Operating Systems.2

Question 1

magnetic disks rotation rate

Answer 1

60-250 times/second

Question 2

transfer rate

Answer 2

rate of data flow between drive and computer

Question 3

positioning time/ random-access time

Answer 3

seek time + rotational latency

Question 4

seek time

Answer 4

time taken to move head to desired cylinder (usually 3-12ms)

Question 5

rotational latency

Answer 5

time for desired sector to rotate under disk head. based on spindle speed (60/RPM)

Question 6

head crash

Answer 6

when disk head comes in contact with disk surface

Question 7

connection between drive and computer

Answer 7

drive attached to computer using I/O bus- busses vary.

host controller in computer uses bus to talk to disk controller built into drive or storage array

Question 8

storage range for hard disks

Answer 8

30GB - 3TB

Question 9

access latency/average access time

Answer 9

average seek time + average latency

Question 10

average I/O time

Answer 10

average access time + (amount to transfer/transfer rate) + controller overhead

Question 11

SSDs advantages

Answer 11

• non volatile (used like a hard drive)
• more reliable than HDDs
• faster
• no moving parts so no seek time/rotational latency

Question 12

SSDs disadvantages

Answer 12

• lower capacity
• expensive per MB
• last less longer
• busses too slow: can connect directly to PCI for example

Question 13

how are disk drives addressed

Answer 13

large 1-dimensional arrays of logical blocks- logical block = smallest unit of data transfer

Question 14

low level formatting

Answer 14

creates logical blocks on physical media. divides disk into sectors so disk controller can read and write

Question 15

mapping of array onto disks

Answer 15

1D array of logical blocks mapped into sectors of the disk sequentially. first sector 0, then rest of sectors on the track, then rest of tracks in the cylinder then rest of cylinders from outermost to innermost

Question 16

sector 0

Answer 16

first sector of first track on outermost cylinder

Question 17

logical to physical address is easy unless

Answer 17

bad sectors. (non constant num. of sectors per track)

Question 18

how are disks attached to system

Answer 18

an array of disks are attached and have controllers

Question 19

disk scheduling

Answer 19

OS responsible for using hardware efficiently: fast access time + disk bandwidth

Question 20

disk bandwidth

Answer 20

total bytes transferred/total time between first request and completion of last transfer

Question 21

OS disks scheduling

Answer 21

maintains queue of requests per disk or device. disk is busy- request is added to queue as drive controller has a buffer. optimisation algorithms used when a queue exists. Several algorithms to service I/O operations in queue.

Question 22

first come first serve (FCFS)

Answer 22

requests serviced in the order they arrive

Question 23

(SSTF) shortest seek time first

Answer 23

request with minimum seek time from current head position is serviced next. form of SJF scheduling: can cause starvation of requests

Question 24

SCAN aka elevator algorithm

Answer 24

start from head and go all the way to the other end and reverse till all requests are serviced.

Question 25

C-SCAN

Answer 25

more uniform wait time than SCAN. Head goes all the way to the other end of the disk then returns straight to beginning of disk without servicing any requests on return trip- cylinders treated as a circular queue: wraps from last cylinder to first cylinder.

Question 26

LOOK/C-LOOK

Answer 26

LOOK is to SCAN as C-LOOK is to C-SCAN. Difference is, instead of going to the very end of the disk, stop at last request in one direction before reversing/returning to beginning

Question 27

what are the best algorithms for systems that place a heavy load on disk

Answer 27

SCAN, C-SCAN: less starvation

Question 28

performance is dependent on

Answer 28

number and type of requests

Question 29

reasonable choice for default algorithm

Answer 29

SSTF and LOOK

Question 30

each sector holds what information

Answer 30

header info, data and ECC (usually 512 bytes but can be selectable)

Question 31

to use a disk to hold files

Answer 31

OS needs to record its own data structures on disk- partition disks into cylinders- each treated as a logical disk. logical formatting: creating a file system

Question 32

boot block

Answer 32

initialises system: loads bootstrap loader program stored in boot blocks of boot partition

Question 33

swap space

Answer 33

Virtual memory uses (raw i.e. no file system) disk space as an extension to main memory

Question 34

file

Answer 34

abstract data type: sequence of words/bytes: meaning is decided by programs/OS

Question 35

file attributes

Answer 35

name: only information in human readable form identifier: unique tag identifies file in system type: needed for systems that supports different file types size: current file size location: pointer to where file is stored in system protection: controls who can read/write/execute time/date/user id: data for protection/security/usage monitoring (info kept in directory structure)

Question 36

file operations

Answer 36

create, delete, write/read (at pointer locations), seek, truncate (delete contents but keep attributes), open(Fi)/close(Fi)- search directory structure on disk for file entry Fi and move the content of entry to memory (+v.v.) -must open to carry out any operation

Question 37

how to manage open files

Answer 37

open file table: tracks open files file pointer: points to last read/write location per process that has the file open file count: counter of number of times a file is open- to allow removal of data when file is closed by the last process

Question 38

file locking

Answer 38

at critical section: makes sure noone else has access to the file - shared lock: reading file, many people can have access to file - exclusive lock: writing a file - mandatory: OS denies access depending on locks held and requested - advisory: process can find status of locks and decide what to do

Question 39

sequential-access file

Answer 39

when reading/writing: read/write one logical block at a time until output indicates end of file to go backwards: must rewind all the way to the start (read next/write next/ reset)

Question 40

direct access

Answer 40

file is fixed length logical records. can read/write directly to relative block numbers (n) read n/write n/position to n/read next/write next/rewrite n

Question 41

how to implement sequential file access as direct access

Answer 41

``` (reset) cp = 0 read cp cp = cp + 1 write cp cp = cp + 1 ```

Question 42

file access methods: indexing

Answer 42

have an index of a file in memory for fast determination of location of the data to be operated on. if too large: index in memory of the index on disk

Question 43

directory structure

Answer 43

a collection of nodes containing information about all files- directory structure+files are both stored on disk