COMP5102 Lecture 3

1. phones off(please) COMP5102 Lecture 3 Operating Systems (OS) File systems

2. Lecture Outline • File Types • Files information • File Access • File Structure • Disk Construction • File Allocation Tables Comp5102/5122 - L3

3. 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 Comp5102/5122 - L3

4. File Types – Name, Extension Comp5102/5122 - L3

5. Unix File Types • Four basic types: • Regular 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. Comp5102/5122 - L3

6. Unix File Types …. • Device 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, Comp5102/5122 - L3

7. Files info. In a given Directory • The `a' (``all'') and `l' (``long'') options of the ls command can give us a lot of information about files in a specified directory: • bash-2.05$ ls -al • total 448 • drwx--x--x 3 msaf csstf 4096 Nov 17 10:44 . • drwx--x--x 69 msaf csstf 86016 Nov 17 10:39 .. • -rwx--x--x 1 msaf csstf 83012 Nov 5 2002 a.out • -rwx--x--x 1 msaf csstf 14964 Oct 9 2002 prg1 • -rw------- 2 msaf csstf 343 Oct 9 2002 prog1.c Comp5102/5122 - L3

8. Files info. … • The output is separated into six columns: • 1st column - access permissions (see below) • 2nd column - number of file entries (in the case of directory files) • 3rd column - owner • 4th column - size in bytes • 5th column - date and time of last modification • 6th column – file name Comp5102/5122 - L3

9. File Access Control • In Unix, all files are protected under some access control mechanism • the owner of a file can deny access of his files to others. • e.g. drwxr-xr-x. • The meanings of the flags are shown below: • file type: • d (directory) • - (ordinary file) • l (symbolic link) Comp5102/5122 - L3

10. File Access … • permissions for the owner: • r (read) • w (write) • x execute • r-x read and execute permission are granted, but not write permission, for other users. • To change or delete a file you need to have write permission • To view a file you need to have read permission Comp5102/5122 - L3

11. Some File Commands • chmod • change access permissions of any of your own file. • u user (i.e. owner) • g group • o others • + add permission • - remove permission • r read • w write • x execute Comp5102/5122 - L3

12. e.g. • chmod ugo+rw prog2 • adds read and write permission for all users to the prog2 file or directory of the person issuing this command. • In some cases it is useful for a user to deny himself/herself permission to write to a file, • e.g. to make sure the file not deleted by mistake. Comp5102/5122 - L3

13. du and df • The du command displays the sizes in kilobytes of all files in the specified directory, and the total of all those sizes; • The df command displays the amount of unused space left in your disk systems. • diff • displays line-by-line differences between two ASCII files. If for example, you have two versions of a C source file but don't remember how the new version differs from the old one, you could type • diff oldprog.c newprog.c Comp5102/5122 - L3

14. Wild Cards • There are two wild-card characters in Unix, `*' and `?'. rm *.c rm x?b.c rm test[1-3].c Comp5102/5122 - L3

15. 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 Comp5102/5122 - L3

16. 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 Comp5102/5122 - L3

17. 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 Comp5102/5122 - L3

18. 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 1.8 - 5.25 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 Comp5102/5122 - L3

19. Read / Write Heads r/w heads arms actuator side view • 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 • 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 Comp5102/5122 - L3

20. 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 sectorholds 512 bytes Comp5102/5122 - L3

21. 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 = 2113929216 bytes = 2.11 gigabytes Comp5102/5122 - L3

22. 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 Comp5102/5122 - L3

23. 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: Comp5102/5122 - L3