Disk Drives
CSIT 301 (Blum)
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Hard disk accessing
• Seek time is the time required to position
the head to the selected cylinder.
– Typical seek times are in milliseconds (ms)
• Latency is the wait time until the correct
sector rotates past the head.
– Also milliseconds.
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Ordering the to-do list
• Because the hard drive is slower than the
processor and memory, there may be a back up of
tasks for it to perform. The order in which it
performs these tasks can greatly affect its
efficiency.
• One ordering is a simple FIFO (first-in, first-out)
ordering. The tasks (reads and writes) are queued
up and the first task requested is the first task
performed.
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More Sophisticated Orderings
• Seek-Time Optimization (a.k.a. Elevator
seeking):
– Seek time involves the radial positioning of the
head. The tasks are ordered based on their
radial positioning to minimize seek time.
• Access-Time Optimization (a.k.a.
multiple command reordering):
– Takes into account both radial and angular
positioning to minimize access time which
includes seek time and latency.
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Ordering Comparison
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Locating data
• The platters are dense with data, if the head is
even the slightest bit off target, one will be reading
the wrong data.
• The actuator uses a voice coil. The voice coil uses
a feedback mechanism to locate the data.
– Feedback means taking some output and putting it back
in as input. In this case the head is reading information
from the platter. Some of that information is telling the
head where it is on the platter.
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Servo Information
• The information on the disk about location
on the disk is called servo information.
– It is not user’s data but data about the disk’s
operation.
• The servo information can be placed
– In dedicated sectors (wedge servo)
– On dedicated platter side (dedicated servo)
– Spread throughout (embedded servo)
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Low-level format
• Low-level formatting (LLF), a.k.a. physical
formatting, establishes the tracks, sectors, etc.
and writes the servo information.
• For a hard drive, this is done once by the
manufacturer.
• For a floppy disk, this can be done by the user.
• Any previous data is lost when a low-level format
is performed on a disk.
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Our old enemy heat
• When objects heat up, they tend to expand
(thermal expansion).
• If the heat in a hard drive is not distributed
uniformly, then different platters are at
different temperatures and thus have
expanded different amounts.
• This is why dedicated servo is not as good
as embedded servo.
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Thermal recalibration
• Every once in awhile the hard disk reads
itself just to check on track positioning and
so forth.
– You may hear the hard drive spinning even
though the PC is neither reading it nor writing
to it.
– This is called thermal recalibration.
– The amount of thermal calibration necessary
was reduced by manufacturers moving to
embedded servo techniques.
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A gentle breeze
• While reading or writing the platters spin, which
causes a breeze. The actuator arm is designed like
a wing that floats in this breeze.
• When the drive spins down (stops spinning), the
breeze stops blowing and the actuator arm must
come in for a landing. It comes in contact with the
platter.
• We do not want there to be any data written in the
region where the arm lands.
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The landing zone is for …
• There are special regions on the disk called
landing zones where no data is written and where
the head comes to rest when the drive spins down.
• Positioning the head into the landing zone is called
head parking.
• There is a BIOS parameter that informs the system
where the landing zones are. But in modern drives
the drive is designed to return automatically to the
landing zone (even if the power is lost).
• Some drives are designed so that the head never
lands.
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Landing Zone Setting in BIOS
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Spindle Speed Revisited
• In addition to accurately positioning the head, the
platter must spin at a very accurate speed.
• The standard spindle speed for a long time was
3600 RPM (revolutions per minutes), which
corresponds to 60 cycles per second or 60 Hz,
which is same frequency as the AC power that
comes out of a standard US wall socket.
• But since latency is connected to spindle speed,
the spindle speeds have been raised.
• The higher speeds tend to debut with SCSI drives
and then filter down to ATA/IDE drives.
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Spindle speed and drive type
Source: PCGuide.com
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Spindle Speed Issues
• Smaller platters are easier to spin faster
– Think of an ice skater spinning
• Platters must spin together, so it is easier to
spin fewer platters.
• The platter must spin at a steady speed and
not vibrate (we don’t want a head crash).
• Heat and power: spinning faster requires
more power and generates more heat.
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Power Issue
• Because of momentum (objects in motion tend to
remain in motion), it takes more power to start the
drive spinning than to keep it spinning.
– This is one place where the 12V connection is used
– “Power management” reduces the power when a device
is not being used. But spinning up a hard drive requires
more power than maintaining the rotation, so
continually starting and stopping could cost power.
• Also the starting and stopping could be detrimental
to the drive.
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Power Issue (Cont.)
• When a computer has multiple drives, e.g. a
master/slave set up, the slave may delay its
spin up until the master has completed its
spin up so they are not pulling on the power
simultaneously.
– One may have to use a particular jumper setting
to ensure this delay.
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Cover and Base Casting
Don’t open the
case (unless your
room is clean).
The screws in the
case are unusual to
prevent you from
opening it.
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Yes to Air / No to Dust
• The hard drive cannot be completely sealed off, it
needs air to keep it from overheating.
• On the other hand, the slightest spec of dust can
cause a head crash.
• A hard drive needs an air filtration and circulation
system.
– The circulation occurs naturally in that the spinning
disks cause the air to flow.
– There is a filter to keep dirt out. It does not need
changing.
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Heat Issues Revisited
• Recall that moving air (convection) is the primary
mechanism for cooling computer devices and that
the spinning platters get the air moving inside hard
drives.
• Thus typical hard drives (ATA/IDE and SCSI with
moderate spindle speeds) can use passive cooling.
• High speed SCSI drives may need active cooling
(i.e. fans).
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Drive cooler
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Bay cooler
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Retail vs. OEM
• Retail
– Hard Disk Drive, Installation Instructions,
Drivers and/or Overlay Software, Mounting
Hardware, Interface Cable, Warranty Card
• OEM (original equipment manufacturer)
– Hard Disk Drives and Jumpers (maybe)
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Geometry Review
• A platter is divided into concentric circles called
tracks.
• The tracks are further divided into arcs called
sectors.
– A sector holds 512 bytes of data.
– There may be additional bytes for servo information.
– There may be additional bytes for error detection and
correction.
– In zoned bit recording (ZBR) outer tracks have more
sectors than inner tracks.
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Data transfer rate is track dependent
• The spindle rotates at a specified angular
speed (e.g. 7200 RPM).
• The data transfer rate depends on this speed.
• But since there are more sectors and thus
more data at larger radii, the data transfer
rate can be track dependent.
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Zone Dependent Data Transfer Rate
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Interleaving
• When storing a file larger than a sector, it is
sometimes faster to store it on nonconsecutive sectors
– In one-to-one interleaving, the sectors are
placed consecutively around a track.
– In two-to-one interleaving, every other sector is
written to
–…
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Interleaving (cont.)
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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.
• It depends how fast the controller is. Modern
controllers are very fast and 1-to-1 interleaving is
the norm.
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What’s the delay?
• Modern controllers are so fast there is effectively
no delay between writing one sector and the next
and thus interleaving is not necessary.
• But if writing requires switching to a different
platter (same cylinder) there is a small delay.
• And if the writing requires placing data on more
than one track/cylinder, there is a more substantial
delay in repositioning the head so as to write to the
next track.
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Cylinder and Head Skew
• The sector numbering is staggered to
account for delay.
• There is staggering from platter to platter
within the cylinder. This is known as head
skew.
• There is staggering from track to track on a
given platter. This is known as cylinder
skew.
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Cylinder and Head Skewing
Staggering on a given platter (cylinder skew) and
staggering from platter-to-platter (head skew).
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Three-Step Process to Use Hard Disk
• To prepare a hard disk for use, there are
three steps:
– Low-level (or physical) formatting
– Partitioning
– High-level (or logical) formatting
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Low-Level Formatting
• For hard drives, low-level formatting is done by
the manufacturer.
• It establishes the tracks, sectors and so on.
• It writes the servo information onto the disk.
• Any information on a disk is lost if it is low-level
formatted.
• There are pseudo-low-level formatting utilities
that one can run which effectively write 0’s to all
of the sectors and replace bad sectors with spares.
– This is distinct from true low-level formatting which
establishes the sectors.
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Partitioning
• Partitioning separates the disk into logical pieces.
• The standard motivations for partitioning are:
– To have multiple operating systems on the same disk
(e.g. a “dual boot system”)
– To improve disk efficiency, to minimize “slack” and so
on.
– To separate system files from user files (from virtual
memory).
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Drive Letters and Partitions
• Drive letters A and B are reserved for
floppies. (It was standard for early PCs to
have two floppy disk drives.)
• The computer begins assigning hard drive
partitions drive letters starting with C.
• All primary drives are assigned letters first.
• Next letters are assigned to logical volumes
within the extended partition.
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Partitions
• A drive must have at least one partition.
• A drive can have at most four partitions.
– These are known as the primary partitions.
• One of these is designated as “active” and under
normal circumstances one boots from the active
drive.
– One can achieve the effect of having more than
four partitions on one hard drive by designating
one as an extended partition (instead of a
primary partition). The extended partition can
then be divided into logical partitions or
logical volumes.
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Primary versus Logical
• “Boot-ability”: A primary partition is bootable and
can be set as the active partition.
– Typical schemes reserve primary partitions to act as
potential active partitions. That is, if the space is to be
used for non-operating-system files, the space will be set
up as a logical volume instead of a primary partition.
• Drive Letter Assignment: All primary partitions are
assigned drives before any logical partition is
assigned a drive letter.
– Adding (not replacing) a hard drive can result in a
reordering of one’s drives if one has logical partitions.
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Control Panel/Administrative Tools/Computer
Management/Disk Management
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Parts of a Primary Partition
• A primary partition will contain some
combination of the following three things:
– System partition: the files need to start the
operating system
– Boot partition: the operating system files
– General purpose partition: any other files
(programs and data)
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Master Boot Record
• The first thing read from the hard drive when a
computer starts is the Master Boot record
(MBR).
– It is located at cylinder 0, head 0, and sector 1.
• The MBR consists of:
– Master Partition Table
• This table is limited in size and only has information about the
primary partitions (not any logical volumes) and which
primary partition is active
– Master Boot Code
• A small program to start the boot process
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Volume Boot Sector
• Each partition has a volume boot sector (or
partition boot sector) which contains
information about:
– The size of the partition
– Volume Boot Code:
• The Master Boot Code calls the Volume Boot Code
of the active partition. The Volume Boot Code starts
loading the operating system.
– Location of the File Allocation Table (FAT)
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Boot sector virus
• The master boot code and the volume boot
code are the first software code executed
after the BIOS (firmware) has started the
computer when it is powered up.
• This very low-lying code is susceptible to
viruses known as boot sector infectors.
– Why you should not have a floppy in the drive
when you boot (unless you mean to boot from
the floppy).
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High-level formatting
• After one partitions a disk, the next level of
preparing it to be used is to high-level (or
logical) formatting.
• Logical formatting provides
–
–
–
–
–
Partition boot sector
System ID byte (identifies partition)
Information for the filesystem
Data on bad sectors
Lose data (maybe)
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Formatting a drive
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References
• PC Hardware in a Nutshell, Thompson and
Thompson
• http://www.pcguide.com
• All-in-One A+ Certification, Meyers and
Jernigan
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