locations on the drive. The CHS information for the drive is detected by the system BIOS and passed on to the operating system.

The first sector of the disk is called the MBR, or Master Boot Record. It is the most important sector on the disk because it contains the bootloader code and the partition table (the table containing pointers to beginning and end of the logical partitions on the disk). The BIOS gets the system's hardware ready, and then executes the bootloader code. The bootloader code and the bootloader program load the kernel and turn over control of the system to the kernel. Then, Linux is on its way to providing us with one of the best operating system experiences in the world.

The MBR sector is 512 bytes long; the first 446 bytes contain the bootloader code. The next 64 bytes contain the partition table, and the final 2 bytes contain a special code (the hexadecimal values of 55 and AA, respectively) that identifies that sector as the MBR. More details about the MBR can be found Chapter 13, "Backing Up."

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