Disk Fundamentals
More than one platter
(round cylinders)
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.ht
m
• A 36-GB, 10000 RPM, IBM SCSI server
Hard Drive
– http://www.pcguide.com/ref/hdd/index.htm
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)
• http://www.pcguide.com/ref/hdd/geom/for
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
– 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_Partiti
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.