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