Lecture Operating systems: A concept-based approach (2/e): Chapter 12 - Dhananjay M. Dhamdhere

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Chapter 12: Implementation 
of File Operations

Dhamdhere: Operating Systems—
A Concept­Based Approach, 2 ed

Slide No: 1
Copyright © 2008


Input Output Control System (IOCS)

•

The IOCS consists of two layers that provide efficient file
processing and efficient device performance
– Access Methods layer
* Each access method provides efficient processing of files with a
specific file organization, e.g., sequential file organization and direct
file organization

– Physical IOCS layer
* Performs I/O operations on devices
* Ensures efficient device performance

Chapter 12: Implementation 
of File Operations

Dhamdhere: Operating Systems—
A Concept­Based Approach, 2 ed

Slide No: 2
Copyright © 2008


Physical organizations in Access methods and
Physical IOCS

•  The physical IOCS reads data from disk into buffers or disk cache/file cache
   maintained in memory (or writes data), ensuring high device throughput
•  The access method moves the data between buffers or caches and the 
   address space of the process, ensuring efficient file processing
Chapter 12: Implementation 
of File Operations

Dhamdhere: Operating Systems—
A Concept­Based Approach, 2 ed

Slide No: 3
Copyright © 2008


Policies and Mechanisms

•

Policy

– A guiding principle for implementing a functionality (e.g., prioritybased scheduling)
* It invokes mechanisms to perform various actions required to
implement the functionality

•

Mechanism
– Specific action in implementing a functionality

Chapter 12: Implementation 
of File Operations

Dhamdhere: Operating Systems—
A Concept­Based Approach, 2 ed

Slide No: 4
Copyright © 2008


Layers of File system and IOCS

•  M: Mechanism module, P: Policy module
•  A policy module invokes mechanism modules of the same layer,
   which may invoke policy and mechanism modules of the lower layer
Chapter 12: Implementation 
of File Operations

Dhamdhere: Operating Systems—
A Concept­Based Approach, 2 ed

Slide No: 5
Copyright © 2008


Policies and mechanisms in file system
and IOCS layers

Chapter 12: Implementation 
of File Operations

Dhamdhere: Operating Systems—
A Concept­Based Approach, 2 ed

Slide No: 6
Copyright © 2008


Model of a computer system

•  The I/O subsystem has an independent data path to memory
•  Devices are connected to device controllers, which are connected to the
   DMA; a device is identified by the pair (controller id, device id)
•  The DMA, a device controller, and a device implement an I/O operation
Chapter 12: Implementation 
of File Operations

Dhamdhere: Operating Systems—
A Concept­Based Approach, 2 ed

Slide No: 7

Copyright © 2008


Access and data transfer time in an I/O operation

The total time required to perform an I/O operation = ta + tx

Chapter 12: Implementation 
of File Operations

Dhamdhere: Operating Systems—
A Concept­Based Approach, 2 ed

Slide No: 8
Copyright © 2008


Error detection approaches

•  Parity bits
•  Cyclic 
   redundancy
   checksum 
   (CRC)

Chapter 12: Implementation 
of File Operations

Dhamdhere: Operating Systems—
A Concept­Based Approach, 2 ed

Slide No: 9
Copyright © 2008


Disk data organization

•

Data should be organized such that it can be accessed
efficiently
– Notion of a Cylinder
* Consists of identically positioned tracks on all platters of a disk
 All of its tracks can be accessed for the same position of disk
heads
 Its use reduces disk head movement
 Put adjoining data of a file on tracks in the same cylinder

– Data staggering techniques
* A disk rotates a bit while disk heads are being readied or moved to
access a new track. Make sure that data to be accessed passes
under the read / write heads after their movement is completed
Chapter 12: Implementation 
of File Operations

Dhamdhere: Operating Systems—
A Concept­Based Approach, 2 ed

Slide No: 10
Copyright © 2008

A cylinder in a disk with several platters

Tracks in a cylinder can be accessed by electronically switching to use of
the disk head positioned on a track; it does not involve movement of heads

Chapter 12: Implementation 
of File Operations

Dhamdhere: Operating Systems—
A Concept­Based Approach, 2 ed

Slide No: 11
Copyright © 2008


Data staggering techniques

•

Sector interleaving
– Background: Older disks used to contain a buffer to store data
read from the disk or data to be written on it
* In a read operation, the data was first read from a disk sector into the
buffer
* Data from the buffer was then transferred to memory
* Now the disk was ready for a new operation
* But the next sector had already passed under the head!

– Technique: A few sectors are skipped while recording adjoining
file records or file data
* The number of sectors skipped is called the interleaving factor (inf)

Chapter 12: Implementation 
of File Operations

Dhamdhere: Operating Systems—
A Concept­Based Approach, 2 ed

Slide No: 12
Copyright © 2008


Sector interleaving

(a) No interleaving; adjacent records in a file occupy adjoining disk sectors
(b) Interleaving factor = 2; two sectors exist between adjacent file records

Chapter 12: Implementation 
of File Operations

Dhamdhere: Operating Systems—
A Concept­Based Approach, 2 ed

Slide No: 13
Copyright © 2008


Variation of throughput

with sector interleaving factor (inf)

•  The disk is assumed to contain 8 sectors on a track
•  inf  = 0: Consecutive file records cannot be read in the same disk revolution
•  inf  = 2: Consecutive file records can be read in the same disk revolution
•  Peak throughput is obtained by reading the same track over and over
Chapter 12: Implementation 
of File Operations

Dhamdhere: Operating Systems—
A Concept­Based Approach, 2 ed

Slide No: 14
Copyright © 2008


Variation of throughput with interleaving factor

Inf  = 2 provides the best peak throughput

Chapter 12: Implementation 
of File Operations

Dhamdhere: Operating Systems—
A Concept­Based Approach, 2 ed

Slide No: 15
Copyright © 2008

Data staggering techniques

•

Head skew
– The disk requires some time to switch from reading data of one
track to data of another track in a cylinder (Head switch time)
* Some sectors/records/blocks pass under the head during this time
* Head skewing: Stagger the data on tracks
 The first sector of a new track should pass under the head only
after its disk head is ready to read

•

Cylinder skew
– The disk rotates while the disk heads move to tracks of an
adjoining cylinder
* Cylinder skewing: Data is skewed analogous to head skew

Chapter 12: Implementation 
of File Operations

Dhamdhere: Operating Systems—
A Concept­Based Approach, 2 ed

Slide No: 16
Copyright © 2008


Redundant Array of Inexpensive Disks (RAID)

•
•

An array of cheap disks is used instead of a single disk
Different arrangements are used to provide three benefits
– Reliability
* Store data redundantly (remember stable storage?)
* Read / write redundant data records in parallel

– Fast data transfer rates
* Store file data on several disks in the RAID
* Read / write file data in parallel

– Fast access
* Store two or more copies of data
* To read data, access the copy that is accessible most efficiently
Chapter 12: Implementation 
of File Operations

Dhamdhere: Operating Systems—
A Concept­Based Approach, 2 ed

Slide No: 17
Copyright © 2008


Disk striping

•

•

A disk strip contains data (it is like a sector or disk block)
A disk stripe is a collection of identically positioned strips
on different disks in the RAID
– Data written on strips in a disk stripe can be read in parallel
* This arrangement provides high data transfer rates

Chapter 12: Implementation 
of File Operations

Dhamdhere: Operating Systems—
A Concept­Based Approach, 2 ed

Slide No: 18
Copyright © 2008


RAID levels

•

Different RAID organizations (called RAID levels) provide
different benefits
– RAID 0: Disk striping
* High data transfer rates

– RAID 1: Disk mirroring
* Identical data is written on two disks
* To read, the copy accessible faster is accessed

– RAID 0+1
* Disk striping as in RAID 0, each stripe is mirrored

– RAID 1+0
* Disks are first mirrored, then striped
* Provides better reliability than RAID 0+1
Chapter 12: Implementation 
of File Operations

Dhamdhere: Operating Systems—
A Concept­Based Approach, 2 ed

Slide No: 19
Copyright © 2008


RAID levels
– Level 2: Error correction codes
* Data and redundant bits are recorded on different disks
* For example, (12,8) Hamming code

– Level 3: Bit interleaved parity
* Similar to level 2, but uses a single parity disk
* Device controller detects error, parity bit is used to correct it

– Level 4: Block interleaved parity
* Strip contains consecutive bytes; parity strip contains parity bits

– Level 5: Block interleaved distributed parity

* Like level 4, but parity strips are spread on several disks

– Level 6: P+Q redundancy

Chapter 12: Implementation 
of File Operations

Dhamdhere: Operating Systems—
A Concept­Based Approach, 2 ed

Slide No: 20
Copyright © 2008


Physical IOCS and Device Drivers

•

The Physical IOCS
– Supports device-level I/O operations
* Provides functionalities for starting an I/O operation, checking its
status, etc. (Note: a device is identified by the pair (cu, d))

– Performs device-level performance optimization
* Schedules I/O operations on a device in such a manner that the
device throughput is maximized
* Reduces disk head movement in a disk

•

The Physical IOCS provides this support for all classes of
I/O devices
– A device driver in a modern OS provides such support for a
specific class of I/O devices; e.g., a disk driver

Chapter 12: Implementation 
of File Operations

Dhamdhere: Operating Systems—
A Concept­Based Approach, 2 ed

Slide No: 21
Copyright © 2008


Overview of device-level I/O

•

I/O instructions and I/O commands
– I/O instruction
* An I/O instruction initiates an I/O operation; it involves the CPU, DMA, device
controller and device
* An I/O operation may consist of several distinct tasks
 e.g., reading from a disk involves positioning disk heads, locating a
sector and performing data transfer
* Individual tasks in I/O are performed through ‘I/O commands’
* An I/O instruction indicates the (cu, d) pair identifying a device and specifies
the commands that describe the tasks to be performed

– I/O command
* Performs a specific task in an I/O operation
 e.g., positioning disk heads, reading data, etc.
 Does not involve the CPU
Chapter 12: Implementation 
of File Operations

Dhamdhere: Operating Systems—
A Concept­Based Approach, 2 ed

Slide No: 22
Copyright © 2008


Computer features supporting device-level I/O

•

The following features in a computer are used for I/O
– Initiating an I/O operation
* Instruction ‘I/O-init (cu, d), command address’ initiates an I/O opn
* I/O commands are assumed to occupy consecutive memory locations

– Checking device status
* Instruction ‘I/O-status (cu, d)’ provides status information regarding
the device (cu, d)

– Performing I/O operations
* Device-specific commands may be used to perform I/O

– Handling interrupts
* Interrupt hardware performs switching of PSWs

Chapter 12: Implementation 
of File Operations

Dhamdhere: Operating Systems—
A Concept­Based Approach, 2 ed

Slide No: 23
Copyright © 2008


Device-level I/O without OS support

•

We consider a program that is executing in a raw
machine, i.e., without OS support
– When the program initiates an I/O operation, it should execute the
next instruction only after the I/O operation completes
* CPU and the I/O channel operate in parallel
* Hence the CPU should do nothing until the I/O operation completes.
It is achieved as follows
 An IO_FLAG is associated with the I/O operation
 It is set by the interrupt servicing routine when the I/O operation
completes
 The CPU loops until IO_FLAG is set to the value ‘I/O complete’

Chapter 12: Implementation 

of File Operations

Dhamdhere: Operating Systems—
A Concept­Based Approach, 2 ed

Slide No: 24
Copyright © 2008


Device level I/O

•  Before initiating an I/O operation, the IO_FLAG is set to ‘I/O in progress’
•  The IO_init instruction sets a condition code regarding its outcome
•  If the I/O device is busy, the IO_init instruction is attempted repeatedly
•  The CPU loops at PROCEED until the I/O operation completes
Chapter 12: Implementation 
of File Operations

Dhamdhere: Operating Systems—
A Concept­Based Approach, 2 ed

Slide No: 25
Copyright © 2008