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Operating Systems
I/O Management and File Systems
I/O Management and File Systems
Topics
1. I/O Management and File Systems
2. Where I/O Fits In
3. I/O Device Types
4. Device Type Features
5. Architectural Support for I/O in
Systems
6. Evolution of I/O Functions
7. Bus Support for DMA Control
Topics (continued)
8. DMA and Busy Cycles
9. DMA Configurations
10. DMA Channels
11. Buffering
12. Buffering Performance
13. Anatomy of a Disk
14. Raw Disk Sector Addressing
15. Access Performance of Disks
16. Disk Scheduling Policies
Topics (continued)
17. Disk Free Space Management
18. Disk Block Allocation Methods
19. Contiguous Allocation
20. Linked Allocation
21. File Allocation Tables
22. Indexed Schemes
23. Unix File System Layout
24. Unix INODE Usage
Where I/O Fits In
I/O devices (also called peripherals) are
the third part of the von Neumann
architecture.
Peripherals are responsible for permitting
the computer to exchange information
with its external environment.
CPU
PC
IR
MAR
MBR
I/O AR
I/O BR
ALU
Memory
Instructions
Instructions
Data
Data
Data
Buffers
(Persistent Store and
interface devices)
Peripherals
0
1
I/O Device Types
I/O devices may communicate with either
devices or users. Devices can be
categorized according to what they
interface with:
1. Human Readable
2. Machine Readable
3. Data Communication
Device Type Features
Devices differ according to their:
1. Data rate
2. Application
3. Control complexity
4. Unit of transfer
5. Data Representation
6. Error Conditions
7. Storage media
8. Removeability of media
Architectural Support for I/O in
Systems
Historically the choices for I/O support
included:
1. Programmed I/O
2. Interrupt Driven I/O
3. Direct Memory Access (DMA)
The current trend for block oriented
devices is DMA.
Evolution of I/O Functions
Traditionally users make a system call to
get I/O.
The following methods are used:
1. Direct Processor Controlled I/O
2. Polling Software Module Level Control
3. Interrupt Driven Software Module
Level Control
4. Interrupt Driven DMA
Evolution of I/O Functions (continued)
5. Separate I/O Processor using Main
Memory
6. Separate I/O Processor with Local
Memory
Disk controllers and modern network
cards all into the last category.
Bus Support for DMA Control
The data bus typically needs to be
augmented with additional lines to
support DMA.
Data
Count
Data
Lines
Address Lines
DMA Req
DMA ACK
INTR
Figure 2:
Read
A typical DMA
Write
Block Diagram
Data
Register
Address
Register
Control
Logic
DMA and Busy Cycles
The bus scheduling for DMA access is
typically more flexible than for an
interrupt. DMA just has to block memory
access, interrupts have to avoid
instruction restart whenever possible.
DMA vs. Interrupt Scheduling
Instruction Cycle
Processor Processor Processor Processor Processor Processor
Cycle
Cycle
Cycle
Cycle
Cycle
Cycle
Fetch
Decode
Fetch
Execute
Instruction Instruction Operand Instruction
DMA
Breakpoints
Store
Result
Process
Interrupt
Interrupt
Breakpoints
DMA Configurations
DMA can either share a common bus with
the CPU or it can be detached, or it can
have its own bus.
Some Popular DMA Configurations
CPU
DMA
Module
CPU
...
I/O
Memory
(a) Single-bus, detached DMA
CPU
DMA
Module
DMA
Module
Memory
I/O
I/O
I/O
(b) Single-bus, integrated DMA-I/O
System Bus
CPU
DMA
Module
Memory
I/O Bus
I/O
(c) I/O bus
I/O
I/O
DMA Channels
Systems requiring sustained I/O
bandwidth may give DMA controllers
dedicated channels, with instruction
streams flowing to the controllers
along the channels. Multiplexors can can
be used to select the I/O channel.
Data and
Address Channel
to Main Memory
Selector
Channel
Control Signal
Path to CPU
I/O
Controller
I/O
...
Controller
To
Memory
To CPU
(a) Selector
Multiplexor
Channel
I/O
Controller
I/O
Controller
I/O
Controller
I/O
Controller
Buffering
Hardware and software can use buffering
to store data in fast memory pending I/O
to overcome latency.
User
Process
Operating
System
I/O Device
Operating
System
I/O Device
In
(a)No buffering
User
Process
Move
(b)Single buffering
In
Operating
System
User
Process
Move
(c)Double buffering
I/O Device
In
Operating
System
User
Process
I/O Device
Move
(d)Circular buffering
..
.
In
Buffering Performance
Let
• M be the time for a memory copy,
• C be the the computing time between input requests,
and
• T be the time to transfer a block out to the peripheral.
The cost of unbuffered I/O is T+C.
The cost of a buffered I/O is max(C,T)+M.
Buffering Performance (continued)
Historically M<<T however recent trends
in high performance computing have
determined that excessive copying
degrades performance in practice.
In general buffering levels out I/O
performance over short term variations.
Anatomy of a Disk
Hard disks (also called Winchester disks)
are often the large capacity media of
choice in modern systems.
Hard Disk Components
Track t
Arm
Cylinder
Rotation
Read/write
heads
Raw Disk Sector Addressing
Typically the sectors on a disk are
assigned sequential addresses along the
tracks, let be the block address of a
sector.
Parameter
Meaning
b
i
j
k
s
t
The block address (to find)
cylinder of sector
surface of sector
position of sector within its track
number of sectors per track
track/cylinder number
Table 1: Parameters of Sector Computation
b=k+sx(j+ixt)
[1]
Access Performance of Disks
Seek time is the time to move the arm to
the desired track. Let Ts be the seek time,
n be the number of tracks traversed, m
be the time to traversing a single track,
and s be the startup time, then:
Ts=m x n + s
[2]
Access Performance of Disks (continued)
Transfer time is the time it takes to scan
the information off the disk,
letting T be the transfer time, and b be the
number of bytes to transfer, N be the
sector size in bytes, and r be the
revolutions per seconds, then:
T=
(3)
b
rN
Access Performance of Disks (continued)
The average access time, Ta is:
1
b
Ta = Ts +
+
2r
rN
(4)
Disk Scheduling Policies
In practice disk access times are highly
sensitive to the previous head position.
Most disk scheduling algorithms focus
on this.
Some Disk Schedules
0
25
50
75
100
125
150
175
199
(a) FIFO
Time
0
25
50
75
100
125
150
175
199
(b) SSTF
Time
0
25
50
75
100
125
150
175
199
(c) SCAN
Time
0
25
50
75
100
125
150
175
199
(d) C-SCAN
Time
Disk Free Space Management
Available space needs to be found
quickly for storage. Typically blocks are:
1. Corrupted --- Hardware failure, not
available for use
2. Free
3. Used
Disk Free Management (continued)
Typically either bitmaps are used, or a
list structure, sometimes with run length
encoding (assuming larger contiguous
spaces). The list structure may be
contiguous or linked.
The lists are stored starting in a fixed
block within the partition (for uniform
access at boot time), and may chain into
other blocks.
Disk Block Allocation Methods
1. Contiguous
2. Linked List (and FAT)
3. Indexed
Contiguous Allocation
This requires that a file n blocks long
occupy n contiguous blocks. External
fragmentation and file placement are
difficult problems.
count
0
1
2
3
4
5
6
7
8
9
10 11
12 13 14 15
16 17 18 19
20 21 22 23
24 25 26 27
28 29 30 31
A Contiguous
Allocation
Scheme
directory
File
Count
mail tr
mail
list
list
f
Start Length
0
14
19
28
6
2
3
6
4
2
Linked Allocation
Each block maintains a pointer to the
next block, with an entry in the system
directory block for the head of each file.
Free blocks get placed on a special free
linked list. Performance can be bad due
to excessive seeks.
A Linked Allocation
Scheme
0
4
8
1
5
9
2
6
3
directory
7
10 11
12 13 14 15
16 17 18 19
File Start End
Jeep
9
25
key
20 21 22 23
10
1
24 25 26 27
16
-1
28 29 30 31
25
File Allocation tables
File allocation tables use a linked list
stored in a table with one entry per block
in a dedicated block on the partition. This
is used in MS-DOS, OS/2 and MS
Windows (probably NT too). Redundant
storing of the links on disk provides a
recovery mechanism.
File Allocation Table
Scheme (FAT)
Directory
entry
test . .
name
0
.
217
Start
block
217
618
339 End-of-file
618
No. of disk blocks -1
339
FAT
Indexed Schemes
Each file has an index block which is an
array of pointers to contiguous disk
regions. When a file is allocated all index
pointers are initialized to. As space is
needed, blocks are appended to the index
tree structure.
Some options include:
1. Linked
2. Multilevel Indexed
3. Combined
Unix File System Layout
The Unix file system dedicates low order
blocks in a partition for:
1. Boot Block --- For bootable
partitions.
2. Superblock --- The root of the file
system's Inode hierarchy.
3. Inodes --- Indices to blocks on disk
for the file system.
Unix File system Partitions
Disk
drive
partition
filesystem
i-list
partition
partition
Directory blocks and
data blocks
boot
block(s)
super block
i-node i-node
...
i-node
17.6 - Unix INODE Usage
The INODES form a Btree of the blocks
in a file.
directory blocks and data blocks
i-list
data data
lock lock
i-node i-node
directory
block
i-node
1st data block
2nd data block
3rd data block
i-node
data
lock
directory
block
i-node # filename
i-node # filename
Unix INODE Usage
Directory blocks contain Inode
information for files in the directory. The
Unix file system is a directed rooted
graph (but not a tree or DAG). Partitions
can be added by mounting them (i.e.
inserting them into the directory
structure). MS-DOS does not have
mounting.
Figure 14: Inodes and directories in
the Unix file system
directory blocks and data blocks
directory
block
i-list
2549
1267
directory
block
1267
i-node
#
.
..
.
..
2549 testdir
i-node
0
i-node
1267
i-node
2549
data
block