Chapter 13 14: I/O Systems and Mass-Storage Structure

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Chapter 1314 I/O Systems and Mass-Storage
Structure

• I/O Hardware
• Application I/O Interface
• Kernel I/O Subsystem
• Disk Structure
• Disk Scheduling
• Disk Management
• Swap-Space Management

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I/O Hardware

• Incredible variety of I/O devices
• Common concepts
• Port
• Bus (daisy chain or shared direct access)
• Controller (host adapter)
• I/O instructions control devices
• Devices have addresses, used by
• Direct I/O instructions
• Memory-mapped I/O

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A Typical PC Bus Structure
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Interrupts

• CPU Interrupt request line triggered by I/O
  device
• Interrupt handler receives interrupts
• Maskable to ignore or delay some interrupts
• Interrupt vector to dispatch interrupt to correct
  handler
• Based on priority
• Some unmaskable
• Interrupt mechanism also used for exceptions

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Interrupt-Driven I/O Cycle
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Intel Pentium Processor Event-Vector Table
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Direct Memory Access

• Used to avoid programmed I/O for large data
  movement
• Requires DMA controller
• Bypasses CPU to transfer data directly between
  I/O device and memory

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Six Step Process to Perform DMA Transfer
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Application I/O Interface

• I/O system calls encapsulate device behaviors in
  generic classes
• Device-driver layer hides differences among I/O
  controllers from kernel
• Devices vary in many dimensions
• Character-stream or block
• Sequential or random-access
• Sharable or dedicated
• Speed of operation
• read-write, read only, or write only

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A Kernel I/O Structure
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Characteristics of I/O Devices
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Block and Character Devices

• Block devices include disk drives
• Commands include read, write, seek
• Raw I/O or file-system access
• Memory-mapped file access possible
• Character devices include keyboards, mice, serial
  ports
• Commands include get, put
• Libraries layered on top allow line editing

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Blocking and Nonblocking I/O

• Blocking - process suspended until I/O completed
• Easy to use and understand
• Insufficient for some needs
• Nonblocking - I/O call returns as much as
  available
• User interface, data copy (buffered I/O)
• Implemented via multi-threading
• Returns quickly with count of bytes read or
  written
• Asynchronous - process runs while I/O executes
• Difficult to use
• I/O subsystem signals process when I/O completed

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Kernel I/O Subsystem

• Scheduling
• Some I/O request ordering via per-device queue
• Some OSs try fairness
• Buffering - store data in memory while
  transferring between devices
• To cope with device speed mismatch
• To cope with device transfer size mismatch
• To maintain copy semantics

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Kernel I/O Subsystem

• Caching - fast memory holding copy of data
• Always just a copy
• Key to performance
• Spooling - hold output for a device
• If device can serve only one request at a time
• i.e., Printing
• Device reservation - provides exclusive access to
  a device
• System calls for allocation and deallocation
• Watch out for deadlock

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Life Cycle of An I/O Request
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Disk Structure

• Disk drives are addressed as large 1-dimensional
  arrays of logical blocks, where the logical block
  is the smallest unit of transfer.
• The 1-dimensional array of logical blocks is
  mapped into the sectors of the disk sequentially.
• Sector 0 is the first sector of the first track
  on the outermost cylinder.
• Mapping proceeds in order through that track,
  then the rest of the tracks in that cylinder, and
  then through the rest of the cylinders from
  outermost to innermost.

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Disk Scheduling

• The operating system is responsible for using
  hardware efficiently for the disk drives, this
  means having a fast access time and disk
  bandwidth.
• Access time has two major components
• Seek time is the time for the disk are to move
  the heads to the cylinder containing the desired
  sector.
• Rotational latency is the additional time waiting
  for the disk to rotate the desired sector to the
  disk head.
• Minimize seek time
• Seek time ? seek distance
• Disk bandwidth is the total number of bytes
  transferred, divided by the total time between
  the first request for service and the completion
  of the last transfer.

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Disk Scheduling (Cont.)

• Several algorithms exist to schedule the
  servicing of disk I/O requests.
• We illustrate them with a request queue (0-199).
• 98, 183, 37, 122, 14, 124, 65, 67
• Head pointer 53

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FCFS
Illustration shows total head movement of 640
cylinders.
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SSTF

• Selects the request with the minimum seek time
  from the current head position.
• SSTF scheduling is a form of SJF scheduling may
  cause starvation of some requests.
• Illustration shows total head movement of 236
  cylinders.

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SSTF (Cont.)
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SCAN

• The disk arm starts at one end of the disk, and
  moves toward the other end, servicing requests
  until it gets to the other end of the disk, where
  the head movement is reversed and servicing
  continues.
• Sometimes called the elevator algorithm.
• Illustration shows total head movement of 208
  cylinders.

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SCAN (Cont.)
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C-SCAN

• Provides a more uniform wait time than SCAN.
• The head moves from one end of the disk to the
  other. servicing requests as it goes. When it
  reaches the other end, however, it immediately
  returns to the beginning of the disk, without
  servicing any requests on the return trip.
• Treats the cylinders as a circular list that
  wraps around from the last cylinder to the first
  one.

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C-SCAN (Cont.)
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C-LOOK

• Version of C-SCAN
• Arm only goes as far as the last request in each
  direction, then reverses direction immediately,
  without first going all the way to the end of the
  disk.

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C-LOOK (Cont.)
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Selecting a Disk-Scheduling Algorithm

• SSTF is common and has a natural appeal
• SCAN and C-SCAN perform better for systems that
  place a heavy load on the disk.
• Performance depends on the number and types of
  requests.
• Requests for disk service can be influenced by
  the file-allocation method.
• The disk-scheduling algorithm should be written
  as a separate module of the operating system,
  allowing it to be replaced with a different
  algorithm if necessary.
• Either SSTF or LOOK is a reasonable choice for
  the default algorithm.

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Disk Management

• Low-level formatting, or physical formatting
  Dividing a disk into sectors that the disk
  controller can read and write.
• To use a disk to hold files, the operating system
  still needs to record its own data structures on
  the disk.
• Partition the disk into one or more groups of
  cylinders.
• Logical formatting or making a file system.
• Boot block initializes system.
• The bootstrap is stored in ROM.
• Bootstrap loader program.
• Methods such as sector sparing used to handle bad
  blocks.

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MS-DOS Disk Layout
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Swap-Space Management

• Swap-space Virtual memory uses disk space as an
  extension of main memory.
• Swap-space can be carved out of the normal file
  system,or, more commonly, it can be in a separate
  disk partition.
• Swap-space management
• 4.3BSD allocates swap space when process starts
  holds text segment (the program) and data
  segment.
• Kernel uses swap maps to track swap-space use.
• Solaris 2 allocates swap space only when a page
  is forced out of physical memory, not when the
  virtual memory page is first created.