ITFN 2601 Introduction to Operating Systems

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ITFN 2601Introduction to Operating Systems

• Lecture 12
• Disk Access

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Agenda

• Disc Types
• Magnetic
• RAID
• Optical
• Disk Arm Scheduling
• Error Handling
• Stable Read/Writes

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Magnetic Disks

• Heads
• Which Platter to Read
• Cylinders
• Ring Number
• Sectors
• Rotational Location

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RAID

• Redundant Array of Inexpensive Disks
• Allows for the amalgamation of many drives
• Eliminates danger of a Single Large Expensive
  Disk
• Built-in error mechanism
• Built-in recovery mechanism
• No Error/Recovery in RAID-0 and RAID-2

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RAID Levels

• 6 Levels
• RAID-0 Write-Striping
• RAID-1 Write-Mirror
• RAID-2 Byte-Striping
• RAID-3 Byte-Striping w/Parity
• RAID-4 Write-Striping w/Parity
• RAID-5 Write-Striping w/Parity-scattering

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RAID 0

• Each write is issued to a different drive
• Less wait for write-completion
• Prefers drives being equivalent

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RAID 1

• Writes are issued as in RAID 0
• Writes are identically issued to backup-discs
• Highly size dependent

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RAID 2

• As RAID 0, except write is broken into bytes
• Normal writes are in block-sized chunks
• Each byte is issued to a different drive

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RAID 3

• Writes are broken and issued as RAID 2
• One disc is Parity
• XOR of the bytes being written on that pass
• One bit per byte

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RAID 4

• As RAID 0
• Each parity bit represents one strip
• Parity will have n bits, where n is the number of
  drives (minus the parity drive)

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RAID 5

• Based on RAID 4
• If the Parity drive fails, all recovery data is
  lost!
• Staggers Parity across drives
• If any drive is lost, its data is recovered from
  Parity

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Optical Devices

• Optical Disks (LaserDisc)
• 30cm diameter
• Compact Disc
• Audio IS-10149 Red Book (1980)
• Computer CD-ROM Yellow Book (1984)

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Physical Construction

• Data is encoded in Pits/Lands
• Pit On
• Land Off

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Red/Yellow Specifications

• Symbols
• 8bits-gt14bits, Encoding Error Correction
• Frames
• 42 Symbols (24 data bytes 18 ECC)
• Sectors
• 98 Frames
• 2048 data bytes
• 288 ECC

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CD-R/RW

• Standard computer CD is CD-ROM
• Disc is extruded plastic
• CD-Recordable Orange Book (1989)
• Disc has dye that is activated by light
• CD-ReWritable
• Dye has two states (transparent opaque)
• Drive has three lasers

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DVD

• Digital Video Disc (or Versatile)
• Spiral is smaller
• Pits/Lands are smaller
• Smaller laser
• Holds up to 4.7G (vs 650M)
• 4 types
• Single/Double Sided
• Single/Double Layered

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Low Level Formatting

• Performed by the manufacturer
• Dictates the structure of the drive
• Writes each track with information about the
  track
• Skips over bad sectors

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Disk Format

• Moving the head is slow
• Disc would have to spin back
• Cylinder Skew
• Transfer rate may be slow
• Interleaving sector s

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Disk Arm Scheduling

• Rotation and Head movement are bad
• First Come First Serve
• Shortest Seek First
• Identical to SJF Process Scheduling
• Elevator Algorithm
• Keep moving in one direction
• Turn around at end of requests

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FCFS
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SSF
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Elevator (bit set UP)
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Error Handling

• Bad sectors
• Each drive has n sectors per track
• Only m are available for use (m lt n)
• The rest are backup
• If a sector has repeated read/write errors
• Copy everything into the next backup sector
• Mark the original as bad

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Stable Storage

• If an error occurs during a write
• Old data is gone
• New data isnt there ?
• Stable storage ensures data integrity
• Relies on the fact that errors are uncommon
• Like RAID 1

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Stable Writes

• Stable Writes
• Write the data on main-disc
• Repeat until the write is successful
• Write the data on secondary-disc
• Stable Reads
• Read from the main-disc
• Repeat some number of times
• Read from secondary-disc (if unsuccessful)

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Crash Recovery

• Valid data always exists
• May not have last write, but is not invalid
• Usually acceptable

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Summary

• Magnetic vs. Optical storage
• Various levels or RAID
• Disk Arm Scheduling
• FCFS
• SSN
• Elevator
• Stable Storage affords integrity