I/O Management and Disk Scheduling

1
I/O Management and Disk Scheduling

• Chapter 11

2
I/O Driver

• OS module which controls an I/O device
• hides the device specifics from the above layers
  in the OS/kernel
• translates logical I/O into device I/O (logical
  disk blocks into track, head, sector)

• performs data buffering and scheduling of I/O
  operations
• structure several synchronous entry points
  (device initialization, queue I/O requests, state
  control, read/write) and an asynchronous entry
  point (to handle interrupts)

3
Typical driver structure

• driver_strategy(request)
• if (empty(request-queue))
• driver_start(request)
• else
• add(request, request-queue)
• block_current_process reschedule()
• driver_start(request)
• current_request request
• start_dma(request)
• driver_ioctl(request)

driver_init() driver_interrupt(state) /
asynchronous part / if (stateERROR)
(retriesltMAX) driver_start(current_re
quest) return
add_current_process_to_active_queue if (!
(empty(request_queue)) driver_start(get_next
(request_queue))
4
User to Driver Control Flow
read, write, ioctl
user
kernel
ordinary file
special file
File System
character device
block device
Buffer Cache
Character queue
driver_read/write
driver-strategy
5
I/O Buffering

• before an I/O request is placed the
  source/destination of the I/O transfer must be
  locked in memory
• I/O buffering data is copied from user space to
  kernel buffers which are pinned to memory
• works for character devices (terminals), network
  and disks
• buffer cache a buffer in main memory for disk
  sectors
• character queue follows the producer/consumer
  model (characters in the queue are read once)
• unbuffered I/O to/from disk (block device) VM
  paging for instance

6
Buffer Cache

• when an I/O request is made for a sector, the
  buffer cache is checked first
• if it is missing from the cache, it is read into
  the buffer cache from the disk
• exploits locality of reference as any other cache
• usually replacements done in chunks (a whole
  track can be written back at once to minimize
  seek time)
• replacement policies are global and controlled by
  the kernel

7
Replacement policies

• buffer cache organized like a stack replace from
  the bottom
• LRU replace the block that has been in the cache
  longest with no reference to it (on reference a
  block is moved to the top of the stack)
• LFU replace the block with the fewest references
  (counters which are incremented on reference and
  blocks move accordingly)
• frequency-based replacement define a new section
  on the top of the stack, counter is unchanged
  while the block is in the new section

8
Least Recently Used

• The block that has been in the cache the longest
  with no reference to it is replaced
• The cache consists of a stack of blocks
• Most recently referenced block is on the top of
  the stack
• When a block is referenced or brought into the
  cache, it is placed on the top of the stack

9
Least Frequently Used

• The block that has experienced the fewest
  references is replaced
• A counter is associated with each block
• Counter is incremented each time block accessed
• Block with smallest count is selected for
  replacement
• Some blocks may be referenced many times in a
  short period of time and then not needed any more

10
Application-controlled File Caching

• two-level block replacement responsibility is
  split between kernel and user level
• a global allocation policy performed by the
  kernel which decides which process will give up a
  block
• a block replacement policy decided by the user

• kernel provides the candidate block as a hint to
  the process
• the process can overrule the kernels choice by
  suggesting an alternative block
• the suggested block is replaced by the kernel

• examples of alternative replacement policy
  most-recently used (MRU)

11
Sound kernel-user cooperation

• oblivious processes should do no worse than under
  LRU
• foolish processes should not hurt other processes
• smart processes should perform better than LRU
  whenever possible and they should never perform
  worse

• if kernel selects block A and user chooses B
  instead, the kernel swaps the position of A and B
  in the LRU list and places B in a placeholder
  which points to A (kernels choice)
• if the user process misses on B (i.e. he made a
  bad choice), and B is found in the placeholder,
  then the block pointed to by the placeholder is
  chosen (prevents hurting other processes)

12
Disk Performance Parameters

• To read or write, the disk head must be
  positioned at the desired track and at the
  beginning of the desired sector
• Seek time
• time it takes to position the head at the desired
  track
• Rotational delay or rotational latency
• time its takes for the beginning of the sector
  to reach the head

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Disk Performance Parameters

• Access time
• Sum of seek time and rotational delay
• The time it takes to get in position to read or
  write
• Data transfer occurs as the sector moves under
  the head

14
Disk I/O Performance

• disks are at least four orders of magnitude
  slower than the main memory
• the performance of disk I/O is vital for the
  performance of the computer system as a whole
• disk performance parameters

• seek time (to position the head at the track)
  20 ms
• rotational delay (to reach the sector) 8.3 ms
• transfer time 1-2 MB/sec

• access time (seek time rotational delay) gtgt
  transfer time for a sector
• therefore the order in which sectors are read
  matters a lot

15
Disk Scheduling Policies

• Seek time is the reason for differences in
  performance
• For a single disk there will be a number of I/O
  requests
• If requests are selected randomly, we will get
  the worst possible performance

16
Disk Scheduling Policies

• First-in, first-out (FIFO)
• Process request sequentially
• Fair to all processes
• Approaches random scheduling in performance if
  there are many processes

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Disk Scheduling Policies

• Priority
• Goal is not to optimize disk use but to meet
  other objectives
• Short batch jobs may have higher priority
• Provide good interactive response time

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Disk Scheduling Policies

• Last-in, first-out
• Good for transaction processing systems
• The device is given to the most recent user so
  there should be little arm movement
• Possibility of starvation since a job may never
  regain the head of the line

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Disk Scheduling Policies

• Shortest Service Time First
• Select the disk I/O request that requires the
  least movement of the disk arm from its current
  position
• Always choose the minimum Seek time

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Disk Scheduling Policies

• SCAN
• Arm moves in one direction only, satisfying all
  outstanding requests until it reaches the last
  track in that direction
• Direction is reversed

21
Disk Scheduling Policies

• C-SCAN
• Restricts scanning to one direction only
• When the last track has been visited in one
  direction, the arm is returned to the opposite
  end of the disk and the scan begins again

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Disk Scheduling Policies

• N-step-SCAN
• Segments the disk request queue into subqueues of
  length N
• Subqueues are process one at a time, using SCAN
• New requests added to other queue when queue is
  processed
• FSCAN
• Two queues
• One queue is empty for new request

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Disk Scheduling Policies

• usually based on the position of the requested
  sector rather than according to the process
  priority
• shortest-service-time-first (SSTF) pick the
  request that requires the least movement of the
  head
• SCAN (back and forth over disk) good
  distribution
• C-SCAN(one way with fast return)lower service
  variability but head may not be moved for a
  considerable period of time
• N-step SCAN scan of N records at a time by
  breaking the request queue in segments of size at
  most N
• FSCAN uses two subqueues, during a scan one
  queue is consumed while the other one is produced

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RAID

• Redundant Array of Independent Disks (RAID)
• idea replace large-capacity disks with multiple
  smaller-capacity drives to improve the I/O
  performance
• RAID is a set of physical disk drives viewed by
  the OS as a single logical drive
• data are distributed across physical drives in a
  way that enables simultaneous access to data from
  multiple drives
• redundant disk capacity is used to compensate the
  increase in the probability of failure due to
  multiple drives
• size RAID levels (design architectures)

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RAID Level 0

• does not include redundancy
• data is stripped across the available disks

• disk is divided into strips
• strips are mapped round-robin to consecutive
  disks
• a set of consecutive strips that map exactly one
  strip to each array member is called stripe

strip 2
strip 0
strip 3
strip 1
strip 7
strip 6
strip 5
strip 4
...
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RAID Level 1

• redundancy achieved by duplicating all the data
• each logical disk is mapped to two separate
  physical disks so that every disk has a mirror
  disk that contains the same data

• a read can be serviced by either of the two disks
  which contains the requested data (improved
  performance over RAID 0 if reads dominate)
• a write request must be done on both disks but
  can be done in parallel
• recovery is simple but cost is high

strip 1
strip 0
strip 0
strip 1
strip 3
strip 2
strip 3
strip 2
...
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RAID Levels 2 and 3

• parallel access all disks participate in every
  I/O request
• small strips (byte or word size)
• RAID 2 error correcting code (Hamming) is
  calculated across corresponding bits on each data
  disk and stored on log(data) parity disks
  necessary only if error rate is high
• RAID 3 a single redundant disk which keeps the
  parity bit
• P(i) X2(i) X1(i) X0(i)
• in the event of failure, data can be
  reconstructed but only one request at the time
  can be satisfied

X2(i) P(i) X1(i) X0(i)
b0
b1
b2
P(b)
...
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RAID Levels 4 and 5

• independent access each disk operates
  independently, so multiple I/O request can be
  satisfied in parallel
• large strips
• RAID 4 for small writes 2 reads 2 writes
• example if write performed only on strip 0
• P(i) X2(i) X1(i) X01(i)
• X2(i) X1(i) X0(i) X0(i) X0(i)
• P(i) X0(i) X0(i)
• RAID 5 parity strips are distributed across all
  disks

P(0-2)
strip 2
strip 1
strip 0
P(3-5)
strip 5
strip 4
strip 3
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UNIX SVR4 I/O