Fall 2000 M.B. Ibáñez

Lecture 24File-System III

File System Implementation

Fall 2000 M.B. Ibáñez

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

Fall 2000 M.B. Ibáñez

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

Fall 2000 M.B. Ibáñez

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

Fall 2000 M.B. Ibáñez

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 lengthFrom Operating Systems. Internals and Design Principles. W. Stallings. Prentice Hall

Contiguous File Allocation

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 32 33 34

File Allocation Table

File Name Start Block Length

FileAFileBFileCFileDFileE

2 39 5

18 830 226 3

FileA

FileB

FileC

FileE

FileD

From Operating Systems. Internals and Design Principles. W. Stallings. Prentice Hall

Fall 2000 M.B. Ibáñez

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 localityFrom Operating Systems. Internals and Design Principles. W. Stallings. Prentice Hall

Chained File Allocation

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 32 33 34

File Allocation Table

File Name Start Block Length

... ... ...

......FileB 5

...1

FileB

From Operating Systems. Internals and Design Principles. W. Stallings. Prentice Hall

Fall 2000 M.B. Ibáñez

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

Indexed Allocation with Block Portions

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 32 33 34

File Allocation Table

File Name Index Block

...

...

...

...FileB 24

FileB

1831428

From Operating Systems. Internals and Design Principles. W. Stallings. Prentice Hall

Indexed Allocation - Var Length Portions

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 32 33 34

File Allocation Table

File Name Index Block

...

...

...

...FileC 24

Start Block Length

12814

341

FileB

From Operating Systems. Internals and Design Principles. W. Stallings. Prentice Hall

Fall 2000 M.B. Ibáñez

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

Fall 2000 M.B. Ibáñez

UNIX (4K bytes per block)

From Operating System Concepts Silbershatz & Galvin

Fall 2000 M.B. Ibáñez

Free-Space Management• Bit vector (n blocks)

• Block number calculation

…

0 1 2 n-1

bit[i] = 0 block[i] free

1 block[i] occupied

(number of bits per word) *(number of 0-value words) +offset of first 1 bit

From Operating System Concepts Silbershatz & Galvin

Fall 2000 M.B. Ibáñez

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

• CountingFrom Operating System Concepts Silbershatz & Galvin

Fall 2000 M.B. Ibáñez

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 block[i] to have a situation where bit[i] = 1 in memory and bit[i] = 0 on disk.

– Solution:

• Set bit[i] = 1 in disk.

• Allocate block[i]

• Set bit[i] = 1 in memoryFrom Operating System Concepts Silbershatz & Galvin

Fall 2000 M.B. Ibáñez

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