CSC414 Disk Geometry Computer Disk Geometry Physical Disk Geometry Logical Areas of a Disk System - Disk Hardware - Boot Record Fundamentals - Physical Geometry - File Access Tables (FAT) - Physical Sectors - Root Directory - Data Area - Physical Formatting Other Storage Formats Digital Forensics Center - Partitioning - RAID Department of Computer Science and Statics THINK BIG WE DO - Logical Formatting - Optical Disk U R I - Tape

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Disk Hardware Disk Hardware

Floppy Disks Hard Disk Drives Cover Disk 8" Sealed Housing Card

Case

5.25" 3.5" Floppy Floppy Disk Disk Spindle Hub IDE Platter (Integrated Disk Electronics)

Disk Hardware Disk Hardware

Hard Disk Drives Hard Disk Drives

Actuator Axis Platter Actuator Actuator Spindle Arm Spindle Head Head

Actuator Axis Actuator Arm Side 0 Head Platters Side 1 Data is stored Ribbon Cable Head Platters Actuator on both sides to Controller Side 2 Side 3 Ribbon Cable Head to Controller Physical Geometry Physical Geometry

Physical Format • Track 0 Physical Format Sector - Low level formatting Floppy disks have • Track 1 - Low level formatting 80 tracks per side - Tracks • Track 2 - Tracks - Concentric circles (placed by manufacturer) - Concentric circles (placed by manufacturer) - Cylinders - Cylinders - Matching tracks on each side of the platters - Matching tracks on each side of the platters - Sectors 4KB file (4096 bytes) Cylinder - Smallest addressable unit requires 8 sectors ( 4096 = 8 * 512) Cylinder 0 is made up of - Commonly 512 bytes Platter 0 - Side 0 - Track 0 Platter 0 - Side 1 - Track 0 - Newer operating systems support 4KB sectors Platter 1 - Side 2 - Track 0 Platter 1 - Side 3 - Track 0 - Smallest addressable unit 4096 bytes - Physical sector numbering begins with 1 Cylinder

Physical Geometry Physical Geometry

Zone 0 Disk Layout Sector Disk Layout - Constant Angular Velocity (CAV) - Multiple zone recording Zone 1 For our calculations, - Number of bits on each track is the same - Zone Bit Recording (ZBR) we'll assume we are working with - Bits are denser towards the center. - Zone-CAV Zone 2 CAV formatted - Recording (Z-CAV) drives. - Platter spins the same speed for every track Zone 3 - Constant Linear Velocity (CLV) - Compromise of CAV and CLV - All tracks have the same physical length and - Disk divided into zones number of bits - Cylinders in different zones have a - Constant speed reading data off a track different number of sectors - Drive speeds up when accessing tracks close to - Number of sectors in a particular zone the center is constant Both layouts have the - Drive slows down when accessing towards the same number of sectors - Data is buffered so the data rate to the outer edge (Track 0) per track I/O interface is constant Sectors

Physical Sectors Physical Sectors

Time to Read Data Sector Time to Read Data to Read avgSeek = time to move from track 0 to track n/2 - Seek Time - For a disk with n cylinders or tracks avgLatency = ! * 1/rotational speed - Time required to move head from - Average Seek Time (avgSeek) xferTime = 1/(numSectorsPerTrack * rotationalSpeed) one track to another - average time to move from one track to totalAccessTime = avgSeek + avgLatency + xferTime - Latency another - Average Latency Time (avgLatency) - Time required for disk to rotate to beginning of correct sector - average time to rotate to the beginning of the sector - Transfer Time - Transfer time (xferTime) - Time required to transfer a block of - should be constant for CAV layout data to the disk controller buffer - Total Time to Access a Disk Sector A sector can also - Total of average seek time, average referred to as block. latency time and sector transfer time Physical Sectors Physical Sectors

- Gap - head switching recovery period Sectors contain 512 data bytes Sector Sectors contain 512 data bytes - Sync and Mark - synchronizing head - Exact number of bytes per sector - Exact number of bytes per sector movement with rotation depends on disk controller and depends on disk controller and - Sector ID and CHS Info - identification operating system operating system for the sector (Cylinder, Head, Sector) - For current floppy and hard disks: - For current floppy and hard disks: - CRC - Cyclical Redundancy Check - Actual bytes per sector: 571 bytes total - Actual bytes per sector: 571 bytes total - Check value used to verify bits

Pre-Index Gap Sector ID CHS Info Data CRC Sector ID CRC 512 Bytes of Data 01011010111100001010101010110001010100101...... 0101011100010101 Header Data

Disk Capacity

Disk Capacity Calculation

- totalSectors = cylinders * sectorsPerTrack * platters * headsPerPlatter - totalCapacity = totalSectors * bytesPerSector Tracks or Sectors Heads Per Bytes per Total Disk Platters Cylinders per Track Platter Sector Capacity Disk Geometry Low Density 80 9 1 2 512 720 KB Floppy High Density 80 18 1 2 512 1.44 MB Floppy Seagate Wren 3 969 36 5 2 512 175 MB Digital Forensics Center Department of Computer Science and Statics THINK BIG WE DO

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