1. Operating Systems File systems
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CSCI2413 Lecture 7
Operating Systems
File systems

2. Lecture Outline File management File Types File Access File Structure
Unix file types
File Access
File Structure
Disk Construction
File Allocation Tables
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CSCI L7

3. File Management
Files exist to store information and allow it to be retrieved later.
The OS must set up a management system to provide facilities for general users to carry out a range of file handling processes such as:
create a file,
delete a file,
copy a file
identify a file type,
name a file,
rename a file,
display the contents of a file,
create/remove/list logical groups of files e.g. folders, directories
specify locations of files
protect files from unauthorised access.
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CSCI L7

4. File Management
Also programmers will want to carry out file handling processes such as:
Open: a file
Close: a file
Read: data from a file
Write: data to a file
Seek: a particular place in a file for subsequent data transfer.
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CSCI L7

5. File System, Hierarchical or Tree structure
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6. File Types A file may have a specific type or purpose, e.g.
regular files, directories, character/block special files, pipes
Some systems have specific types for things like
ASCII (text) files
executable (machine code) files
File types may be enforced
MS-DOS executable must have ‘.exe’ extension
or not (they are just a hint to the user)
UNIX text files may have ‘.txt’ extension
File types may have associations
Apple Mac types associate a file with its creator
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7. File Types – Name, Extension
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8. Unix File Types Four basic types: Regular Files Directory Files
may contain text, a program, or other data,
either in ASCII, in the numerical range 0 to 127,
i.e. in the 7-bit range
or a binary file, whose bytes can be of all possible values 0 to 255, in the 8-bit range.
Directory Files
A directory containing files within a directory
E.g. directory x may contain a, b and c, and that b is a directory, containing files u and v. b can be viewed as a directory, containing files and as a file itself.
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9. Unix File Types …. Device Files Link Files
In Unix, physical devices (printers, terminals etc.) are represented as ``files.''
In his way, the same read() and write() functions used to read and write real files can also be used to read from and write to these devices.
Link Files
new name for another file
suppose we have a file X, and type ln X Y If we then run ls, it will appear that a new file, Y, has been created, as a copy of X,
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10. File Access Methods There are two distinct classes of access method
sequential access
suited to magnetic tape devices
the information in a file (bytes or records) can only be written at the end (appended) or read in sequential order
random access
suited to hard disks
bytes or records can be read or written in any order
sometimes this is implemented by each read and write operation specifying a position parameter, or
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CSCI L7

11. File System Structure
Hard disks provide the bulk of secondary storage on which a file system is maintained
to improve input / output efficiency, I/O transfers are performed in units of blocks
Each block consists of one or more physical hard disk sectors
typical physical sector sizes vary from 32 to 4K bytes
usually, particularly on PC’s, sectors are 512 bytes
Hard disks have two important characteristics
blocks can be read, modified and written (in place)
blocks can be directly accessed in any order
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CSCI L7

12. File System Layers
In order to provide convenient and efficient user access to the disk, the operating system imposes a file system structure
this generally comprises a number of layers
to insulate the logical structure from device dependencies
application programs
logical file system
(files, directories, etc)
basic file system
(block read & write)
I/O control
(interrupts, device drivers)
hardware devices
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13. Disk Construction A hard disk consists of a number of platters
made of aluminium or glass
with a thin magnetic film on the surface
common platter diameters range from inches
Information is stored magnetically on both surfaces (sides) of each platter
The platters rotate at a fixed rate on the spindle
e.g. modern PC drives rotate at 5,400 or 7,200 rpm
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14. Read / Write Heads
Each surface has a read/write head to access the magnetic fields
the heads are mounted on arms and their movement is controlled by an actuator
all the heads move together
side view
r/w heads
arms
actuator
The heads do not touch the surface, but ‘fly’ a very small distance above the surface
the fly height
typically between around 0.2 to 0.07 mm
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15. The Disk Surface
The surface of a disk is logically divided into concentric circular tracks
1,000’s are now common
The surface is also divided into segments
a track through a particular segment is a sector
note: often the term sector is confusingly used to refer to both segments and sectors!
on typical PC disks, each sector holds 512 bytes
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16. data capacity of a disk is described best by:
Disk Capacity = Cylinders * Heads * Sectors * (# bytes / disk block)
A typical example for a 2.1 GB IDE disk on a Linux-based PC would be: C/H/S = 1024/64/63
Disk Capacity = 1024 * 64 * 63 * 512 bytes/block = bytes
= 2.11 gigabytes
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17. File Allocation Tables
The FAT is an index array of linked-list block numbers kept in its own disk block
the FAT contents are critical to finding all file blocks, so it is (usually) held twice at the start of each partition
the root directory immediately follows the FATs
The directory entry holds the start block of a file
the FAT entry indexed by that block number contains the block number of the next block in the chain
there is a special value to signify end of file
Special values signify free blocks and bad blocks
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18. File Systems in Unix
In Unix, the files are organized into a tree structure with a root named by the character '/'.
The first few levels of the tree look like this:
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19. Summary
File system, The system that an operating system or program uses to organize and keep track of files. For example, a hierarchical file system is one that uses directories to organize files into a tree structure. Although the operating system provides its own file management system, you can buy separate file management systems. These systems interact smoothly with the operating system but provide more features, such as improved backup procedures and stricter file protection.
© De Montfort University, 2004/5
CSCI L7