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Sunday, June 30, 2013

Explain the single and two level directory structures

About Directory Structure
- Directory structure is referred to the way that the operating system follows for displaying the files and file system to the user in the field of computing. 
- A hierarchical tree structure is used for displaying the files in a typical way. 
- The special kind of string the file name uses or the unique identification of a particular file that is stored in the computer’s file system. 
- Before the 32 bit operating systems actually came in to the scenario; short names of about 6 to 14 characters in size were used for the file names. 
However, the modern operating systems give permission for file names of longer length i.e., of 250 character and that too per path name element. 
- The drive:\ is the root directory in the operating systems such as the OS/2, windows and DOS for example, “C:\”. 
- The “\” is the directory separator but the forward slash “/” is also internally recognized by the operating system.
- A drive letter is use for naming the drives either physically or virtually. 
- This also implies there does not exist a root directory that is formal. 
- Rather, we have root directories in each drive that are independent of each other. 
- However, one virtual drive letter can be formed by combining in to one. 
- This is done by keeping a RAID setting of 0 for the hard drive. 
- The file system hierarchy standard is used by the operating systems such as the UNIX and other UNIX like systems. 
- This is the most common form for the directory structures used by the UNIX operating systems. 
- It is under the root directory “/” that all the files and the directories are stored even if they are actually present on a number of different physical devices.

About Single – level Directory
- This is the simplest of the directory structures. 
- All files are stored in the same directory itself because it is quite easy to understand as well as support. 
- The first computer of the world i.e., the CDC 6600 also operated on just one directory and it could be used by a number of users at the same time. 
- There are significant limitations of the single-level directory. 
- These limitations come in to play when there are more than one users using the system or when the system has to deal with a large number of files. 
- All the files have to be assigned unique names since they are all stored under the same directory. 
- No two files can have the same file name. 
- It may become difficult to keep the names of the files in mind if they are large in number.

About Two–level Directory
- The limitations of the single level directory structure can be overcome by creating an individual directory for every user. 
- This is the most standard solution for the problems of the single level directories. 
- In this two-level directory structure, a UFD or user file directory is made for every user. 
- The structure of all the user file directories is almost the same, but the difference is that only the files of the individual user are stored in one.
- When a user tries to log in or when he starts a task, the system searches for the MFD or master file directory. 
- The name of the user or his/ her account number is used for indexing the MFDs in the operating system. 
- Each of those entries points to the UFD belonging to that user. 
- When a reference is made to some file, the system only searches for the user file directory for example, when a file has to be deleted or created. 

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