Websites with interesting info • Putting a Window in a Disk Drive • http://www.bit-tech.net/article/66/1 • Looking at OLD disk drives • http://www.duxcw.com/digest/guides/hd/hd2.htm • A 36-GB, 10000 RPM, IBM SCSI server Hard Drive • http://www.pcguide.com/ref/hdd/index.htm
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.
Low-Level Formatting (which is not the same as servo-formatting) • http://www.pcguide.com/ref/hdd/geom/formatLow-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 • http://www.pcguide.com/ref/hdd/op/logic.htm • Control Circuitry • http://www.pcguide.com/ref/hdd/op/logic.htm • (link at bottom) • The Flow of Information over the Hard Drive Interface • http://www.pcguide.com/ref/hdd/op/logic.htm • Firmware • http://www.pcguide.com/ref/hdd/op/logic.htm
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. • http://www.pcguide.com/ref/hdd/file/struct_Partitions.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.