Module 5.0: I/O and Disks

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Module 5.0 I/O and Disks

• I/O hardware
• Polling and Interrupt-driven I/O
• DMA
• Bock and character devices
• Blocking and nonblocking I/O
• Secondary storage
• Disk Hardware
• Disk Scheduling
• RAID

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I/O Hardware

• Incredible variety of I/O devices
• Common concepts
• Port serial, parallel, usb
• Bus (daisy chain or shared direct access)
• PCI bus, IDE Bus
• SCSI (up to 16 devices)
• 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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Polling vs. Interrupt-driven I/O

• Polling or (Programmed I/O)
• Determines state of device
• command-ready
• busy
• error
• Busy-wait cycle to wait for I/O from device
• Interrupt-driven
• (covered earlier)

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Interrupt-driven I/O Cycle
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Direct Memory Access

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

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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
ATAPI Advanced Technology Attachment Packet
Interface (for mass storage devices) SCSI Small
Computer Standard Interface
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Block and Character Devices

• Block devices include disk drives
• Information is stored in fixed-size blocks, each
  block with its own address
• Commands include read, write, seek
• file-system access
• Memory-mapped file access possible
• Character devices include keyboards, mice, serial
  ports, printers
• A stream of characters is sent or received
  without regard to any block structure
• Commands include get, put
• Other Devices
• Varying enough from block and character to have
  own interface
• E.g., Network devices, clocks, timers, etc.

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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)
• Extremely useful and popular with ULTs
• Does not block a ULT, and thus does not prevent
  other ULTS from running
• Returns quickly with count of bytes read or
  written
• Example search in Mozilla FireFox
• Asynchronous - process runs while I/O executes
• Difficult to use
• I/O subsystem signals process when I/O completed
• Used in C

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Secondary Storage

• Secondary Storage is often arranged in a
  hierarchy transparent to the user with
  fast/expensive storage at the top and slow/cheap
  storage at the bottom.

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Disk Definitions
Volume a single storage unit, e.g. CD,
diskette, tape, partition Tracks multiple
concentric circles containing data Sectors
subsection of a track Blocks subsection of a
sector created during formatting
(512-4096B) Cylinder all tracks of same
diameter in the disk pack Fixed heads one r/w
head per track Moveable heads one r/w head per
surface Access time seek time latency time
(15-60ms) block transfer time (1-2ms)
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Disk Hardware

• Seek time -- time to get r/w head to desired
  track
• Rotation/latency delay wait time for desired
  data to spin under r/w head
• Block transfer time time to read or write block
  of data

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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. (assuming no
  interleaving).
• Block interleaving is used to allow some time for
  the disk controller to transfer data to memory.
  The controller can not keep up with the
  continuous bit streams from the disk.

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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.
• Minimize seek time. The other two factors are
  fixed.
• Reason for differences in performance
• 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.
• The idea is to improve both access time and
  bandwidth by scheduling the servicing of the disk
  I/O in a good order.

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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 Shortest Seek Time First

• 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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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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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-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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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.
• Each sector/block (256,512,1024) has header,
  trailer, data
• Header contains sector number
• Trailer contains ECC (Error-Correcting Code)
• 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
• Holds data structure for NTFS or FAT
• Boot block initializes system.
• The bootstrap is stored in ROM.
• Bootstrap loader program.

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Booting from a Disk in Windows 2000
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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.

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Disk Performance and Reliability

• Technology to improving processor performance is
  higher than improving secondary storage.
• Several improvements in disk-use techniques
  involve the use of multiple disks working
  cooperatively. RAID technology, Redundant Array
  of Independent Disks.
• RAID schemes improve performance and improve the
  reliability of the storage system by storing
  redundant data.
• Set of physical disk drives viewed by the OS as a
  single logical drive
• Data is distributed across the physical drives of
  an array
• Redundant disk capacity is used to store parity
  information
• Seven RAID configuration levels (RAID level 0 to
  RAID level 6).

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Data Mapping for RAID level 0

• RAID 0 is non-redundant, the other 6 levels are.
• Data is striped across available disks
• High data transfer rate
• Strips 0, 1, 2, and 3 can be accessed in parallel
• High I/O request rate
• Servicing multiple I/O requests in parallel,
  e.g., requests for strip 0 and strip 3 can be
  serviced simultaneously.

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RAID level 1

• Redundancy is achieved using mirroring
  (duplication of data on other disks.)
• Expensive
• A read can be serviced by either disks (the least
  access time)
• A write requires updating two strips
• Recovery is simple

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RAID level 2

• All member disks participate in the execution of
  every I/O request. (Gives high transfer rate but
  not I/O request rate).
• Spindles and heads are all synchronized to the
  same position
• Strips are very small, a single byte or a word
• Redundancy is done through Hamming Code (able to
  correct single-bit error and detect double bit
  errors).
• Requires smaller number of disks compared to RAID
  level 1
• RAID level 3 requires only one extra disk. It
  uses parity checks.

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RAID Levels