Structure of File Systems

1
Structure of File Systems

• CP2088
• Operating Systems Programming

2
Summary

• Contents
• File system requirements
• Microsoft file systems - FAT and NTFS
• C/C Access to File System

3
What does user want from a file system?

4
Directories of files

• Typical directory entries
• name
• type
• dir, exe, link, ...
• pointer to start of file on device
• size
• protection status
• time/date - creation and update

5
File Operations

• File access
• opening, closing
• sequential access
• read/write, seek ...
• direct access
• read n/write n (n record number)
• File and directory operations
• create, delete, rename, search ...

6
File/Directory Operations

• File access
• opening, closing
• sequential access
• read/write, seek ...
• direct access
• read n/write n (n record number)
• File and directory operations
• create, delete, rename, search

7
Disk Space Allocation

• Basic requirements
• Disc space used efficiently
• Fast access of file content
• Three basic approaches to allocation of free
  space to files.
• Contiguous allocation
• Linked allocation
• Indexed

8
Allocation Strategy

• All rely on allocation of fixed size blocks on
  disc.
• 1 block 1 sector
• efficient use of disc free space
• management of free space becomes inefficient - to
  much time spent in block allocation
• Solution
• 1 block 2n sectors - cluster

9
Linking

• Used by FAT 16/32
• Directory entry points at first block
• Pointer in block to next block in file
• For
• No fragmentation
• No prior knowledge of file size needed
• Against
• File can be spread across disc - slow access
• Direct access slow - why?
• You have to look through blocks to locate the
  physical disk block containing record 'n'
• Recovery of file difficult if link lost - power
  failure

10
Linkage

0
1
2
3
4
5
6
7
8
9
14
-1
26
10
11
12
13
14
15
16
17
18
19
-1
8
19
5
20
21
22
23
24
25
26
27
28
29
3
12
file start
22
11
Free Space

• List of all free blocks on disc must be kept.
• List should be cached
• Free-space list searched for free block to
  allocate to file.
• Blocks added to list when file deleted.
• Should first free block found be allocated to
  file?

12
Typical File System approach
application
high level I/O requests

directs request to file system driver
virtual file system
understands directory/file structure
logical file system
maps logical blocks to track/sector
logical device driver
physical device driver
controls device
device
13
Windows

• FAT 16
• MS-DOS, Win95/98/NT/2000
• FAT 32
• Win95 (OSR 2), NT using 3rd party driver, Win2000
• Windows NT File System (NTFS)
• Win NT/2000, Win 95 using 3rd party driver

14
FAT 16

• Partition layout - set of logical sectors
• 1 logical sector 1 physical sector
• Disc space allocation unit cluster
• cluster 2n sectors
• Disc allocation/management variant of linked
  allocation - file allocation table
• FAT
• array of cluster numbers
• one entry per cluster
• entry 0 - cluster free
• FAT cached in memory

15
FAT32

• 32 bit cluster numbers
• more files
• smaller clusters - less wasted space
• bigger partitions
• No security - user access
• Root directory ordinary cluster chain - no limit
  on size
• Limited error recovery
• can use backup copy of FAT

16
NTFS

• 64 bit cluster numbers
• more files
• smaller clusters - less fragmentation
• bigger partitions
• Secure - limiting access to files
• Recovery - transaction logging
• Journaling File System

17
C/C Access to File System

• Directory Information
• _findfirst() provides information about the
  first instance of a filename that matches the
  file specified in the filespec argument.
• Header ltio.hgt
• long _findfirst( char filespec, struct
  _finddata_t fileinfo )

18
struct _finddata_t

• struct _finddata_t
• unsigned attrib
• time_t time_create / -1 for FAT
  file systems /
• time_t time_access / -1 for FAT
  file systems /
• time_t time_write
• _fsize_t size
• char name260

19
FindNext

• Once we have a file match then we can search for
  any other match.
• Find the next name, if any, that matches the
  filespec argument in a previous call to
  _findfirst, and then alters the fileinfo
  structure contents accordingly.
• int _findnext( long handle, struct _finddata_t
  fileinfo )

20
Reading a File.

• Many options available
• The simplest being
• FILE fopen( const char filename, const char
  mode )
• File must first be opened in the correct mode
• Read till we get to the EOF marker
• Closed

21
Reading Continued

• size_t fread( void buffer, size_t size, size_t
  count, FILE stream )
• size_t is the actual number of bytes read.
• buffer must be bigger than we expect.
• size is the size of the element to read
• count - Maximum number of elements expected.

22
Alternative File commandsusing int handles not
streams

• int _open( const char filename, int oflag , int
  pmode )
• int _read( int handle, void buffer, unsigned int
  count )
• int _write( int handle, const void buffer,
  unsigned int count )
• Do not mix these up with _fopen etc..

23
Windows Options

• In Windows we have to use the specific Windows
  functions.
• HANDLE CreateFile( LPCTSTR lpFileName, // file
  name
• DWORD dwDesiredAccess, // access mode
  DWORD dwShareMode, // share mode
  LPSECURITY_ATTRIBUTES lpSecurityAttributes,
  DWORD dwCreationDisposition, // how to create
  DWORD dwFlagsAndAttributes, // file attributes
  HANDLE hTemplateFile // handle to template
  file
• )

24
Windows Continued

• The idea of handling files etc., using an object
  Handle.
• Therefore the file handling is far more complex.
• Come to the Level 3 module to learn more.

25
Writing to file

• size_t fwrite( const void buffer, size_t size,
  size_t count, FILE stream )
• size_t is the actual number of bytes written.
• buffer Contains the data to write
• size is the size of the element to write
• count - Maximum number of elements to write.

26
Summary

• Requirements of a File System
• Directories of files
• Basic structure
• File allocation table techniques
• Disk space allocation
• Free Space.

27
Summary

• Fat 16, Fat 32 and NTFS
• File System APIs
• Opening a File
• Writing a File
• Reading a File
• Looked at different levels of API interface.