Chap3. Secondary Storage and System Software

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File Structures by Folk, Zoellick, and Ricarrdi
Chap3. Secondary Storage
and System Software
서울대학교 컴퓨터공학과
객체지향시스템연구실
(SNU-OOPSLA-LAB)
김 형 주 교수
File Structure
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Chapter Objectives





Describe the organization of typical disk drives, including basic units of
organization and their relationships
Identity and describes the factors affecting disk access time, and describe
methods for estimating access times and space requirements
Describe magnetic tapes, identify some tape applications, and investigate
the implications of block size on space requirements and transmission
speeds
Identify fundamental differences between media and criteria that can be
used to match the right medium to an application
Describe in general terms the events that occur when data is transmitted
between a program and a secondary storage device

Introduce concepts and techniques of buffer management

Illustrate many of the concepts introduced in the chapter, especially system
software concepts, in the context of UNIX
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Contents
3.1 Disks
3.2 Magnetic Tape
3.3 Disk versus Tape
3.4 Introduction to CD-ROM
3.5 Physical Organization of CD-ROM
3.6 CD-ROM Strengths and Weaknesses
3.7 Storage as a Hierarchy
3.8 A Journey of a Byte
3.9 Buffer Management
3.10 I/O in UNIX
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3.1 Disks
Disks



Random Access Device
Disk device - DASD(Direct Access Storage Devices)
Magnetic disk





hard disk - most common
floppy disk - inexpensive, little capacity
magnetic tape - serial access
Iomega Zip (100 M byte), Jaz (1 G byte)
Nonmagnetic disk


optical discs - increasingly important for secondary storage
e.g. CD, DVD
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3.1 Disks
Organization of Disks(1)

Components of Disk

Tracks

Sectors


Disk pack



the smallest addressable portion of a disk
a collection of a lot of platters
Cylinder (vertical collection of tracks)
 Arms are moving together
Seeking


r/w arm movement
the slowest part of reading data from disk
File Structure
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3.1 Disks
Organization of Disks(2)
Track
Sector
Gaps
Surface of disk showing tracks and sectors
File Structure
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3.1 Disks
Organization of Disks(3)
Spindle
Platters
Read/write heads
Boom
Schematic illustration of disk drive
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3.1 Disks
Estimating Capacities(1)


Cylinder --> Track --> Sector --> Byte
Track capacity
= # of sectors per track * bytes per sector

Cylinder capacity
= # of tracks per cylinder * track capacity

Drive capacity
= # of cylinders * cylinder capacity
** Inner track, Outer track? Same capacity?
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3.1 Disks
Estimating Capacities(2)

Ex) file with 20,000 fixed-length data records
on 300MB disk





# of bytes per sector = 512
# of sectors per track = 40
# of tracks per cylinder = 11
# of cylinders = 1,331
How many cylinders does the file require if each data
record requires 256 bytes?
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3.1 Disks
Organizing Tracks by Sector(1)

Two ways to organize data on a disk


by sector & by user-defined block
Physical placement of sectors



logical view and physical view
adequate way to view a file logically, but always not good
way to store sectors physically
interleaving
—
—
File Structure
problems occur due to the gap between disk revolution speed
and disk controller speed
high performance disks (with high speed disk controller) now
offer 1:1 interleaving
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3.1 Disks
Organizing Tracks by Sector(2)
• Interleaving
If interleaving factor is 3
.
.
Disk
.
4
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.
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23
3
33
22 2
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21
1
11
3.1 Disks
Organizing Tracks by Sector(3)

Clusters (for improving file access speed)
 One file consists of one or more clusters







One cluster is a fixed number of contiguous sectors
Cluster is the smallest unit of space
Contiguous sectors per cluster and Mapping table
No additional seek (arm movement) for one cluster
Very useful when access granularity is medium
large cluster vs. small cluster
FAT (File Allocation Table)
 contains a list of all the clusters in file
 each cluster entry --> physical location of the cluster
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3.1 Disks
Organizing Tracks by Sector(3)
• FAT(File Allocation Table)
- A table containing mappings to the physical locations of all the
clusters in all files on disk storage
File allocation table(FAT)
Cluster
number
1
2
3
.
.
.
File Structure
Cluster
location
The part of the FAT
pertaining to our file
2
.
.
.
1
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3.1 Disks
Organizing Tracks by Sector(4)

Extents (further attempt for file access speed)




Contiguous clusters per extent and Mapping table
File may have several extents
Very useful when accessing the whole file sequentially
Many extents of a file increase amount of seeking
file extents
extent
extent
extent
extent
extent
extent
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3.1 Disks
Organizing Tracks by Sector(5)

Fragmentation




the space that goes unused within a cluster, block, track, or
other unit of physical storage
internal fragmentation: loss of space within a sector or cluster
trade-offs in use of large cluster sizes
ex) size of sector is 512 bytes and size of all
records in file is 300 bytes
 we may enforce one record / sector
 we may allow span sectors
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3.1 Disks
Organizing Tracks by Block(1)



User-defined size of blocks can be fixed or variable
No sector spanning ==> No internal fragmentation
Blocking factor

the number of records that are to be stored in each block

In block-addressing, each block of data is
accompanied by one or more subblocks containing
extra information about the data block

Note: the “block” has a different meaning in the context of the
UNIX I/O system
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3.1 Disks
Sector organization vs.
Block organization
Sector 1
Sector 2
Sector 3
Sector 4
Sector 5
111 111 111 111 111 222 223 334 444 555
(a) sector organization
111111
111111
111 222 22 333 4 444 555
(b) block organization
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3.1 Disks
Organizing Tracks by Block(2)

Subblocks


count subblocks
 contain the num of bytes in the accompanying data block
key subblocks
 contain key for last record in data block
Count
subblock
Data
subblock
Count
Data
subblock subblock
Count subblock
Count
Key
Data
subblock subblock subblock
Count
Key
Data
subblock subblock subblock
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Key subblock
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3.1 Disks
Nondata Overhead(1)


Stored during preformatting
Sector-addressable disks (physical)



sector/track address, condition, gaps, and synchronization
marks
of no concern to programmer
Block-organized disks (user-defined)


some of nondata for programmer
more nondata provided with blocks than with sectors
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3.1 Disks
Nondata Overhead(2)

Ex) block-addressable disks



20,000 bytes/track
subblock, interblock gap equivalent to 300 bytes/block
If we want to store a file containing 100-byte records, how
many records can be stored per track if blocking factor is 10
or 60 ?

if blocking factor is 10, then 20000 / ( (10*100)+300) = 15
blocks ==> 150 records

if blocking factor is 60, 180 records(=3 blocks)
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3.1 Disks
Nondata Overhead(3)

Block-organized scheme



larger blocking factor (many records in a block)
 less amount of nondata block
 more efficient use of storage
pros.
 flexibility -> savings in time & efficiency
cons.
 internal fragmentation problem
 necessary for programmer and OS to do extra data
organization
 sector interleaving cannot be used to improve performance
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3.1 Disks
Cost of Disk Access

Disk Access Time


Seek time




time to move arm to locate cylinder
more costly in multi-user environment
average seek time for a particular file operation
Rotational delay


Seek time + Rotational delay + Transfer time
time taken for disk to rotate correct sector under read/write
arm
Transfer time (for actual data)
number of bytes transferred
number of bytes on a track
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rotation time
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Timing Computation

Seagate Cheetah 9 Gbyte drive
 Minimum
seek (track-to-track): 0.78msec
 Maximum seek: 19 msec
 Average seek: 8 msec
 Rotation delay: 3 msec
 Read one track: 6 msec
 Read one cluster: 0.28 msec

Read 8704K size file
 sequentially
access the whole file: 1.7 sec
 randomly access the whole file:
9.25 sec
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3.1 Disks
Effect of Block Size on Performance (In UNIX)






UC Berkeley CSRG team
Many experiments in BSD UNIX System
Trade-off between block size, fragmentation, access time
Doubling block size to 1,024 bytes improves performance by
more than a factor of 2
In 512-byte block size:
the slow throughput & low internal fragmentation (6.9%)
In 4,096-byte block:
the fastest throughput & internal fragmentation (45.6%)
==> Invent the cluster concept!
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3.1 Disks
Disk as Bottleneck



Disk performance is increasing, but still slow!
Even high-performance network is faster than disk
(5 M/ sec HD is slower than 100M bps LAN)
Disk bound jobs


CPU and network must wait
Solution techniques



multiprogramming
striping - parallel I/Os
avoid accessing disk
 RAM disk, disk caches - locality
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3.2 Magnetic Tape
Magnetic Tape
A set of parallel tracks
 9 tracks - parity bit
 Frame

 one-bit-wide

slice of tape
Interblock gaps
 permit
File Structure
stopping and starting
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3.2 Magnetic Tape
Nine-track tape
Track
Frame
0
1
1
0
1
0
0
1
0
Gap
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Data block
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Gap
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3.2 Magnetic Tape
Estimating Tape Length(1)
There is an interblock gap for each data block
 Space requirement s
s = n * ( b + g )

b
is the physical length of a data block
 g is the length of an interblock gap
 n is the number of data blocks
Tape density
 Tape speed
 Size of interblock gap

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3.2 Magnetic Tape
Estimating Tape Length(2)

Ex) one million 100-byte records
 6,250
BPI tape
 0.3 inches of interblock gap
 How
much tape is needed?
 when
blocking factor is between 1 and 50
Nominal recording density
 Effective recording density:

 num
File Structure
of byte per block / num of inches for block
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3.2 Magnetic Tape
Estimating Data Transmission Times

Factors of data transmission rate
 interblock
gaps
 effective recording density
 nominal recording density
 speed of r/w head
 time to start/stop the tape
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3.3 Disk versus Tape
Disk VS. Tape

Disk




Random access
Immediate access
Expensive seek in
sequential processing
Tape



Sequential access
Long-term storage
No seek in sequential
processing
 Decrease in cost of disk and RAM
 More RAM space is available in I/O buffers,
so disk I/O decreases
 Tertiary storage for backup: CD-ROM, tape ...
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3.4 Introduction to CD-ROM
Introduction to CD-ROM

CD-ROM: Compact Disc Read-Only Memory






Can hold over 600MB(200,000 pages)
Easy to replicate
Useful for publishing or distributing medium
But, not storing and retrieving data
CD-ROM is a child of CD audio
CD audio provides



High storage capacity
Moderate data transfer rate
But, against high seek performance
Poor seek performance
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3.4 Introduction to CD-ROM
History of CD-ROM: Videodisc

Videodisc technology developed in late 1960's and early
1970's




The goal was to store movie
A number of methods for storing video signals
1. Use a needle to respond mechanically to grooves in a disc like
a vinyl LP record
2. Use optical storage
Many companies were fighting which approach should become a
standard
VideodiskLaserVisionCD audioCD-ROM
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3.4 Introduction to CD-ROM
History of CD-ROM: LaserVision

LaserVision





Emerged as the winner of standard battles about standard
Support CLV(Constant Linear Velocity) and CAV(Constant Angular
Velocity) format
Fast seek performance by using CAV format
Data are stored in analog form
Why did they fail?
 Earlier disputes over the physical format of the video disc, many
incompatible encoding scheme and error correction techniques
were used by many firms
 No standard format  the market never grew
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3.4 Introduction to CD-ROM
History of CD-ROM: CD-ROM

CD-ROM








Philips and Sony developed CD-ROM in 1984 in order to store music
on a disc
Use a digital data format
The development of CD-ROM as a licensing system results in
widely acceptance in the industry
Promised to provide a standard physical format
Any CD-ROM drive can read any sector which they want
Computer applications store data in a file not in terms of sector, thus,
file system standard should be needed
In late 1985, videodisc/digital data industry moved into CD-ROM
industry
In early summer of 1986, an official standard for organizing files was
worked out
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3.5 Physical Organization of CD-ROM
Physical Organization of Master Disk
 Master


Disc
Formed by using the digital data, 0 or 1
Made of glass and coated that is changed by the laser beam
 Two
part of CD-ROM
 Pit



The areas that is hit by the laser beam
Scatter the light
Land


Smooth, unchanged areas between pits
Reflect the light
light
laser beam
land
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pit
36
3.5 Physical Organization of CD-ROM
Encoding Scheme of CD-ROM

Encoding scheme




Constraint




The alternating pattern of high- and low-density reflected light is the signal
1 : transition from pit to land and back again
0s : the amount of time between transitions
The limits of the resolution of the optical pickup (generic hardware limit),
there must be at least two 0’s between any pair of 1’s (no two adjacent 1s)
We cannot represent all bit patterns, thus, we need translation scheme
We need at least 14 bits to represent 8 bits under this constraint
EFM(eight to fourteen modulation) coding is used
110
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000000012
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10000100000000EFM
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3.5 Physical Organization of CD-ROM
Format of CD-ROM

CD audio chose CLV format instead of CAV format
because



CD audio requires large storage space
CD audio is played from the beginning to the end sequentially
Format of CD-ROM







CLV(Constant Linear Velocity)
A single spiral pattern
Same amount of space for each sector
Capability for writing all of sectors at the maximum density
Rotational speed is slower in reading outer edge than in inner edge
Finding the correct speed though trial and error
Characteristics
 Poor seek performance
 No straightforward way to jump to a specified location
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3.5 Physical Organization of CD-ROM
Constant Angular Velocity Disk

Magnetic disk usually uses CAV(Constant Angular Velocity)
 Concentric tracks and pie-shaped sectors
 Data density is higher in inner edge than in outer edge
 Storage waste: total storage is less than a half of CLV
 Spin the disc at the same speed for all positions
 Easy to find a specific location on a disk good seek
performance
CLV
File Structure
CAV
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3.5 Physical Organization of CD-ROM
Addressing of CD-ROM

Addressing



Track density varies thus, each second of playing time on a CD is
divided into 75 sectors




Magnetic disk: cylinder/track/sector approach
CD-ROM: a sector-addressing scheme
75 sectors/sec, 2 Kbytes/sector
At least one-hour of playing time
Maximum capacity can be calculated: 600 Mbytes
60 min * 60 sec/min * 75 sectors/sec = 270,000 sectors
We address a given sector by referring minutes, second, and
sector of play

16:22:34 means 34th sector in the 22nd second in the 16th minutes of play
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3.5 Physical Organization of CD-ROM
Fundamental Design of the CD disc








Initially designed for delivering digital audio information
Store audio data in digital form
Wave patterns should be converted into digital form
Measure of the height of the sound: 65,536 different gradation(16 bits)
Sampling rate: 44.1 kHz, because of 2 times of 20,000 Hz upto which
people can listen
16 bits sample, 44,100 times per second, and two channel for stereo
sound, we should store 176,400 bytes per seconds
Storage capacity of CD is 75 sectors per seconds, we have 2,352 bytes
per sector
CD-ROM divides this raw sector as shown in the following figure
12 bytes
synch
File Structure
4 bytes
sector ID
2,048 bytes
user data
4 bytes
error
detection
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8 bytes
null
276 bytes
error
correction
41
3.6 CD-ROM Strengths and Weaknesses
File Structure Problem of CD-ROM


The most important problem in using CD-ROM as a storage
media
Strongness and weakness of CD-ROM




Strong aspects of CD-ROM
 Data transfer rate: 75 sectors/sec
 Storage capacity : over 600 Mbytes
 Inexpensive to duplicate and durable
Weak aspects of CD-ROM
 Poor seek performance (weak random access)
 Magnetic disk: 30 msec, CD-ROM : 500 msec
Comparison of access time of a large file from several media
 RAM: 20 sec, Disk: 58 days, CD-ROM: 2.5 years
We should have a good file structure avoiding seeks to an even
greater extent that on magnetic disk
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3.7 Storage as a Hierarchy
Storage as a Hierarchy

Primary storage



Secondary storage



1/1,000,000,000 sec
semi-conductors: registers, RAM, RAM disk, and disk cache
1/1,000 sec
magnetic disks(DASD), tape and mass storage(Serial)
Off-line storage (archival & back-up)


10 sec
removable magnetic disks, optical discs, tapes
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3.8 A Journey of a Byte
A Journey of a Byte(1)

What happens when a program writes a byte to a file
on a disk?

WRITE(TEXT, c, 1);
User Program ---> OS File manager -->
I/O buffer
--->
Disk controller
File Structure
I/O processor --->
---> Disk
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3.8 A Journey of a Byte
A Journey of a Byte(2)
Logical layer
1
The program asks the OS to write the contents of the variable c to the
next available position in TEXT.
2
The OS passes the job on to the file manager
3
The file manager looks up TEXT in a table containing information about
it, such as whether the file is open and available for use, what types of
access are allowed, if any, and what physical file the logical name TEXT
corresponds to.
4
The file manager searches a FAT for the physical location of the sector
that is to contain the byte.
5
The file manager makes sure that the last sector in the file has been
stored in a system I/O buffer in RAM, then deposits the ‘P’ into its
proper position in the buffer.
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3.8 A Journey of a Byte
A Journey of a Byte(3)
6
The file manager gives instructions to the I/O processor about where
the byte is stored in RAM and where it needs to be sent on the disk
7
The I/O processor finds a time when the drive is available to receive
the data and puts the data in proper format for the disk. It may also
buffer the data to send it out in chunks of the proper size for the disk
8
The I/O processor sends the data to the disk controller.
9
The controller instructs the drive to move the r/w head to the proper
track, waits for the desired sector to come under the r/w head, then
sends the byte to the drive to be deposited, bit-by-bit, on the surface
of the disk.
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Physical layer
3.8 A Journey of a Byte
What is buffer?

Definition - the part of main memory available for storage of
copies of disk blocks

Program buffers vs. System I/O buffers

Buffer manager

subsystem responsible for the allocation for blocks

goal:

minimize the number of disk access

utilize the memory space effectively
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3.9 Buffer Management
Buffer Management

Buffer bottlenecks




What if program performs both input and output on one
character at a time, and only one I/O buffer is available?
At least two buffer - for input and output
I/O bound jobs: wait for I/O completion
Buffering Strategies for Performance


use more than one buffer
I/O system processes next block when CPU is processing
current one
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3.9 Buffer Management
Buffering Strategies(1): Multiple buffering

Double buffering




swapping roles of two buffers after I/O finished
allows O/S operating on one buffer while the other buffer is
being loaded or emptied
any number of buffers can be used
Buffer pooling


takes from a pool of available buffers
decides which buffer to take from a buffer pool
 take buffer by LRU
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3.9 Buffer Management
Double Buffering
I/O buffer 1
(a)
To disk
Program data area
I/O buffer 2
I/O buffer 1
(b)
Program data area
I/O buffer 2
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To disk
50
3.9 Buffer Management
Buffering Strategies(2): Move

Move mode (using both system buffer & program
buffer)



& Locate mode
moving data from one place in RAM to another before they
can be accessed
sometimes, unnecessary data moves
Locate mode (using system buffer only or program
buffer only)


perform I/O directly between secondary storage and program
buffer (program’s data area)
system buffers handle all I/Os, but program uses locations
through pointer variable
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3.9 Buffer Management
Move mode & Location mode
Move
mode
Locate
mode
File Structure
program’s
data area
user’s
program
location
(pointer)
system
buffer
disk
system
buffer
disk
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3.9 Buffer Management
Buffering Strategies(3): Scatter/Gather IO



A file with many blocks and each block with header
and data
Need to put the headers in one buffer and the data in
a different buffer ==> may occur complication
Scatter-input mode


a single READ can scatter data into a collection of buffers
Gather-output mode

a single WRITE can gather several buffers and output
File Structure
SNU-OOPSLA Lab
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3.9 Buffer Management
Scatter Input & Gather Output
scatter
input
buffer 1
buffer 2
gather
output
buffer 1
disk
buffer 3
buffer 2
File Structure
read()
disk
SNU-OOPSLA Lab
write()
54
3.10 I/O in UNIX
I/O in UNIX
PROCESS
user programs
KERNEL
character
I/O system
(terminals,
printers, etc.)
block device drivers
File Structure
libraries shell commands
I/O system
block I/O
system
(normal
files)
disk
<Kernel I/O structure>
character device drivers
disk... consoles printers...
SNU-OOPSLA Lab
system call
interface
network
I/O system
(sockets)
network interface drivers
...networks...
55
3.10 I/O in UNIX
UNIX Kernel(1)




The central part of UNIX operating system
View all I/O as operating on a sequence of bytes
Make all operations below top layer independent of
application’s logical view of file
Data structures related to unix files




file-descriptor table: owned by user process
open-file table: owned by kernel
index-node: a kind of FAT (one inode for each file in use)
index-node table: owned by kernel
File Structure
SNU-OOPSLA Lab
56
3.10 I/O in UNIX
UNIX Kernel(2)

File descriptor table



associates each file descriptor to open file table
every process has its own file descriptor table
Open file table


entries for every open file
file structures: r/w mode, offset, reference count
File Structure
SNU-OOPSLA Lab
57
3.10 I/O in UNIX
UNIX Kernel(3)
descriptor table
file
descriptor
file table
entry
o(keyboard)
1(screen)
2(error)
3(normal)
4(normal)
5(normal)
.
.
.
File Structure
to open file
table
SNU-OOPSLA Lab
58
3.10 I/O in UNIX
UNIX Kernel(4)
open file table
R/W
mode
write
# of
processes
using it
1
Offset
of next
access
ptr to
write
routine ......
......
100
inode
table
entry
to inode
table
write() routine
for this type
of file
File Structure
SNU-OOPSLA Lab
59
3.10 I/O in UNIX
UNIX Kernel(5)

Inode(Index node)
 data
structure used to describe a file
 when a file is opened, a copy of inode is loaded
into RAM for rapid access
 has a list of disk blocks of the file
 this list is UNIX counterpart to FAT

Device driver
 I/O
processor program invoked by kernel
performing I/O for devices
File Structure
SNU-OOPSLA Lab
60
3.10 I/O in UNIX
Inode Structure
device
permissions
owner’s userid
file size
.
.
.
block count
An inode
file
allocation .
.
table
.
File Structure
SNU-OOPSLA Lab
61
3.10 I/O in UNIX
Hard/Soft Link & File types
 Linking
 hard
file names to files
link: direct reference
pointer from a directory to the inode of a file
 ln src-file target-file

 soft(symbolic)
link: pathnames
link a file name to another file name
 ln -s src-file target-file

 File types
 normal file (governed by block IO system)
 special file: stream, signal control of device (governed by
character IO system)
 sockets: endpoints of IPC (governed by network IO system)
File Structure
SNU-OOPSLA Lab
62
Let’s Review !!!
3.1 Disks
3.2 Magnetic Tape
3.3 Disk versus Tape
3.4 Introduction to CD-ROM
3.5 Physical Organization of CD-ROM
3.6 CD-ROM Strengths and Weaknesses
3.7 Storage as a Hierarchy
3.8 A Journey of a Byte
3.9 Buffer Management
3.10 I/O in UNIX
File Structure
SNU-OOPSLA Lab
63
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