Hard disk DriveHard disk DriveHard disk DriveHard disk Drive

Head positioning Of a voice coil Based System

• Voice coil based systems use a special information previously written on disk surface ( at the time of manufacturing).

• This special index information on the disk surface is called “Servo”.

• Voice coil actuators use this servo information as an index to accurately move the R/W head to any particular location

• It is a closed loop system• This type does not require frequent low level

formatting as required in the stepper motor based HDD

Closed loop servo

Read the Servo/ index information

Is it the reqd. Position

Move the head to next position

StopYes

No

Closed Loop

Types of servo• There are 3 types of servo for

controlling the voice coil based head positioning system (relative position of information wrt the head position)

• Wedge servo• Embedded servo• Dedicated servo

Wedge servo

• It appears in a wedge like shape on the disk surface.

• The servo information is written in pre- index gap i.e gap between end and beginning of the track

• Most controllers need not be specially configured for these drives but some may need to be configured to work with these drives

Problem with Wedge servo

• Servo information appears only once per rotation

• If the head is looking for some information, then the drive needs several rotations for the head to properly find the required information

Embedded servo• Servo information is kept at the

beginning of each sector• This design allows the head

positioning system to receive the current position many times in a single rotation. This makes it precise and faster

Dedicated Servo

• One side of one platter exclusively used for servo information

• As the head on the servo side is not made for R/W operation, no low level format program or controller can change servo info

• It will have odd number of R/W heads. They are high capacity, high speed HDD.

• It allows positional accuracy and highest possible speed

HDD features• Characteristics of HDD are :• Size• Capacity• Interleave• Head skew• Cylinder skew

Form Factor• Drives are available in different

widths and heights• Form factor is the basic unit of

measurement of size of the drive• Form factor is decided by the

platter width• 5.25 inch platter has 5.25 inch

form factor• 3.5 inch platter has 3.5 inch form

factor

Storage capacity• The storage may be given as

formatted capacity in million bytes or in megabytes.

• It may be given as unformatted capacity in million bytes or in megabytes

Disk Geometry• HDD stores data as magnetic information on

its surface. If the data is stored continuously one next to another, then at the time of retrieval, the disk has to be scanned for getting the required data.

• To arrange data properly on the disk surface , it is divided into Heads/side, tracks, sectors, cylinders etc..

• When info is written on some side, sector, track etc are saved in an index like place called directory and FAT

• When data is required, comp. looks at the entry in directory and FAT and locates the side, track and sector

Sides and Heads

• HD can contain more than one platter

• Each side can be used for read and write operation

• Each side has a separate R/W head• All the heads are connected to a

single head rack which makes R/W heads move over the disk drives. Head numbers begin from head 0, head1 and so on

Track• Each side of HDD platter’s surface is

divided into concentric circles called tracks

• They are magnetic information written during formatting of HDD

• Outermost track is called track 0. The innermost will have the highest number

Sector

• A track is a big area to store data( 5000 bytes)

• Hence tracks are divided into sectors • The formatting program divides disk

surface into sectors by writing magnetic pattern on disk surface

• Different HDD capacities have different number of tracks

• 512 byte data can be stored in each sector. Sector no. starts from 1

Cylinder• Same tracks of different platters

form an imaginary cylinder like structure

• Data is stored cylinder by cylinder• All tracks on a cylinder are written

and then the R/W head moves to the next Cylinder . This reduces movement of R/W head and increases the speed of read and write operation

Calculation of storage capacity

• Total number of sectors = Total sides x total tracks per side x Total sectors per track

• Total Storage capacity = Total number of sectors x 512

Interleave• If the sectors are numbered

sequentially from 1 - max number, the disc read operation will be very slow

• Consider disc read of a complete track (sectors1 ,2,3 etc)

• The drive starts reading when R/W head reaches Sector1

• After reading, drive sends this data to the controller. CRC test is done. During this the disk is rotating

Interleave

• By the time this process is completed, the R/W disk passes the 2nd sector.

• Now when the 2nd sector instruction is given, the head is in the 3rd / 4th sector. Now the disc has to complete one full rotation to come to 2nd sector again

• This will make the read process slow

Interleave Factor• The solution to this to put the 2nd sector

in 3rd or 4th position, so that the head will position itself at the correct location when it receives the instruction to read the 2nd sector

• Numbering the sectors out of order with leaving a gap of one or more sectors in sector numbering is called Interleaving.

• Now a days , we have 1:1 interleave with IDE and SCSI interfacing which support them

Interleave factor

Zone bit recording

• A new recording scheme is used by current high capacity IDE and SCSI HDDs to store more sectors in outer track compared to the number in the inner track

• This method is called zone bit recording• In this method, the platter is divided into

number of zones, each zone will have a fixed number of sectors / track

• The controller used with the drive has one additional job of converting the odd number of sectors/ track in different tracks into standard no. of sectors/track

Zone bit recording

Write Pre-compensation

• It is useful for drives using standard track, sector format

• Drives using zone bit recording do not require any write pre- compensation

• The magnetic particles used to write on the disk surface have north and south poles

• Like poles repel and unlike poles attract• In outer surface of hard disk platter,

magnetic particles are far apart to be affected by the attraction and repulsion of magnetic particles

Write Pre-compensation

• In the inner tracks of the disk drive, the density of the magnetic are very high and adjacent particles start to attract and repel.

• This will force to change the information written on the disk

• To compensate for this shift of data particles due to attraction and repulsion, the drive can write the data apart or closer than the required position

Write Pre-compensation

• The particles will slowly shift to the required position because of attraction and repulsion

• This process of writing the data closer or farther to compensate for attraction or repulsion of magnetic particles is called Write pre-compensation

• The cylinder from which this pre-compensation is started is called pre-compensation cylinder. This value will be used by all the cylinders that are towards the centre of the drive

Master Boot Record

• MBR contains a small program to load and start the active / bootable partition

• This area also contains information about all four primary partitions on HDD, their starting sector, ending sector, size etc.in the form of partition table record

• The MBR is located at Cylinder 0, head 0 sector1 and partition boot sector are located at the beginning of each partition volume

• Max number of all type of partition together allowed by DOS is 24(c-z)

Cluster• When OS writes some information on the

hard disk, it does not allocate the space sector wise, instead uses a new unit of storage called “Cluster”

• Clusters are the minimum space allocated by DOS when storing any information on the disk

• Even to store only one byte long information on the disk requires minimum one cluster area on the disk surface

Cluster• A cluster can be made up of one or more

sectors, it depends on disk type being used. • This reduces the size of FAT that DOS uses to

keep track of the used and the empty disk space

• First cluster no. is taken as 2• Clusters are used to allocate the storage area

for data area only, FAT and directory areas are not allocated according to the cluster size

Default Cluster size for different volume sizes

Volume size

FAT 16 Cluster

FAT 32 cluster

NTFS cluster

7MB – 16MB 2 KB Not supported 512 bytes

17MB- 32 MB 512 bytes Not supported 512 bytes

33 MB- 64MB 1 KB 512 bytes 512 bytes

65MB – 128MB 2KB 1 KB 512 bytes

129 MB – 256 MB 4 KB 2 KB 512 bytes

257 KB- 512 MB 8 KB 4 KB 512 bytes

513 MB -1024 MB 16 KB 4 KB 1 KB

1o25 MB-2 GB 32 KB 4 KB 2 KB

2 GB -4 GB 64 KB 4KB 4KB

4 GB – 8 GB Not supported 4KB 4KB

8GB -16 GB Not supported 8 KB 4KB

16 GB- 32 GB Not supported 16 KB 4KB

32 GB- 2 TB Not supported Not supported 4 KB

Logical Structure of HDD

Physical Sector Partition1(C:)

(Logical sector 1)Partition 2 (D:)(logical sector 2)

DOSBoot record

FAT 1

FAT 2 Root Direc-tory

Data Area

DOSBoot record

FAT 1

FAT 2

Root Direc-tory

Data Area

Logical structure• It is the function of DOS FDISK and

FORMAT command to create logical areas on the disk drive

• FDISK creates Master Boot Record (MBR)

• FORMAT command creates DOS boot record, FAT area, Root directory area and empty data area

Formatting• Hard Disk drive requires a low level

formatting and a high level formatting to make it useful for data storage

• Low level formatting magnetically divides the disk into tracks and sectors

• High level formatting is done on hard disk to make the disk DOS compatible by writing DBR, FATs and empty root directory information on the drive

Low level formatting• It is called physical formatting.• To find out if a drive is low level

formatted or not , DOS FDISK program is used. If FDISK program recognizes HDD, the drive has already been low level formatted

• Most HDD currently available are Low level formatted from factory itself.

Functions of low level formatting

• Dividing the disk surface into tracks and sectors

• Establishing interleave factor• Marking identification information

on each track and sector• Marking defective sectors

High level formatting• After low level formatting and partitioning,

final step for preparing the hard disk drive for use is high level format the drive

• High level format program need to only create File Allocation Table, directory system etc..so that the DOS can use the HDD for storing and storing files

• The command A:\> FORMAT C:/S

Partitioning• It was a new concept introduced by IBM

when they launched 10 MB HDD• At that time 10 MB was considered to

be a large storage area and decided to use it for 2 different operating system

• Partitioning means dividing the drive into logical parts or volumes

• DOS/ Windows FDISK.EXE is used to partition a HDD. Big size partition is wastage of space

PartitioningConsider the table:• If partition size is 100 MB

– Cluster size is 2 KB• For partition size 4000

MB – cluster size is 64 KB• On 100 MB disc – 500

files will require 500x 2 KB = 1MB of space

• On 400MB disc 500 files will require 500 x 64 KB = 32MB of space

Size of logical Drive (MB)

Cluster size in KB

0-15 4

16- 127 2

128 -255 4

256 -511 8

512 - 1023 16

1024 – 2047 32

2049- 4095 64

Partition

• Partitioning is the middle process

Configure the system BIOS setup

Low level formatting

Partitioning (FDISK)

High level formatting

Partitioning• It is done to have more than one operating system on

the same drive• To have more than one logical drive• Partition program FDISK writes a MBR in the first

physical sector i.e Cylinder 0, head 0 and Sector 1FDISK can be used for :• Creating a primary DOS partition• Creating an extended DOS partition• Creating logical drives in extended partition• Deleting partitions• Display partition data