Transcript 슬라이드 1 - Chonbuk

Chap3. Secondary Storage and System Software
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
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
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
Organization of Disks(1)

Components of Disk
 Tracks
 Sectors
 the smallest addressable portion of a disk
 Disk pack
 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
Organization of Disks(2)
Track
Sector
Gaps
Surface of disk showing tracks and sectors
Organization of Disks(3)
Spindle
Platters
Read/write heads
Boom
Schematic illustration of disk drive
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?
Estimating Capacities(2)

Ex) file with 50,000 fixed-length data records on 2.1G disk
 # of bytes per sector = 512
 # of sectors per track = 63
 # of tracks per cylinder = 16
 # of cylinders = 4092

How many cylinders does the file require if each data record requires 256 bytes?
 50,000/2 = 25,000 sectors
 63x16 = 1008 sectors in a cylinder
 25,000/1008 = 24.8 cylinders
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
— 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
Organizing Tracks by Sector(2)
• Interleaving
If interleaving factor is 3
.
.
Disk
.
4
.
.
34
23
3
33
22 2
32
21
1
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 for a file
 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
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
.
.
.
Cluster
location
The part of the FAT
pertaining to our file
2
.
.
.
1
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
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
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
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
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
Key
Data
subblock subblock subblock
Count
Data
subblock subblock
Count subblock
Count
Key
Data
subblock subblock subblock
Key subblock
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
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)
Cost of Disk Access




Disk Access Time
 Seek time + Rotational delay + Transfer 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
 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
rotation time
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
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!
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
Magnetic Tape

A set of parallel tracks

9 tracks - parity bit

Frame
 one-bit-wide slice of tape

Interblock gaps
 permit stopping and starting
Nine-track tape
Track
Frame
0
1
1
0
1
0
0
1
0
Gap
Data block
Gap
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
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 of byte per block / num of inches for block
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
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 ...
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
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
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
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
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
pit
Encoding Scheme of CD-ROM

Encoding scheme
 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

Constraint
 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
000000012
10000100000000EFM
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
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
CAV
Addressing of CD-ROM

Addressing
 Magnetic disk: cylinder/track/sector approach
 CD-ROM: a sector-addressing scheme

Track density varies thus, each second of playing time on a CD is divided into 75 sectors
 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
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
4 bytes
sector ID
2,048 bytes
user data
4 bytes
error
detection
8 bytes
null
276 bytes
error
correction
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
Storage as a Hierarchy

Primary storage
 1/1,000,000,000 sec
 semi-conductors: registers, RAM, RAM disk, and disk cache

Secondary storage
 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
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
I/O processor --->
---> Disk
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.
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.
Physical layer
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
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
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
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
To disk
Buffering Strategies(2): Move

& Locate mode
Move mode (using both system buffer & program buffer)
 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
Move mode & Location mode
Move
mode
Locate
mode
program’s
data area
user’s
program
location
(pointer)
system
buffer
disk
system
buffer
disk
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
3.9 Buffer Management
Scatter Input & Gather Output
scatter
input
buffer 1
buffer 2
gather
output
read()
disk
buffer 1
disk
buffer 3
buffer 2
write()
3.10 I/O in UNIX
I/O in UNIX
PROCESS
user programs
KERNEL
libraries shell commands
I/O system
block I/O
system
(normal
files)
character
I/O system
(terminals,
printers, etc.)
block device drivers
disk
<Kernel I/O structure>
character device drivers
disk... consoles printers...
system call
interface
network
I/O system
(sockets)
network interface drivers
...networks...
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
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
3.10 I/O in UNIX
UNIX Kernel(3)
descriptor table
file
descriptor
o(keyboard)
1(screen)
2(error)
3(normal)
4(normal)
5(normal)
.
.
.
file table
entry
to open file
table
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
100
ptr to
write
routine ......
......
inode
table
entry
to inode
table
write() routine
for this type
of file
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
3.10 I/O in UNIX
Inode Structure
device
permissions
owner’s userid
file size
.
.
.
block count
file
allocation .
.
table
.
An inode
3.10 I/O in UNIX
Hard/Soft Link & File types

Linking file names to files
 hard 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)
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