Disk Fundamentals

More than one platter (round cylinders)

Websites with interesting info Putting a Window in a Disk Drive – Looking at OLD disk drives – tmhttp:// tm A 36-GB, RPM, IBM SCSI server Hard Drive –

A Hard Drive

Labeled Disk Drive Picture

Tracks A typical hard drive is made up of multiple platters attached to a spindle that rotates at a constant speed. The read/write heads record magnetic pulses, moving in toward the center and out toward the rim as a group, in small steps. The surface of a disk is formatted into invisible concentric bands called tracks. The outside track of a disk is track 0, increasing in number when moving toward the center.

Cylinders, Sectors, Physical Disk Geometry Cylinders refers to all tracks accessible from a single position of the read/write heads. A file is initially stored on a disk using adjacent cylinders. This reduces the amount of movement of the heads. A sector is a 512-byte portion of a track. A hard-disk may have 63 or more sectors per track. Servo-formatting includes separating the sectors using Gray codes. Physical disk geometry consists of the number of cylinders per disk, the number of read/write heads per cylinder, and the number of sectors per track.

Disk Tracks and Sectors

Low-Level Formatting (which is not the same as servo-formatting) matLow-c.htmlhttp:// matLow-c.html

Fragmentation As files become more spread out around a disk, the sectors associated with the files are no longer in contiguous areas of the disk. This slows down the reading and writing of files, and makes the data more susceptible to errors.

Translation to Logical Sector Numbers Hard drive controllers perform a process called translation, the conversion of physical disk geometry to a logical structure that is understood by the operating system. Logical sector numbers are always numbered sequentially, starting at zero.

Hard Drive Controller A Controller – Control Circuitry – –(link at bottom) The Flow of Information over the Hard Drive Interface – Firmware –

Disk Partitions A single hard drive can be divided into one or more logical units (partitions) or volumes. Each formatted partition is represented by a separate drive letter (C, D, E). It can be formatted using one of several file systems.

Drive Partitions Partitions can be primary or extended Up to 4 primary partitions Extended partitions can be divided into up to 24 logical partitions. Primary partitions can be made bootable, logical partitions cannot be made bootable. ons.htmhttp:// ons.htm Diskmgmt.exe Only one primary partition can be active at one time, this is called the system partition.

Master Boot Record (MBR) Created when the first partition is created. Located in the drive’s first logical sector. Contains: –The disk partition table, which describes the sizes and locations of all partitions on the disk. –A small program that locater the partition’s boot sector and transfers control to a program in the sector that loads the operating system.

File Systems So far, we have been dealing with the hardware level of disk storage systems. (platters, sides, tracks, cylinders, and sectors) At the software level are clusters (a mapping of logical sectors) and filenames (a mapping of clusters).

Clusters The smallest unit of space used by a file. Consists of one or more disk sectors. The size of a cluster depends on both type of file system in use and the size of its disk partition. A file system stores each file as a linked sequence sequence of clusters. A chain of clusters is referenced by a file allocation table (FAT) that keeps track of all clusters used by a file.

FAT16 Only available format of hard drives formatted under MS-DOS. Supported by all versions of MS-Windows

NTFS Supported by Windows NT, 2000, and XP Handles very large volumes Default cluster size is 4KB for disks over 2GB Supports unicode filenames up to 255 characters long. …

FAT Each file is represented in the file allocation table as a linked list, called a cluster chain. Each entry contains an integer that identifies the next entry. The eof is the last FAT entry - is a predefined integer that tells the OS that the final entry has been reached. Scandisk can be used to rebuild the FAT and defragment all files.