Project Phase

1
Project Phase 3

• Implementing file redirection can be done using
  the fcntl() system call.
• include ltfcntl.hgt
• include ltunistd.hgt
• The fcntl() subroutine performs controlling
  operations on the open file specified.

2
fcntl()

• Append standard input to a file
• fdo1 open(filename, O_WRONLY O_APPEND)
• close(1)
• fdo2 fcntl(fdo1, F_DUPFD, 1)
• Write standard output to a file
• fdo1 open(filename, O_WRONLY O_TRUNC)
• close(1)
• fdo2 fcntl(fdo1, F_DUPFD, 1)
• Read standard input from a file
• fdo1 open(filename, O_RDONLY)
• close(0)
• fdo2 fcntl(fdo1, F_DUPFD, 0)

3
Handling Signals

• signal(SIGINT,SIG_IGN)
• signal(SIGQUIT,SIG_IGN)

4
Magnetic Disk
read-write head
sector
track
platter
arm
cylinder
5
Disk Speed

• Transfer Rate rate at which data flows between
  the drive and the computer
• Positioning Time
• Seek Time - time to move the arm to a desired
  cylinder
• Rotational Latency time for the desired sector
  to rotate to the disk head
• Bandwidth - total number of bytes transferred
  divided by the total time between the first
  request and the completion of the last transfer.

6
Disk Structure

• Blocks are mapped to disk sectors
• Sector 0 is the first sector of the first track
  on the outermost cylinder.
• Then through the track, then the next track in
  the cylinder, then through the rest of the
  cylinders (outermost to innermost)

7
Disk Rotation

• Constant Angular Velocity (CAV)
• traditional
• bits near outside are spread out compared to
  those near the inside
• Constant Linear Velocity (CLV)
• variable speed disk motor
• bits are evenly spread
• disk spins at a different speed when the disk arm
  is near the center compared to when near the
  outside edge

8
Disk Scheduling

• When a process requests I/O to or from the disk,
  it specifies
• Read or write
• Disk address
• Memory address for the transfer
• Number of sectors
• Disk I/O requests will be placed in a queue if
  they cannot be serviced immediately.
• How do we choose which request to service next?
• Disk Scheduling

9
First-Come, First-Served (FCFS)
Requests (cylinders) 98, 183, 37, 122, 14, 124,
65, 67
(assume head initially is at 53)
0
14
37
53
65
67
98
122
124
183
199
640 head movements
10
Shortest-Seek-Time-First (SSTF)

• Select the request with the minimum seek time
  from the current head position.
• Intuition since we must service this request
  eventually, we might as well service it while the
  disk head is close.

11
First-Come, First-Served (FCFS)
Requests (cylinders) 98, 183, 37, 122, 14, 124,
65, 67
(assume head initially is at 53)
0
14
37
53
65
67
98
122
124
183
199
236 head movements
12
SSTF Problems

• Constantly switching directions slows things
  down.
• Starvation
• Suppose requests continually arrive for cylinders
  near the current position.
• If a request is in the queue that is far away
  from the current head position, it may remain
  pending indefinitely.

13
SCAN Scheduling

• Disk arm moves one direction, servicing requests
  as it moves, until it reaches the end of the
  disk.
• Then it reverses direction, and continues to scan
  and service requests.

14
SCAN
Requests (cylinders) 98, 183, 37, 122, 14, 124,
65, 67
(assume head initially is at 53)
0
14
37
53
65
67
98
122
124
183
199
236 head movements
15
C-SCAN

• Variant of SCAN where the disk head moves from
  one end of the disk to the other, then resets to
  its original position, and repeats.
• Does not service requests as it resets

16
C-SCAN
Requests (cylinders) 98, 183, 37, 122, 14, 124,
65, 67
(assume head initially is at 53)
0
14
37
53
65
67
98
122
124
183
199
383 head movements (200 for reset)
17
LOOK and C-LOOK

• LOOK is a variant of SCAN where the disk head
  only goes as far as the furthest request.
• C-LOOK is a variant of C-SCAN where the disk head
  only moves as far as the furthest request.

18
C-LOOK
Requests (cylinders) 98, 183, 37, 122, 14, 124,
65, 67
(assume head initially is at 53)
0
14
37
53
65
67
98
122
124
183
199
322 head movements
19
Summary

• SSTF or LOOK are often used.
• SCAN and C-SCAN are better suited for systems
  will heavy disk loads since starvation is
  eliminated.
• If disk requests are very sparse, all algorithms
  are about the same.
• Disk manufacturers will often implement a disk
  scheduling algorithm in the controller.

20
RAID

• Redundant Array of Independent Disks
• Read/write speed
• Reliability of data storage
• Redundancy
• Mirroring every write is carried out on each
  disk
• If one disk fails, the data is read from another
  disk

21
Data Striping in RAID

• Data Striping split data among multiple disks.
• Bit-Level split the bits in a byte
• Block-Level split blocks of a file
• Increases throughput by load balancing
• Reduces response time of large accesses

22
RAID Levels

• RAID schemes have been developed that combine
  disk striping with parity bits.
• Provides redundancy and speed increase
• The various schemes are implemented as different
  RAID levels.

23
RAID Levels

• Level 0 block striping
• Level 1 disk mirroring
• Level 2 parity bits and striping
• Level 3 interleaved byte parity bits
• Level 4 interleaved block parity bits
• Level 5 distributed interleaved block parity
  bits
• Level 6 like 5, but uses error-correcting codes
• Level 01 Combination of 0 and 1