Lecture 27
Secondary Storage Structure
Disk
Fall 2000
M.B. Ibáñez
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.
From Operating System Concepts. Silberschatz & Galvin. Addison Wesley
Fall 2000
M.B. Ibáñez
Disk Scheduling
• 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.
From Operating System Concepts. Silberschatz & Galvin. Addison Wesley
Fall 2000
M.B. Ibáñez
Disk Data Layout
Sectors
Tracks
Inter-sector gap
Inter-track gap
Fall 2000
M.B. Ibáñez
From Operating Systems.
Internals and Design Principles.
W. Stallings. Prentice Hall
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 until desired sector is rotated to line up
with the head
From Operating Systems. Internals and Design Principles. W. Stallings. Prentice Hall
Fall 2000
M.B. Ibáñez
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
• Data transfer for an entire file is faster when
the file is stored in the same cylinder and in
adjacent sectors
From Operating Systems. Internals and Design Principles. W. Stallings. Prentice Hall
Fall 2000
M.B. Ibáñez
Disk Scheduling Policies
• First-come, first-served (FCFS)
– process request sequentially
– fair to all processes
– approaches random scheduling in performance
if there are many processes
From Operating Systems. Internals and Design Principles. W. Stallings. Prentice Hall
Fall 2000
M.B. Ibáñez
FCFS
Illustration shows total head movement of 640 cylinders.
Fall 2000
M.B. Ibáñez
From Operating System Concepts. Silberschatz & Galvin. Addison Wesley
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.
Fall 2000
M.B. Ibáñez
From Operating System Concepts. Silberschatz & Galvin. Addison Wesley
SSTF (Cont.)
Fall 2000
M.B. Ibáñez
From Operating System Concepts. Silberschatz & Galvin. Addison Wesley
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.
From Operating System Concepts. Silberschatz & Galvin. Addison Wesley
Fall 2000
M.B. Ibáñez
SCAN (Cont.)
Fall 2000
M.B. Ibáñez
From Operating System Concepts. Silberschatz & Galvin. Addison Wesley
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.
Fall 2000
M.B. Ibáñez
From Operating System Concepts. Silberschatz & Galvin. Addison Wesley
C-SCAN (Cont.)
Fall 2000
M.B. Ibáñez
From Operating System Concepts. Silberschatz & Galvin. Addison Wesley
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.
Fall 2000
M.B. Ibáñez
From Operating System Concepts. Silberschatz & Galvin. Addison Wesley
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.
•
From Operating System Concepts. Silberschatz & Galvin. Addison Wesley
Fall 2000
M.B. Ibáñez
RAID 0 (non-redundant)
strip 0
strip 1
strip 2
strip 3
strip 4
strip 5
strip 6
strip 7
strip 8
strip 9
strip 10
strip 11
strip 12
strip 13
strip 14
strip 15
From Operating Systems. Internals and Design Principles. W. Stallings. Prentice Hall
Fall 2000
M.B. Ibáñez
RAID 1 (mirrored)
Fall 2000
strip 0
strip 1
strip 2
strip 3
strip 4
strip 5
strip 6
strip 7
strip 8
strip 9
strip 10
strip 11
strip 12
strip 13
strip 14
strip 15
strip 0
strip 1
strip 2
strip 3
strip 4
strip 5
strip 6
strip 7
strip 8
strip 9
strip 10
strip 11
strip 12
strip 13
strip 14
strip 15
M.B. Ibáñez
From Operating Systems.
Internals and Design Principles.
W. Stallings. Prentice Hall
RAID 2 (redundancy through
Hamming code)
b0
Fall 2000
b1
b2
b2
M.B. Ibáñez
f0(b)
f1(b)
f2(b)
From Operating Systems.
Internals and Design Principles.
W. Stallings. Prentice Hall
RAID 3 (bit-interleaved parity)
b0
b1
b2
b2
P(b)
From Operating Systems.
Internals and Design Principles.
W. Stallings. Prentice Hall
Fall 2000
M.B. Ibáñez
RAID 4 (block-level parity)
block 0
block 1
block 2
block 3
P(0-3)
block 4
block 5
block 6
block 7
P(4-7)
block 8
block 9
block 10
block 11
block 12
block 13
block 14
block 15
P(8-11)
P(12-15)
Fall 2000
M.B. Ibáñez
From Operating Systems.
Internals and Design Principles.
W. Stallings. Prentice Hall
RAID 5 (block-level distributed
parity)
block 0
block 1
block 2
block 3
P(0-3)
block 4
block 5
block 6
P(4-7)
block 7
block 8
block 12
block 9
P(8-11)
block 10
block 11
P(12-15)
block 13
block 14
block 15
block 16
block 17
block 18
block 19
P(16-19)
From Operating Systems.
Internals and Design Principles.
W. Stallings. Prentice Hall
Fall 2000
M.B. Ibáñez
Data Mapping for RAID Level 0
Array
Physical
Disk 0
Physical
Disk 1
Physical
Disk 2
Physical
Disk 3
strip 0
strip 1
strip 2
strip 3
strip 4
strip 5
strip 6
strip 7
strip 8
strip 9
strip 10
strip 11
strip 12
strip 13
strip 14
strip 15
strip 0
strip 1
strip 2
strip 3
strip 4
strip 5
strip 6
strip 7
strip 8
strip 9
strip 10
strip11
Array
Management
Software
strip 12
strip 13
strip 14
strip 15
Fall 2000
From Operating Systems.
Internals and Design Principles.
W. Stallings. Prentice Hall
M.B. Ibáñez