Disk Drives
Storage vs. Memory
 Because ROM is read only and RAM is
volatile, we need something else to save our
work
 This is storage
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Floppy disk
Hard disk
Optical disk (e.g. CD)
Tape
Floppy Disk
 Developed by IBM in early 1970’s
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8 inch
 Early 1980’s
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5.25 inch
160 KB (single-sided, single density) – 1.2 MB (doublesided, high density)
These were genuinely “floppy”
 Today’s Standard
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3.5 inch
Double sided, high density
1.44 MB
Density
 The amount of information per area in a storage
device
 The higher the density, the larger the amount of
information
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single-density
double-density
high-density
extra-high-density.
 If your drive doesn’t support a certain density, you
cannot use it
Hard disk
 A magnetic disk on which you can store data.
 “Hard” distinguishes it from a soft or floppy
disk.
 A hard disk can hold more and is faster than
a floppy disk.
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A hard disk can store up to several gigabytes
(GB), while a standard floppy holds 1.44
megabytes (MB).
Platters
 A hard disk consists of several (a dozen or
so) platters, round plates covered with
magnetic material, upon which data is
written
Head
 The reading of and writing to the platters is done by
the heads
 Most platters have two read/write heads, one for
each side.
 The heads are all attached to one access arm; they
do not move independently.
 Floppy disk drives have two heads, one for each
side of the disk.
 If the head becomes dirty, it will not work properly.
Check this if your disk drive has problems.
Head crash
 In hard drives, the head normally does not
actually touch the disk; it sits a few
millionths of an inch from the disk.
 Misalignment or dust particles can cause the
head to touch the disk – this is a “head crash”
 In floppy drives, the head touches the disk
during normal operation.
After a crash
 In the event of a crash, you typically lose
data and need to replace both the head and
the disk. (If you’re lucky you will just end up
with a few bad sectors and can continue
using the disk.)
 So keep your computer clean
 Even smoke particles can cause a head crash.
Tracks
 In both hard and floppy
disks, the data is
written in concentric
circular paths known as
tracks
 A typical floppy disk
has 80 (double-density)
or 160 (high-density)
tracks.
track
Tracks (cont.)
 The density of tracks is measured in units of
tracks per inch (TPI).
 Each track is further divided into sectors.
 The location of information is remembered
by noting its track and sector numbers.
Sectors
 Radial lines break the tracks up sectors
Sector
Cylinders
 Each platter has tracks with the same radii.
 All the tracks on different platters but with the same
radius make up a cylinder.
 For example, if a hard drive has
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Four platters
And each platter has 600 tracks
 Then
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There will be 600 cylinders
Each cylinder will have 8 tracks (assuming that each
platter has tracks on both sides).
Clusters
 The Operating System/BIOS can address a
limited number of sectors
 One way to beat this limit was to introduce
clusters which are groups of sectors, then one
addresses clusters instead of sectors
 The operating system assigns a unique
number to each cluster and then keeps track
of files according to which clusters they use.
Lost clusters
 The bookkeeping is done in the File
Allocation Table (FAT). The size of each
cluster depends on the disk's partition size.
 Sometimes the operating system indicates
that a cluster is being used when it is not.
This is called a lost cluster.
 You can free up disk space by reassigning
lost clusters with the ScanDisk utility.
File Allocation Table
 A bookkeeping table that the operating system uses
to locate files on a disk.
 Due to fragmentation, a file may be divided into
many sections scattered all over the disk. The FAT
keeps track of all these pieces.
 In DOS systems, FATs are stored in hidden files,
called FAT files
 The FAT system for older versions of Windows 95
is called VFAT, and the one for new versions of
Windows 95 and Windows 98 is called FAT32.
Scandisk
 DOS/Windows utility that locates errors on hard
disks and is able to correct some of them.
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In DOS, you run ScanDisk by typing scandisk at the
prompt and hitting Enter.
In Windows 95/98, you can run ScanDisk by selecting
Start/Programs/Accessories/System Tools/ScanDisk.
 Among other things, ScanDisk looks for platter
defects and for lost clusters that are sometimes
created when a program aborts.
format
 Formatting prepares a disk for reading and writing.
 When you format a disk, the operating system
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erases all disk’s bookkeeping information
tests the reliability of the sectors
marks bad sectors (e.g. ones with scratches)
creates a new table for bookkeeping information.
 You must format a disk before you can use it.
Format (high level)
 Reformatting a disk does not erase the disk’s
data only the address tables (the pointers to
it)
 If you accidentally reformat a disk containing
useful data, one should be able to recover
most or even all of the information
Format (low level)
 The previous slide talks about high-level
formatting
 There is also low-level formatting, which
sets certain properties of the disk such as the
interleave factor.
 The low-level format also determines what
type of disk controller can access the disk
Format (cont.)
 Most hard disks that you buy have been lowlevel formatted by the manufacturer.
 Thus you don’t have to perform a low-level
format unless you want to change the
interleave factor or make the disk accessible
by a different type of controller.
 Performing a low-level format erases all data
on the disk.
Interleaving
 When storing a file larger than a sector, it is
sometimes faster to store it on nonconsecutive sectors
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In one-to-one interleaving, the sectors are placed
consecutively around a track.
In two-to-one interleaving, every other sector is
written to
Interleaving (cont.)
Interleaving (cont.)
 The purpose of interleaving is to make the
disk drive more efficient. The disk drive can
access only one sector at a time, and the disk
is constantly spinning beneath the head.
 This means that by the time the drive is ready
to access the next sector, the disk may have
already spun beyond it.
Partition
 To divide a storage device into separate sections.
 In DOS systems, you can partition a disk, and each
partition will behave like a separate disk drive.
 Partitioning is particularly useful if you run more
than one operating system. For example, you might
reserve one partition for Windows and another for
UNIX.
Partition
 In addition, partitioning on DOS and Windows
machines can improve disk efficiency. This is
because the FAT system used by these operating
systems automatically assigns cluster size based on
the disk size: the larger the disk, the larger the
cluster.
 Unfortunately, large clusters can result in a wasted
disk space, called slack space. There is an entire
sector of the software industry devoted to building
utilities that let you partition your hard disk.
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FAT32 addresses more clusters
Optical disk
 A type of storage disk in which data is
written and read using laser light.
 A CD is a portable optical disk, the optical
analog of a floppy
 CD’s can hold a lot more data (approx. 6 GB)
than floppies (approx. 1.44 MB)
Three types
 CD-ROM: comes with data on it, cannot be
changed (written to)
 WORM: “write-once, read–many,” comes without
data, can be written to once and only once
 ERASABLE (REWRITABLE): comes without
data, can be written to, then erased and rewritten,
etc. Sometimes called EO (erasable optical) disks
 Different disks and different drives
LIMDOW
 Light Intensity Modulation Direct OverWrite
 Speeds up the writing of data from memory to a CD
 The usual, magnetic-optico (MO) technology,
requires that the disk pass under the head twice -once for erasing and once for writing
 LIMDOW changes the laser's intensity allowing for
both processes to be done in a single pass
Phase-change disk
 A type of rewritable optical disk that employs the
phase change recording method.
 The disk drive writes data with a laser
 The bits are determined by the state of the material
in a given spot: amorphous (disordered) or
crystalline (every aligned).
 Data is read by reflecting light from the disk;
crystalline and amorphous spots reflect light
differently
Tape
 A strip of plastic coated with a magnetic material on
which information can be stored
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Similar to cassette tapes used to record music.
 Cheap, but slow
 Sequential access (as opposed to random access)
 Good for
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long-term storage
backup.
transporting large amounts of data
Controller
 A device that controls data transfer to and from a
peripheral device. For example, disk drives,
monitors, and printers all need controllers.
 Controllers must be designed to communicate with
the appropriate bus (be it ISA (AT), PCI or SCSI).
 Thus your controller conform to the bus
architecture used in your PC.
IDE
 Intelligent Drive Electronics or Integrated Drive
Electronics
 An IDE interface is an interface for storage devices,
in which the controller is integrated into the disk or
CD-ROM drive.
 Although it really refers to a general technology,
most people use the term to refer the ATA
specification, which uses this technology.
ATA
 Short for AT Attachment, a disk drive that
puts the controller on the disk drive itself.
 There are several versions of ATA, all
developed by the Small Form Factor (SFF)
Committee
EIDE
 Short for Enhanced IDE, a newer version of the
IDE storage device interface standard developed by
Western Digital Corporation.
 It supports data rates of between 4 and 16.6 MBps
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Approx 3-4 times faster than the old IDE standard.
 It also increased the storage capacity from 528 MB
to 8.4 GB.
 Because of its lower cost, EIDE has replaced SCSI
in many areas.
SCSI
 Small Computer System Interface. Pronounced
"scuzzy,"
 SCSI drive: a disk drive that works with the SCSI
interface
 SCSI is a parallel interface standard used for
attaching peripheral devices to computers.
 SCSI interfaces provide for faster data transmission
rates (up to 80 megabytes per second) than standard
serial and parallel ports. Also you can attach many
devices to a single SCSI port, so that SCSI is really
an I/O bus rather than simply an interface.
SCSI (Cont.)
 Although SCSI is an ANSI standard, there
are many variations of it, so two SCSI
interfaces may be incompatible.
 For example, SCSI supports several types of
connectors.
SCSI (Cont.)
 SCSI’s were standard in Macintoshes, the iMac
comes with IDE.
 You can attach SCSI devices to a PC by inserting a
SCSI board in one of the expansion slots.
 Many high-end new PCs come with SCSI built in.
Note that the lack of a single SCSI standard means
that some devices may not work with some SCSI
boards.
Varieties of SCSI
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SCSI-1: 8-bit bus, supports data rates of 4 MBps
SCSI-2: Same as SCSI-1, but uses a 50-pin instead of a
25-pin connector, and supports multiple devices.
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This is what most people mean when they refer to plain SCSI.
Wide SCSI: wider cable (168 cable lines to 68 pins) to
support 16-bit transfers.
Fast SCSI: 8-bit bus, but doubles the clock rate to
support data rates of 10 MBps.
Varieties of SCSI
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Fast Wide SCSI: 6-bit bus, supports data rates of 20
MBps.
Ultra SCSI: 8-bit bus, supports data rates of 20 MBps.
SCSI-3: 16-bit bus, supports data rates of 40 MBps.
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Also called Ultra Wide SCSI.
Ultra2 SCSI: 8-bit bus, supports data rates of 40 MBps.
Wide Ultra2 SCSI: 16-bit bus, supports data rates of 80
MBps.
MTBF
 Mean time between failures. MTBF ratings are
measured in hours and indicate the sturdiness of
hard disk drives and printers.
 Typical PC disk drives have MTBF ratings of
approx. 300,000 hours. This means that of all the
drives tested, one failure occurred every 300,000
hours of testing.
 A more useful gauge of a device's lifetime is how
long a warranty the manufacturer offers
Ghost imaging
 Ghost imaging, using ghosting software, is a
method of converting the contents of a hard drive -including its configuration settings and
applications--into an image, and then storing the
image on a server or burning it onto a CD.
 When contents of the hard drive are needed again,
ghosting software converts the image back to
original form.
Ghosting (cont.)
 Companies use ghost imaging when they want to
have identically configured machines with the same
software installed.
 For example, to set up 100 laptops machines, one
could manually set configurations and install
applications on each machine. Ghosting software
(usually contained on a floppy) will retrieve the
ghost image from the server, convert it into its
original form, and copy it onto the machine.
HSM
 Hierarchical Storage Management, a data storage
system that automatically moves data between
high-speed, high-cost devices (hard disk) and lowspeed, low-cost devices (optical disk or tape)
 HSM is a kind of caching. The HSM system
guesses which data can be moved to slower devices
and which data should stay on the hard disks.
RAID
 Redundant array of independent disks
 A type of disk drive that uses two or more
drives in combination for fault tolerance and
performance.
 More for servers than PCs.
RAID levels
 There are number of different RAID levels.
The three most common are 0, 3, and 5:
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Level 0: Provides data striping (spreading out
blocks of each file across multiple disks) but no
redundancy (copies). This improves performance
but does not deliver fault tolerance.
RAID levels (cont)
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Level 1: Provides disk mirroring, in which data is written
to two duplicate disks simultaneously. Disk mirroring is
used commonly in on-line database systems where it's
critical that the data be accessible at all times.
Level 3: Same as Level 0, but also reserves one
dedicated disk for error correction data. It provides good
performance and some level of fault tolerance.
Level 5: Provides data striping at the byte level and also
stripe error correction information. This results in
excellent performance and good fault tolerance.
References
 A+ Certification Training Kit
 http://www.webopedia.com
 http://www.whatis.com