QUESTION BANK - Mahalakshmi Engineering...

MAHALAKSHMI ENGINEERING COLLEGE

QUESTION BANK

Sub Code: CS2411 Sub Name: Operating Systems

Dept: EEE Sem/Year:VII/IV

UNIT-V

PART – A (2 Marks)

1. Define swap space. (AUC NOV’08, NOV ‘10, APR’10)

The main goal for the design and implementation of swap space is to provide the best

throughput for the virtual memory system.

Swap-space — Virtual memory uses disk space as an extension of main memory.

Swap-space can be carved out of the normal file system, or, more commonly, it can be in a

separate disk partition.

2. Write the basic functions which are provided by the hardware clocks and timers.

(AUC APR/MAY2011)

Most computers have hardware clocks and timers that provide three basic functions: 1. Give the current time 2. Give the elapsed time 3. Set a timer to trigger operation X at time T These functions are used heavily by the operating system, and also by time sensitive

applications. The hardware to measure elapsed time and to trigger operations is called a

programmable interval timer.

3. What is polling?

Polling-The host repeatedly reads the busy bit until that bit becomes clear.

The interaction on between the host and controller can be done usinghand shaking concept.

This can be done the following steps.

Determines state of device

command-ready

busy

Error

Busy-wait cycle to wait for I/O from device

4. What is storage-area network? . (AUC APR/MAY2011)

It is a private network among the services and storage units separates from the LAN and

WAN that connects the servers to the clients.

5. What is rotational latency? (AUC NOV 2010)


Rotational latency is the additional time waiting for the disk to rotatethe desired sector to the disk head.

6. What are the advantages of DMA?

DMA can be used with either polling or interrupt software. DMA is particularly

useful on devices like disks, where many bytes of information can be transferred

in single I/O operations.

When used in conjunction with an interrupt, the CPU is notified only after the

entire block of data has been transferred.

For each byte or word transferred, it must provide the memory address and all

the bus signals that control the data transfer. 7. What are the responsibilities of DMA Controller?

The work of moving data between devices and main memory is performed by the CPU as

programmed I/O or is offloaded to a DMA controller. 8. What are the differences between blocking I/O and non-blocking I/O?

Blocking - process suspended until I/O completed

Easy to use and understand

Insufficient for some needs

Non Blocking - I/O call returns as much as available

User interface, data copy (buffered I/O)

Implemented via multi- threading

Returns quickly with count of bytes read or written

9. Define caching

A cache is a region of fast memory that holds copies of data. Access to the cached copy is

more efficient than access to the original. Caching and buffering are distinct functions, but

sometimes a region of memory can be used for both purposes

10. Define spooling. (AUC NOV 2007,10)

A spool is a buffer that holds output for a device, such as printer, that cannot accept

interleaved data streams. When an application finishes printing, the spooling system queues

the corresponding spool file for output to the printer. The spooling system copies the queued

spool files to the printer one at a time.

11. What is the need for disk scheduling? .(AUC APR/MAY2010,13)

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 a. Seek time is the time for the disk are to move the heads to the cylinder containing the desired sector. b. Rotational latency is the additional time waiting for the disk to rotate the desired sector to the disk head.

To Minimize seek time is the major process for disk scheduling. 12. What is low-level formatting?


Before a disk can store data, it must be divided into sectors that the disk controller can read

and write. This process is called low-level formatting or physical formatting. Low-level

formatting fills the disk with a special data structure for each sector. The data structure for a

sector consists of a header, a data area, and a trailer.

13. What is the use of boot block?

For a computer to start running when powered up or rebooted it needs to have an initial

program to run. This bootstrap program tends to be simple. It finds the operating system on

the disk loads that kernel into memory and jumps to an initial address to begin the operating

system execution. The full bootstrap program is stored in a partition called the boot blocks,

at fixed location on the disk. A disk that has boot partition is called boot disk or system disk

14. What is sector sparing?

Low-level formatting also sets aside spare sectors not visible to the operating system. The

controller can be told to replace each bad sector logically with one of the spare sectors. This

scheme is known as sector sparing or forwarding.

15. What is RAID? List out its advantages

Redundant Array of Inexpensive Disks is a series of increasing reliable & expensive ways of

organizing multiple physical hard disks into groups that work as a single logicaldisk.

16. Writable CD-ROM media are available in both 650 MB and 700 MBversions. What is

the principle disadvantage, other than cost, of the 700MB version? .(AUC

NOV/DEC2011)

It can be store more data.

17. Which disk scheduling algorithm would be best to optimize the performance of a RAM

disk? (AUC NOV/DEC2011,13)

Shortest seek time first algorithm is used to optimize the performance of a RAM disk.

18. What is mirroring?

It duplicates the data from one disk onto a second disk using a single disk controller. 19. Give some examples for tertiary storage

1. Low cost is the defining characteristic of tertiary storage. 2. Generally, tertiary storage is built using removable media 3. Common examples of removable media are floppy disks and CD-ROMs..

20. What is seek time? (AUC MAY/JUNE 2012,14)

The seek time is the time for the disk arm to the heads to the cylinder containingdesired

sector.

21. What characteristics determine the disk access speed? (AUC MAY/JUNE 2012)

Access time has two major components


a. Seek time is the time for the disk are to move the heads to the cylinder containing the

desired sector.

b. Rotational latency is the additional time waiting for the disk to rotate the desired sector

to the disk head.

PART-B ( 16 Marks )

1. Explain in detail various disk scheduling algorithms with suitable example. (16)

(AUC NOV 2010), (AUC NOV/DEC2011)

The operating system is responsible for using hardware efficiently — for the disk drives,

this means having a fast access time and disk bandwidth.

2. Access time has two major components

a. Seek time is the time for the disk are to move the heads to the cylinder containing the

desired sector.

b. Rotational latency is the additional time waiting for the disk to rotate the desired sector

to the disk head.

3. Minimize seek time

4. Seek time =seek distance

5. 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.

6. Several algorithms exist to schedule the servicing of disk I/O requests.

Different types of scheduling algorithms are as follows.

1. First Come, First Served scheduling algorithm(FCFS).

2. Shortest Seek Time First (SSTF) algorithm

3. SCAN algorithm

4. Circular SCAN (C-SCAN) algorithm

FCFS

The simplest form of scheduling is first-in-first-out (FIFO) scheduling, which processes

items from the queue in sequential order. This strategy has the advantage of being fair,

because every request is honored and the requests are honored in the order received.

With FIFO, if there are only a few processes that require access and if many of the

requests are to clustered file sectors, then we can hope for good performance.


Priority With a system based on priority (PRI), the control of the scheduling is outside

the control of disk management software.

Last In First Out ln transaction processing systems, giving the device to the most recent

user should result. In little or no arm movement for moving through a sequential file.

Taking advantage of this locality improves throughput and reduces queue length.

Illustration shows total head movement of 640 cylinders.

FCFS disk scheduling

Shortest Seek Time First (SSTF) algorithm

The SSTF policy is to select the disk I/O request the requires the least movement of the

disk arm from its current position. Scan With the exception of FIFO, all of the policies

described so far can leave some request unfulfilled until the entire queue is emptied.

That is, there may always be new requests arriving that will be chosen before an existing

request.

The choice should provide better performance than FCFS algorithm.

1. Selects the request with the minimum seek time from the current head position.

2. SSTF scheduling is a form of SJF scheduling; may cause starvation of some

requests.

3. Illustration shows total head movement of 236 cylinders


SSTF scheduling

Under heavy load, SSTF can prevent distant request from ever being serviced. This

phenomenon is known as starvation. SSTF scheduling is essentially a from of shortest

job first scheduling. SSTF scheduling algorithm are not very popular because of two

reasons.

1. Starvation possibly exists.

2. it increases higher overheads.

3 SCAN scheduling algorithm

The scan algorithm has the head start at track 0 and move towards the highest

numbered track, servicing all requests for a track as it passes the track. The service

direction is then reserved and the scan proceeds in the opposite direction, again picking

up all requests in order.

SCAN algorithm is guaranteed to service every request in one complete pass through

the disk. SCAN algorithm behaves almost identically with the SSTF algorithm. The

SCAN algorithm is sometimes called elevator algorithm.

1. 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.

2. Sometimes called the elevator algorithm.

3. Illustration shows total head movement of 208 cylinders.


4 C SCAN Scheduling Algorithm

The C-SCAN policy restricts scanning to one direction only. Thus, 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.

1. Provides a more uniform wait time than SCAN.

2. The head moves from one end of the disk to the other. Servicing requests as it goes.

When it reaches the other end, however,

a. it immediately returns to the beginning of the disk, without servicing any requests

on the return trip.

3. Treats the cylinders as a circular list that wraps around from the last cylinder to the

firstone.


C-SCAN scheduling

5.C - LOOK Scheduling Algorithm

Start the head moving in one direction. Satisfy the request for the closest track in that

direction when there is no more request in the direction, the head is traveling, reverse

direction and repeat. This algorithm is similar to innermost and outermost track on each

circuit.

1. Version of C-SCAN

2. 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.

C-LOOK Scheduling

Selecting a Disk-Scheduling Algorithm

1. SSTF is common and has a natural appeal

2. SCAN and C-SCAN perform better for systems that place a heavy load on the disk.

3. Performance depends on the number and types of requests.

4. Requests for disk service can be influenced by the file-allocation method.

5. 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.

6. Either SSTF or LOOK is a reasonable choice for the default algorithm.

2. Write short notes on the following :

(i) I/O Hardware (8 Marks)

(ii) RAID structure. (8 Marks) (AUC NOV 2010/MAY 2010)

(i)I/O Hardware

Computer operate variety of I/O devices

Transmission devices (network card, modems)


-interface devices (screen, keyboard, mouse)

port or bus.

-connection point

-It is a set wires and a rigidly defined protocol that specifies a set ofmessages that

can be sent on the wires.

processorcommunicate with the controller by reading and writing bit patterns in these

registers.

instructions triggers bus lines to select the proper device and to move bits intoor

out of a device registers. control devices

-mapped I/O. The device –control registersare

mapped into the address, space of the processor

A Typical PC Bus Structure

Device I/O Port Locations on PCs (partial)


1. Polling

The interaction on between the host and controller can be done usinghand shaking

concept. This can be done the following steps.

Determines state of device

command-ready

busy

Error

Busy-wait cycle to wait for I/O from device

.2. Interrupt

CPU Interrupt request line triggered by I/O device

Interrupt handler receives interrupts

Maskable to ignore or delay some interrupts

Interrupt vector to dispatch interrupt to correct handler

Based on priority

Some unmaskable

Interrupt mechanism also used for exceptions

Interrupt-Driven I/O Cycle

Interrupt vector contains the memory addresses of specialized interrupt Handles.

The purpose of a vectored interrupt mechanism is to reduce the need for a single

interrupt handlerto search all possible sources of interrupts to determine which one

needs service.

RAID structure

To provide redundancy at lower cost by using the idea of disk striping combinedwith

"parity" bits (which we describe next) have been proposed. These schemeshave

different cost-performance trade-offs and are classified according tolevels called RAID

levels..


1. RAID – multiple disk drives provides reliability via redundancy.

2. RAID is arranged into six different levels.

3. Several improvements in disk-use techniques involve the use of multiple disks

workingcooperatively.

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

5. RAID schemes improve performance and improve the reliability of the storage system

bystoring redundant data.

RAID Level 0. RAID level 0 refers to disk arrays with striping at the level ofblocks but

without any redundancy

RAID Level 1. RAID level 1 refers to disk mirroring

RAID Level 2. RAID level 2 is also known as memory-style error-correcting

code(ECC) organization. Memory systems have long detected certain errors by using

parity bits. Each byte in a memory system may have a parity bit associated with it that

records whether the number of bits in the

byte set to 1 is even (parity = 0) or odd (parity = 1). If one of the bits in the byte is

damaged (either a 1 becomes a 0, or a 0 becomes a 1), the parity of the byte changes

and thus will not match the stored parity.


ECC can be used directly in disk arrays via striping of bytes across disks. If one of the

disks fails, the remaining bits of the byte and the associated error-correction bits can be

read from otherdisks and used to reconstruct the damaged data.

RAID Level 3. RAID level 3, or bit-interleaved parity organization. If one of the sectors is

damaged, whether any bit in the sector is a 1 or a 0 by computing the parity of the

corresponding bits from sectors in the other disks. If the parity of the remaining bits is

equalto the stored parity, the missing bit is 0; otherwise, it is 1.RAID level 3 is as good as

level 2 but is less expensive in the number of extra disks requiredRAID level 3 has two

advantages over level 1.


First, the storage overheadis reduced because only one parity disk is

needed for several regulardisks, whereas one mirror disk is needed for

every disk in level 1.

Second,since reads and writes of a byte are spread out over multiple

disks withA/-way striping of data, the transfer rate for reading or writing a

singleblock is N times as fast as with RAID level 1. On the negative side,

RAIDlevel 3 supports fewer I/O’s per second, since every disk has to

participate in every I/O request..

RAID Level 4. RAID level 4, or block-interleaved parity organization, uses block-level

striping, as in RAID 0, and in addition keeps a parity block on a separate disk for

corresponding blocks from A! other disks. If one of the disks fails, the parity block can be

used with the corresponding blocks from the other disks to restore the blocks of the

failed disk. Data-transfer rate for each accessis slower, but multiple read accesses can

proceed in parallel, leading to ahigher overall I/O rate.

The transfer rates for large reads are high, since allthe disks can be read in

parallel; large writes also have high transfer rates, since the data and parity can be

written in parallel. An operatingsystem write of data smaller than a block requires that

the block be read,modified with the new data, and written back. The parity block has to

beupdated as well. This is known as the read-modify-write cycle. Thus, asingle write

requires four disk accesses: two to read the two old blocks andtwo to write the two new

blocks.

RAID Level 5. RAID level 5, or block-interleaved distributed parity, differsfrom level 4 by

spreading data and parity among all N + 1 disks, ratherthan storing data in N disks and

parity in one disk. For each block, one ofthe disks stores the parity, and the others store

data.

RAID Level 6. RAID level 6, also called the P + Q redundancy scheme, is much like

RAID level 5 but stores extra redundant information to guard against multiple disk

failures. Instead of parity, error-correcting codes such as the Reed-Solomon codes are

used.

RAID Level 0 + 1. RAID level 0 + 1 refers to a combination of RAID levels0 and 1. RAID

0 provides the performance, while RAID 1 provides the reliability.

(ii)Kernel I/O Subsystem

Kernels provide many services related to I/O. Several services—scheduling,

buffering, caching, spooling. The I/O subsystem is also responsible for protecting itself

from errant processes and malicious users.

1.I/O Scheduling

Operating-system developers implement scheduling by maintaining a waitqueue

of requests for each device. When an application issues a blocking I/Osystem call, the

request is placed on the queue for that device. The I/O scheduler rearranges the order of

the queue to improve the overall system efficiencyand the average response time

experienced by applications.

The operating system may also try to be fair, so that no one application receives

especially poor service, or it may give priority service for delay-sensitive requests.


When a kernel supports asynchronous I/O, it must be able to keep trackof many

I/O requests at the same time. For this purpose, the operating systemmight attach the

wait queue to a device-status table. The kernel manages this table, which contains an

entry for each I/O device, I/O subsystem improves the efficiency of the computer is by

scheduling I/O operations. Another way is by using storage space in main memory or on

disk via techniques called buffering, caching, and spooling.

2.Buffering

A buffer is a memory area that stores data while they are transferred between

two devices or between a device and an application. Buffering is done for three reasons.

One reason is to cope with a speed mismatch between the producer and consumer of a

data stream.

This double buffering decouples the producer of data from the consumer, thus

relaxing timing requirements between them.

The use of buffering is to adapt between devices that have different data-

transfer sizes. Such disparities are especially common in computer networking, where

buffers are used widely for fragmentation and reassembly of messages.

buffering is to support copy semantics for application I/O.An example will clarify

the meaning of "copy semantics.'' Suppose that an application has a buffer of data that it

wishes to write to disk. It calls the write () system call, providing a pointer to the buffer

and an integer specifying the number of bytes to write

3.Caching

A cache is a region of fast memory that holds copies of data. Access to the

cached copy is more efficient than access to the original .Caching and buffering are

distinct functions, but sometimes a region of memory can be used for both purposes.

4 Spooling and Device Reservation

A spool is a buffer that holds output for a device, such as a printer, that cannot accept

interleaved data streams. Although a printer can serve only one job at a time, several


applications may wish to print their output concurrently. The operating system solves

thisproblem by intercepting all output to the printer. Each application's outputis spooled to a

separate disk file. When an application finishes printing, the

spooling system queues the corresponding spool file for output to the printer.In some operating

systems The spooling system copies the queued spool files to the printer one at a time. In

some operating systems, spooling is managed by a system daemon process. In others, it is

handled by an in-kernel thread.

5 Error Handling

An operating system that uses protected memory can guard against many kinds of hardware

and application errors, so that a complete system failure is not the usual result of each minor

mechanical glitch. Operating systems can often compensate effectively for transient failures.

For instance, a disk read() failure results in a readC) retry, and a network send() error results in

a resendO, if the protocol specifies. some hardware can provide highly detailed error

information, although many current operating systems are not designed to convey this

information to the application.

3. Explain the services provided by a kernel I/O subsystem. (8)(refer Q.no.2)

Explain and compare the C-LOOK and C-SCAN disk scheduling

algorithms. (8) (AUC APR ‘10)

The C-SCAN policy restricts scanning to one direction only. Thus, 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.

This reduces the maximum delay experienced by new requests

1. Provides a more uniform wait time than SCAN.

2. The head moves from one end of the disk to the other. Servicing requests as it goes.When it

reaches the other end, however,

a. it immediately returns to the beginning of the disk, without servicing any requestson the return

trip.

3. Treats the cylinders as a circular list that wraps around from the last cylinder to the firstone.


C-LOOK Scheduling

Start the head moving in one direction. Satisfy the request for the closest track in that direction

when there is no more request in the direction, the head is traveling, reverse direction and

repeat. This algorithm is similar to innermost and outermost track on each circuit.

1. Version of C-SCAN

2. Arm only goes as far as the last request in each direction, then reverses

directionimmediately, without first going all the way to the end of the disk.

4. Explain in detail the salient features of Linux I/O. (10) (AUC APR/MAY 2010)

To the user, the I/O system in Linux looks much like that in any UNIX system.That is, to

the extent possible, all device drivers appear as normal files.


The systemadministrator can create special files within a file system that contain referencesto

a specific device driver, and a user opening such a file will be able to read from and write to

the device referenced.Linux splits all devices into three classes:

block devices,

character devices,

network devices.

Block devices include all devices that allow random access to completely independent,

fixed-sized blocks of data, including hard disks and floppy disks,CD-ROMs, and flash

memory. Block devices are typically used to store file systems, but direct access to a block

device is also allowed so that program scan create and repair the file system.

ablock represents the unit with which the kernel performs I/O. When a block is read into

memory, it is stored in a buffer.The request manager is the layer of software that manages

the reading andwriting of buffer contents to and from a block-device driver.

A separate list of requests is kept for each block-device driver. Traditionally, these requests

have been scheduled according to a unidirectional-elevator (C-SCAN) algorithm that exploits

the order in which requests are inserted in and removed from the per-device lists.

The fundamental problem with the elevator algorithm is that I/O operations concentrated in a

specific region of the disk can result in starvation of requests that need to occur in other

regions of the disk.

The deadlineI/O scheduler used in version 2.6 works similarly to the elevator algorithm

except that it also associates a deadline with each request, thus addressing the starvation

issue. The deadline scheduler maintains asorted queue of pending I/O operations sorted by

sector number. However, it also maintains two other queues—a read queue for read

operations and a write queue for write operations. These two queues are ordered according

to deadline. Every I/O request is placed in both the sorted queue and either thread or the

write queue

Character devices include most other devices, such as mice and keyboards.The fundamental

difference between block and character devices is randomaccess—block devices may be

accessed randomly, while character devices areonly accessed serially. For example, seeking to

a certain position in a file might

be supported for a DVD but makes no sense to a pointing device such as amouse.

The kernel maintains a standard interface to these drivers by means of a set of tty_struct

structures. Each of these structures provides buffering and flow control on the data stream from

the terminal device and feeds those data to a line discipline.

A line discipline is an interpreter for the information from the terminaldevice. The most

common line discipline is the t t y discipline, which glues theterminal's data stream onto the

standard input and output streams of a user'srunning processes, allowing those processes to

communicate directly with theuser's terminal. This job is complicated by the fact that several

such processesmay be running simultaneously, and the t t y line discipline is responsible

forattaching and detaching the terminal's input and output from the variousprocesses connected

to it as those processes are suspended or awakened by theuser.


Network devices are dealt with differently from block and character devices. Users

cannot directly transfer data to network devices; instead, they must communicate

indirectly by opening a connection to the kernel'snetworking subsystem.

5. Describe the important concepts of application I/O interface. (16) (AUC NOV’11)

Structuring techniques and interfaces for the operating system enable I/O devices to be treated in a standard, uniform way. For instance, how an application can open a file on a disk without knowing what kind of disk it is, and how new disks and other devices can be added to a computer without the operating system being disrupted

The actual differences are encapsulated in ken modules called device drivers mat internally are

custom tailored to each device but that export one of the standard interfaces.

The purpose of the device-driver layer is to hide the differences among device controllers from

the I/O subsystem of the kernel, much as the I/O system calls.

Character-stream or block. A character-stream device transfers bytes one by one, whereas a

block device transfers a block of bytes as a unit.

Sequential or random-access. A sequential device transfers data in a fixed order that is

determined by the device, whereas the user of a random-access device can instruct the device

to seek to any of the available data storage locations.

Synchronous or asynchronous.A synchronous device is one that performs data transfers with

predictable response times. An asynchronous device exhibits irregular or unpredictable

response times.

Sharable or dedicated.A sharable device can be used concurrently by several processes or

threads; a dedicated device cannot.

Speed of operation. Device speeds range from a few bytes per second to a few gigabytes per

second.

Read-write, read only, or write only. Some devices perform both input and output, but others

support only one data direction. For the purpose of application access, many of these

differences are hidden by the operating system, and the devices are grouped into a few

conventional types.

Operating systems also provide special system calls to access a few additional devices,

such as a time-of-day clock and a timer. The performance and addressing

characteristics of network I/O differ significantly from those of disk I/O, most operating

systems provide a network I/O interface that is different from the read-write-seek

interface used for disks.

6. (i) Consider the following I/O scenarios on a single-user PC.

(1) A mouse used with a graphical user interface

(2) A tape drive on a multi tasking operating system (assume no device

reallocation is available)

(3) A disk drive containing user files


(4) A graphics card with direct bus connection, accessible through memory-

mapped I/O

For each of these I/O scenarios, would you design the operating system to use

buffering, spooling, caching, or a combination? Would you use polled I/O, or

interrupt driven I/O?

Give reasons for your choices. (8)

(ii) How do you choose a optimal technique among the various disk scheduling

techniques? Explain.(8) (AUC MAY/JUNE 2012)

7. Describe the various levels of RAID. (8) (AUC MAY/JUNE 2012)

RAID structure

To provide redundancy at lower cost by using the idea of disk striping combined with

"parity" bits (which we describe next) have been proposed. These schemeshave

different cost-performance trade-offs and are classified according tolevels called RAID

levels..

1. RAID – multiple disk drives provides reliability via redundancy.

2. RAID is arranged into six different levels.

3. Several improvements in disk-use techniques involve the use of multiple disks

workingcooperatively.

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

5. RAID schemes improve performance and improve the reliability of the storage system

bystoring redundant data.

RAID Level 0. RAID level 0 refers to disk arrays with striping at the level ofblocks but

without any redundancy

RAID Level 1. RAID level 1 refers to disk mirroring

RAID Level 2. RAID level 2 is also known as memory-style error-correcting

code(ECC) organization. Memory systems have long detected certain errors by using

parity bits. Each byte in a memory system may have a parity bit associated with it that

records whether the number of bits in the

byte set to 1 is even (parity = 0) or odd (parity = 1). If one of the bits in the byte is

damaged (either a 1 becomes a 0, or a 0 becomes a 1), the parity of the byte changes

and thus will not match the stored parity.


ECC can be used directly in disk arrays via striping of bytes across disks. If one of the

disks fails, the remaining bits of the byte and the associated error-correction bits can be

read from otherdisks and used to reconstruct the damaged data.

RAID Level 3. RAID level 3, or bit-interleaved parity organization. If one of the sectors is

damaged, whether any bit in the sector is a 1 or a 0 by computing the parity of the

corresponding bits from sectors in the other disks. If the parity of the remaining bits is

equalto the stored parity, the missing bit is 0; otherwise, it is 1.RAID level 3 is as good as

level 2 but is less expensive in the number of extra disks requiredRAID level 3 has two

advantages over level 1.

First, the storage overhead is reduced because only one parity disk is

needed for several regular disks, whereas one mirror disk is needed for

every disk in level 1.

Second, since reads and writes of a byte are spread out over multiple

disks with A/-way striping of data, the transfer rate for reading or writing a

single block is N times as fast as with RAID level 1. On the negative side,

RAID level 3 supports fewer I/O’s per second, since every disk has to

participate in every I/O request..


RAID Level 4. RAID level 4, or block-interleaved parity organization, uses block-level

striping, as in RAID 0, and in addition keeps a parity block on a separate disk for

corresponding blocks from A! other disks. If one of the disks fails, the parity block can be

used with the corresponding blocks from the other disks to restore the blocks of the

failed disk. Data-transfer rate for each accessis slower, but multiple read accesses can

proceed in parallel, leading to ahigher overall I/O rate.

The transfer rates for large reads are high, since allthe disks can be read in

parallel; large writes also have high transfer rates, since the data and parity can be

written in parallel. An operatingsystem write of data smaller than a block requires that

the block be read,modified with the new data, and written back. The parity block has to

beupdated as well. This is known as the read-modify-write cycle. Thus, asingle write

requires four disk accesses: two to read the two old blocks andtwo to write the two new

blocks.

RAID Level 5. RAID level 5, or block-interleaved distributed parity, differsfrom level 4 by

spreading data and parity among all N + 1 disks, ratherthan storing data in N disks and

parity in one disk. For each block, one ofthe disks stores the parity, and the others store

data.

RAID Level 6. RAID level 6, also called the P + Q redundancy scheme, is much like

RAID level 5 but stores extra redundant information to guard against multiple disk

failures. Instead of parity, error-correcting codes such as the Reed-Solomon codes are

used.

RAID Level 0 + 1. RAID level 0 + 1 refers to a combination of RAID levels0 and 1. RAID

0 provides the performance, while RAID 1 provides the reliability.

8. Suppose that a disk drive has 5000 cylinders, numbered 0 to 4999. The drive is

currently serving a request at cylinder 143, and the previous request was at

cylinder 125. The queue of pending requests, in FIFO order, is 86, 1470, 913, 1774,

948, 1509, 1022, 1750, 130 Starting from the current head position, what is the

total distance ((in cylinders) that the disk arm moves to satisfy all the pending

requests, for each of the following disk scheduling

a. FCFS b. SSTFc. SCAN d. LOOK e. C-SCAN

The FCFS schedule is 143, 86, 1470, 913, 1774, 948, 1509, 1022,1750, 130. The total seek distance is 7081. b. The SSTF schedule is 143, 130, 86, 913, 948, 1022, 1470, 1509, 1750,1774. The total seek distance is 1745. c. The SCAN schedule is 143, 913, 948, 1022, 1470, 1509, 1750, 1774,4999, 130, 86. The total seek distance is 9769. d. The LOOK schedule is 143, 913, 948, 1022, 1470, 1509, 1750, 1774,130, 86. The total seek distance is 3319.


e. The C-SCAN schedule is 143, 913, 948, 1022, 1470, 1509, 1750, 1774,4999, 86, 130. The total seek distance is 9813. f. (Bonus.) The C-LOOK schedule is 143, 913, 948, 1022, 1470, 1509,1750, 1774, 86, 130.

The total seek distance is 3363.

9. Compare the performance of C-SCAN and SCAN scheduling, assuming a inform

distribution of requests. Consider the average response time (the time between the

arrival of a request and the completion of that request’s service), the variation in

response time, and the effective bandwidth. How does performance depend on the

relative sizes of seek time and rotational latency?

(16)

10. Write note on

(i) Disk attachment

(ii) Streams

(iii) Tertiary Storage

Disk attachment

Disks may be attached one of two ways:

Host attached via an I/O port

Network-Attached Storage

FigureNetwork-Attached Storage

Storage-Area Network


Storage-Area Networks

(ii) Streams

A stream isa full-duplex connection between a device driver and a user-level process. Itconsists

of a stream head that interfaces with the user process, a driverthat controls the device, and

zero or modules more between the stream

head and the driver end. Each of these components contains a pair of queues-a read queue

and a write queue. Message passing is used to transfer databetween queues.


Modules provide the functionality of STREAMS processing; they are pushedonto a stream by

use of the ioctl () system call.

STREAMS I/0 is asynchronous (or nonblocking) except when the userprocess communicates

with the stream~ head. When writing to the stream,the user process will block, assuming the

next queue uses flow controtuntilthere is room to copy the message.

The benefit of using STREAMS is that it provides a framework for amodular and incremental

approach to writing device drivers and networkprotocols. Modules may be used by different

streams and hence by differentdevices.

For example, a networking module may be used by both an Ethernetnetwork card and a 802.11

wireless network card

(iii)Territory storage

1. Low cost is the defining characteristic of tertiary storage.

2. Generally, tertiary storage is built using removable media

3. Common examples of removable media are floppy disks and CD-ROMs; other types are

available

Removable Disks

1. Floppy disk — thin flexible disk coated with magnetic material, enclosed in a protective plastic

case.

imilar technology is used for removable disks that hold more

than 1 GB.

are at a greater risk

of damage from exposure.

2. A magneto-optic disk records data on a rigid platter coated w

heat is used to amplify a large, weak magnetic field to record a bit.

-optic head flies much farther from the disk surface than magnetic disk head,

and the magnetic material is covered with

a protective layer of plastic or glass; resistant to head crashes.

3. Optical disks do not use magnetism; they employ special materials that are altered by laser

light.

2. WORM Disks

1. The data on read-write disks can be modified over and over.

2. WORM (“Write Once, Read Many Times”) disks can be written only once.

3. Thin aluminum film sandwiched between two glass or plastic platters.

4. To write a bit, the drive uses a laser light to burn a small hole through the aluminum;

information can be destroyed by not altered.

5. Very durable and reliable

6. Read Only disks, such ad CD-ROM and DVD, com from the factory with the data

prerecorded.

3. Tapes

1. Compared to a disk, a tape is less expensive and holds more data, but random access is

much slower.


2. Tape is an economical medium for purposes that do not require fast random access, e.g.,

backup copies of disk data, holding huge volumes of data.

3. Large tape installations typically use robotic tape changers that move tapes between tape

drives and storage slots in a tape library.

a. stacker – library that holds a few tapes

b. silo – library that holds thousands of tapes

4. A disk-resident file can be archived to

QUESTION BANK - Mahalakshmi Engineering...

Documents

Transcript of QUESTION BANK - Mahalakshmi Engineering...