Disk scheduling

Disk Scheduling

Chapter 14
Based on the slides supporting the text and
B.Ramamurthy’s slides from Spring 2001

1

User-Space I/O Software

Layers of the I/O system and the
main functions of each layer 2

Disks
Disk Hardware (1)

Disk parameters for the original IBM PC floppy disk and a
Western Digital WD 18300 hard disk 3

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.

4

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.
5

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

6

FCFS

Illustration shows total head movement of 640 cylinders.
7

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.

8

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.
10

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.

12

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.

14

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.

16

RAID
 RAID – multiple disk drives provides reliability via
redundancy.
 Redundant Array of Independent Disks (RAID)
 RAID is arranged into six different levels.
 Several improvements in disk-use techniques involve the
use of multiple disks working cooperatively.

 Disk striping uses a group of disks as one storage unit.

 RAID schemes improve performance and improve the
reliability of the storage system by storing redundant
data.
 Mirroring or shadowing keeps duplicate of each disk.
 Block interleaved parity uses much less redundancy. 17

Raid Levels(continued)

 Raid levels 0 through 2
 Backup and parity drives are shaded
19

Raid Levels (continued)

 Raid levels 3 through 5
 Backup and parity drives are shaded 20

Raid Levels 6 and 7
 RAID 6:P and Q redundancy
 RAID 7: heterogeneous disks array

21

Disk Attachment
 Disks may be attached one of two
ways:

1.Host attached via an I/O port

2.Network attached via a network
connection

22

Network-Attached Storage(NAS)

23

Storage-Area Network (SAN)

24

Disk scheduling

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