Two-Level Directory

• The standard solution to limitations of single-level directory is to create a separate directory for each user.
• In the two-level directory structure, each user has his own user file directory (UFD). The UFDs have similar structures, but each lists only the files of a single user.
• When a user job starts or a user logs in, the system's master file directory (MFD) is searched.
• The MFD is indexed by user name or account number, and each entry points to the UFD for that user (see Fig. 12).

  Figure 12: Two-level directory structure.

  
• when a user refers to a particular file, only his own UFD is searched (create a file, delete a file?).
• Although the two-level directory structure solves the name-collision problem, it still has disadvantages.
• This structure effectively isolates one user from another.
• Isolation is an advantage when the users are completely independent but is a disadvantage when the users want to cooperate on some task and to access one another's files.
• A two-level directory can be thought of as a tree, or an inverted tree, of height 2.
  • The root of the tree is the MFD.
  • Its direct descendants are the UFDs.
  • The descendants of the UFDs are the files themselves. The files are the leaves of the tree.
• Specifying a user name and a file name defines a path in the tree from the root (the MFD) to a leaf (the specified file).
• Thus, a user name and a file name define a $path$ name. To name a file uniquely, a user must know the path name of the file desired.
• Additional syntax is needed to specify the volume of a file. For instance, in MS-DOS a volume is specified by a letter followed by a colon. Thus, a file specification might be

  C:\userb\test