CSC414ComputerSystemFundamentals

THINK BIG WE DO

U R Ihttp://www.forensics.cs.uri.edu

Digital Forensics CenterDepartment of Computer Science and Statics

Disk GeometryDisk GeometryDisk Geometry

Physical Disk Geometry- Disk Hardware

- Physical Geometry

- Physical Sectors

Disk Formatting- Physical Formatting

- Partitioning

- Logical Formatting

Logical Areas of a Disk- Boot Record

- File Access Tables (FAT)

- Root Directory

- Data Area

Other Storage Formats- RAID

- Optical Disk

- Tape

Disk HardwareFloppy Disks

8" Floppy Disk

5.25" Floppy Disk

3.5" Floppy Disk

Case

Platter

Cover

SpindleHub

Disk HardwareHard Disk Drives

Disk ControllerCard

IDE(Integrated Disk Electronics)

Sealed Housing

Disk HardwareHard Disk Drives

Platter

Spindle

Actuator Arm

Head

Actuator

Actuator Axis

Ribbon Cable to Controller

Disk HardwareHard Disk Drives

Platters

SpindleActuator Arm

Head

ActuatorActuator Axis

Ribbon Cable to Controller

Platters

Head

Head

Head

Side 0

Side 1

Side 2

Side 3

Data is stored on both sides

Physical GeometryPhysical Format- Low level formatting- Tracks- Concentric circles (placed by manufacturer)

- Cylinders- Matching tracks on each side of the platters

Cylinder

Cylinder

• Track 0• Track 1• Track 2

Cylinder 0 is made up of Platter 0 - Side 0 - Track 0Platter 0 - Side 1 - Track 0Platter 1 - Side 2 - Track 0Platter 1 - Side 3 - Track 0

Floppy disks have 80 tracks per side

Physical GeometryPhysical Format- Low level formatting- Tracks- Concentric circles (placed by manufacturer)

- Cylinders- Matching tracks on each side of the platters

- Sectors- Smallest addressable unit- Commonly 512 bytes- Newer operating systems support 4KB sectors- Smallest addressable unit 4096 bytes

- Physical sector numbering begins with 1

Sector

4KB file (4096 bytes) requires 8 sectors ( 4096 = 8 * 512)

Physical GeometryDisk Layout- Constant Angular Velocity (CAV)- Number of bits on each track is the same

- Bits are denser towards the center.

- Platter spins the same speed for every track

- Constant Linear Velocity (CLV)- All tracks have the same physical length and

number of bits

- Constant speed reading data off a track

- Drive speeds up when accessing tracks close to the center

- Drive slows down when accessing towards the outer edge (Track 0)

Sector

Both layouts have the same number of sectors

per track

Physical GeometryDisk Layout- Multiple zone recording- Zone Bit Recording (ZBR) - Zone-CAV - Recording (Z-CAV)

- Compromise of CAV and CLV- Disk divided into zones- Cylinders in different zones have a

different number of sectors- Number of sectors in a particular zone

is constant- Data is buffered so the data rate to the

I/O interface is constant

Zone 0Zone 1

Zone 2Zone 3

Sectors

For our calculations,

we'll assume we are working with CAV formatted

drives.

Physical SectorsTime to Read Data- Seek Time- Time required to move head from

one track to another

- Latency- Time required for disk to rotate to

beginning of correct sector

- Transfer Time- Time required to transfer a block of

data to the disk controller buffer

Sector to Read

A sector can also referred to as block.

Physical SectorsTime to Read Data- For a disk with n cylinders or tracks- Average Seek Time (avgSeek)- average time to move from one track to

another

- Average Latency Time (avgLatency)- average time to rotate to the beginning of

the sector

- Transfer time (xferTime)- should be constant for CAV layout

- Total Time to Access a Disk Sector- Total of average seek time, average

latency time and sector transfer time

avgSeek = time to move from track 0 to track n/2

avgLatency = ! * 1/rotational speed

xferTime = 1/(numSectorsPerTrack * rotationalSpeed)

totalAccessTime = avgSeek + avgLatency + xferTime

Physical SectorsSectors contain 512 data bytes- Exact number of bytes per sector

depends on disk controller and operating system

- For current floppy and hard disks:- Actual bytes per sector: 571 bytes total

Sector

Header Data

Physical SectorsSectors contain 512 data bytes- Exact number of bytes per sector

depends on disk controller and operating system

- For current floppy and hard disks:- Actual bytes per sector: 571 bytes total

- Gap - head switching recovery period

- Sync and Mark - synchronizing head movement with rotation

- Sector ID and CHS Info - identification for the sector (Cylinder, Head, Sector)

- CRC - Cyclical Redundancy Check

- Check value used to verify bits

Pre-Ind

ex Gap

01011010111100001010101010110001010100101... ...0101011100010101

512 Bytes of DataSec

tor ID

Sector

ID CRC

CHS Info

Data CRC

Disk Capacity Calculation- totalSectors = cylinders * sectorsPerTrack * platters * headsPerPlatter

- totalCapacity = totalSectors * bytesPerSector

Disk Capacity

Disk Tracks or Cylinders

Sectors per Track Platters Heads Per

PlatterBytes per Sector

Total Capacity

Low Density Floppy 80 9 1 2 512 720 KB

High Density Floppy 80 18 1 2 512 1.44 MB

Seagate Wren 3 969 36 5 2 512 175 MBTHINK BIG WE DO

U R Ihttp://www.forensics.cs.uri.edu

Digital Forensics CenterDepartment of Computer Science and Statics

Disk GeometryDisk Geometry