Transcript Ceng 334 - Operating Systems

Chapter 4 : File Systems
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What is a file system?
Objectives & user requirements
Characteristics of files & directories
File system implementation
Directory implementation
Free blocks management
Increasing file system performance
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File System
 The
collection of algorithms and data
structures
which
perform
the
translation from logical file operations
(system calls) to actual physical storage
of information
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Objectives of a File System
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Provide storage of data and manipulation
Guarantee consistency of data and minimise
errors
Optimise performance (system and user)
Eliminate data loss (data destruction)
Support variety of I/O devices
Provide a standard user interface
Support multiple users
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User Requirements
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Access files using a symbolic name
Capability to create, delete and change files
Controlled access to system and other users’
files
Control own access rights
Capability of restructuring files
Capability to move data between files
Backup and recovery of files
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Files
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Naming
 Name formation
 Extensions (Some typical extensions are shown below)
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Files (Cont.)
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Structuring
 (a) Byte sequence (as in DOS, Windows & UNIX)
 (b) Record sequence (as in old systems)
 (c) Tree structure (as in some mainframe Oses)
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Files (Cont.)
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File types
Regular (ASCII, binary)
 Directories
 Character special files
 Block special files
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File access
Sequential access
 Random access
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Files (Cont.)
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File attributes
 Read, write, execute, archive, hidden, system etc.
 Creation, last access, last modification
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Files (Cont.)
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File operations
1. Create
2. Delete
3. Open
4. Close
5. Read
6. Write
7. Append
8. Seek
9. Get attributes
10.Set Attributes
11.Rename
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Directories
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Where to store attributes
In directory entry (DOS, Windows)
 In a separate data structure (UNIX)
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Path names
Absolute path name
 Relative path name
 Working (current) directory
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Operations
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Create, delete, rename, open directory, close
directory, read directory, link (mount), unlink
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Directories & Files (UNIX)
Root Directory
Disk A
d1
f1
d2
f2
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/
Linked Branch
d3
f4
Working Directory
f3
f5
d4
Disk B
d5
f6
d6
f7
Working directory : d2
Absolute path to file f2 : /d1/d2/f2
Relative path to file f2 : f2
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Physical Disk Space
Management
Sector
Track
Cylinder
Heads
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Each plate is composed of sectors or physical
blocks which are laid along concentric tracks
Sectors are at least 512 bytes in size
Sectors under the head and accessed without a head
movement form a cylinder
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File System Implementation
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Contiguous allocation
Linked list allocation
Linked list allocation using an index (DOS file
allocation table - FAT)
i-nodes (UNIX)
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Contiguous Allocation
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The file is stored as a contiguous block of data
allocated at file creation
(a) Contiguous allocation of disk space for 7 files
(b) State of the disk after files D and E have been removed
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Contiguous Allocation (Cont.)
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FAT (file allocation table) contains file name,
start block, length
Advantages
Simple to implement (start block & length is
enough to define a file)
 Fast access as blocks follow each other
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Disadvantages
Fragmentation
 Re-allocation (compaction)
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Linked List Allocation
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The file is stored as a linked list of blocks
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Linked List Allocation (Cont.)
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Each block contains a pointer to the next block
FAT (file allocation table) contains file name, first block
address
Advantages
 Fragmentation is eliminated
 Block size is not a power of 2 because of pointer
space
Disadvantages
 Random access is very slow as links have to be
followed
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Linked list allocation using an
index (DOS FAT)
FAT (File allocation table)
0
Disk size
1
EOF
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Free
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5
4
Free
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7
6
Bad
7
1
…
n
Free
3
5
7
1
File blocks
First block address is in
directory entry
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Linked list allocation using an
index (Cont.)
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The DOS (Windows) FAT is arranged this way
All block pointers are in FAT so that don’t take
up space in actual block
Random access is faster since FAT is always in
memory
16-bit DOS FAT length is (65536+2)*2 =
131076 bytes
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Problem
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16-bit DOS FAT can only accommodate 65536
pointers (ie., a maximum of 64 MB disk)
How can we handle large disks such as a 4 GB
disk?
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i (index)-nodes (UNIX)
File mode
Number of links
UID
GID
File size
Time created
Time last accessed
Time last modified
10 disk block numbers
Single indirect block
Double indirect block
Triple indirect block
Indirect blocks
Data blocks
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i-nodes (Cont.)
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Assume each block is 1 KB in size and 32 bits (4
bytes) are used as block numbers
Each indirect block holds 256 block numbers
First 10 blocks : file size <= 10 KB
Single indirect : file size <= 256+10 = 266 KB
Double indirect : file size <= 256*256 +266 =
65802 KB = 64.26 MB
Triple indirect : file size <= 256*256*256 +
65802= 16843018 KB = ~16 GB
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Directory Implementation
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DOS (Windows) directory structure
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UNIX directory structure
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DOS (Windows) Directory
Structure (32 bytes)
8 bytes
3 1
File name Ext A
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Reserved
2 2 2
T D P
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Size
Attributes (A,D,V,S,H,R)
Time of creation
Date of creation
Pointer to first data block
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UNIX Directory Structure
(16 bytes)
2 bytes
I-node #
14 bytes
File name
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The Windows 98 Directory
Structure
•Extended MS DOS Directory Entry
•An entry for (part of) a long file name
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The Windows 98 Directory
Structure
An example of how a long name is stored in Windows 98
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Path Name Lookup :
/usr/ast/mbox
Root (/) i-node
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Root directory
file block 245
1
.
1
..
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bin
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dev
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lib
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etc
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usr
i-node 60 of
/usr/ast/mbox
Blocks of
file
i-node 6 of /usr
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/usr/ast directory
file block 406
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.
6
..
60
mbox
92
books
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src
/usr directory
file block 132
6
.
1
..
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prog
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stu
51 html
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ast
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genc
i-node 26 of /usr/ast
406
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Two ways of handling long file names
in a Directory
In-line
In a heap
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Shared Files
/
d1
f1
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d2
f2
f3
File f2 is shared by two paths (users!) and there is
one physical copy.
The directories d1 & d2 point to the same i-node
with link count equal to 2
Deletion is done by decrementing the link count.
When it reaches zero the file is deleted physically
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Disk Space Management
Block size
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Dark line (left hand scale) gives data rate of a disk
Dotted line (right hand scale) gives disk space efficiency
All files 2KB
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How to Keep Track of Free Disk
Blocks
 Linked
list of disk blocks
 Bit maps
 Indexing as used in DOS FAT
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Linked List of Disk Blocks
•Allocation is simple.
•Delete block number from free blocks list
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Bit Maps
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The bit map is implemented by
reserving a bit string whose length
equals the number of blocks
A ‘1’ may indicate that the block
is used and ‘0’ for free blocks
If the disk is nearly full then the
bit map method may not be as
fast as the linked list method
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Increasing File System
Performance
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Disks (floopies, hard disks, CD ROMS) are
still slow when compared to the memory
Use of a memory cache may speed the disk
transfers between disk and process
Blocks are read into the cache first.
Subsequent accesses are through the cache
Blocks are swapped in & out using
replacement algorithms such as FIFO, LRU
System crashes may cause data loss if
modified blocks are not written back to disk
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Where to Put the Current “File
Position” Field
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The file position field is a 16 or 32 bit variable
which holds the address of the next byte to be
read or written in a file
 Put it in the i-node
 Put it in process table
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File Position Field in i-node
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If two or more processes share the same file,
then they must have a different file position
Since i-node is unique for a file, the file position
can not be put in the i-node
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File Position Field in Process
Table
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When a process forks, both the parent and the
child must have the same file position
Since the parent and the child have different
process tables they can not share the same file
position
So, we can not put in process table
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Solution
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Use an intermediate table for file positions
Process tables
File positions table
parent
position
child
i-node
of
file
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