Lecture 24 File-System III

1
Lecture 24File-System III

• File System Implementation

2
Secondary Storage Management

• Space must be allocated to files
• Must keep track of the space available for
  allocation
• Space is allocated as one or more contiguous
  units or portions
• From Operating Systems. Internals and Design
  Principles. W. Stallings. Prentice Hall

3
Preallocation

• Need the maximum size for the file at the time of
  creation
• Difficult to reliably estimate the maximum
  potential size of the file
• Tend to overestimated file size so as not to run
  out of space
• From Operating Systems. Internals and Design
  Principles. W. Stallings. Prentice Hall

4
Portion Size

• Contiguity of space increases performance
• Large number of small portions increases the size
  of tables needed
• Fixed-size simplifies the reallocation of space
• Variable-size minimizes waste of unused storage
• From Operating Systems. Internals and Design
  Principles. W. Stallings. Prentice Hall

5
Methods of File Allocation

• Contiguous allocation
• single set of blocks is allocated to a file at
  the time of creation
• only a single entry in the file allocation table
• starting block and length of the file
• Fragmentation will occur
• Will become difficult to find contiguous blocks
  of sufficient length
• From Operating Systems. Internals and Design
  Principles. W. Stallings. Prentice Hall

6
Contiguous File Allocation
File Allocation Table
FileA
File Name
Start Block
Length
0
1
2
3
4
FileA
2
3
FileB
9
5
5
6
7
8
9
FileC
18
8
FileB
FileD
30
2
10
11
12
13
14
FileE
26
3
15
16
17
18
19
FileC
20
21
22
23
24
FileE
25
26
27
28
29
FileD
30
31
32
33
34
From Operating Systems. Internals and Design
Principles. W. Stallings. Prentice Hall
7
Methods of File Allocation

• Chained allocation
• allocation on basis of individual block
• each block contains a pointer to the next block
  in the chain
• only single entry in the file allocation table
• starting block and length of file
• No fragmentation
• Any free block can be added to the chain
• No accommodation of the principle of locality
• From Operating Systems. Internals and Design
  Principles. W. Stallings. Prentice Hall

8
Chained File Allocation
File Allocation Table
FileB
File Name
Start Block
Length
0
1
2
3
4
...
...
...
FileB
5
1
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
32
33
34
From Operating Systems. Internals and Design
Principles. W. Stallings. Prentice Hall
9
Methods of File Allocation

• Indexed allocation
• file allocation table contains a separate
  one-level index for each file
• the index has one entry for each portion
  allocated to the file
• the file allocation table contains block number
  for the index

From Operating Systems. Internals and Design
Principles. W. Stallings. Prentice Hall
10
Indexed Allocation with Block Portions
File Allocation Table
FileB
File Name
Index Block
0
1
2
3
4
...
...
5
6
7
8
9
FileB
24
...
...
10
11
12
13
14
15
16
17
18
19
1
8
20
21
22
23
24
3
14
25
26
27
28
29
28
30
31
32
33
34
From Operating Systems. Internals and Design
Principles. W. Stallings. Prentice Hall
11
Indexed Allocation - Var Length Portions
File Allocation Table
FileB
File Name
Index Block
0
1
2
3
4
...
...
5
6
7
8
9
FileC
24
...
...
10
11
12
13
14
15
16
17
18
19
Start Block
Length
20
21
22
23
24
1
3
28
4
14
1
25
26
27
28
29
30
31
32
33
34
From Operating Systems. Internals and Design
Principles. W. Stallings. Prentice Hall
12
UNIX File Management

• Files are streams of bytes
• Types of files
• ordinary - contents entered by user or program
• directory - contains list of file names and
  pointers to inodes (index nodes)
• special - used to access peripheral devices
• named - named pipes
• From Operating Systems. Internals and Design
  Principles. W. Stallings. Prentice Hall

13
UNIX (4K bytes per block)
From Operating System Concepts Silbershatz
Galvin
14
Free-Space Management

• Bit vector (n blocks)

• Block number calculation

From Operating System Concepts Silbershatz
Galvin
15
Free-Space Management (Cont.)

• Bit map requires extra space. Example
• block size 212 bytes
• disk size 230 bytes (1 gigabyte)
• n 230/212 218 bits (or 32K bytes)
• Easy to get contiguous files
• Linked list (free list)
• Cannot get contiguous space easily
• No waste of space
• Grouping
• Counting
• From Operating System Concepts Silbershatz
  Galvin

16
Free-Space Management (Cont.)

• Need to protect
• Pointer to free list
• Bit map
• Must be kept on disk
• Copy in memory and disk may differ.
• Cannot allow for blocki to have a situation
  where biti 1 in memory and biti 0 on
  disk.
• Solution
• Set biti 1 in disk.
• Allocate blocki
• Set biti 1 in memory
• From Operating System Concepts Silbershatz
  Galvin

17
Directory Implementation

• Linear list of file names with pointer to the
  data blocks.
• simple to program
• time-consuming to execute
• Hash Table linear list with hash data
  structure.
• decreases directory search time
• collisions situations where two file names hash
  to the same location
• fixed size
• 
  
  From Operating System Concepts Silbershatz
  Galvin