CS 537 - Quiz #9

	UNIVERSITY OF WISCONSIN-MADISON Computer Sciences Department
CS 537 Spring 2000		A. Arpaci-Dusseau
	Quiz #9: Wednesday, April 26

Name: Student ID #:

File Systems

Problem 1: Disk Characteristics (40 points)

Consider a disk with the following characteristics:

Number of surfaces: 16
Number of sectors / cylinder: 4096
Number of tracks / surface: 2048
Number of bytes / sector: 512

a) How many platters does this disk have?

The top and bottom surface of a platter is usually not active in a disk (i.e., there is no disk arm). Therefore, there are probably 9 platters in this disk.

Credit will also be given for saying there are 8 platters.

b) How many sectors per track?

You need to know the relationship between tracks and cylinders to answer this problem: there is one track on each surface in each cylinder. Since there are 4096 sectors/cylinder and there are 16 surfaces, there are:

  4096 sectors      1 cylinder
  ------------  *  ------------  = 2^12 / 2^4 = 2^8 = 256 sectors/track
   1 cylinder       16 tracks

c) What is the total size (i.e., capacity) of this disk? For full credit, please show your work.

4096 sectors    2048 cylinders      512 bytes
------------ *                   * ------------  = 2^32 bytes = 4 GB
 1 cylinder                           sector

Problem 2: Open Files (60 points)

Consider two independent processes A and B that have just started executing along with per-process and system open file tables with the following initial contents: Open Tables

a) Each of the following pieces of data must be kept in either the per-process open file table or the system open file table. Determine which of the two locations is most appropriate for each piece of information. (Hint: This isn't a memorization question. You should be able to reason through each case.)

Mode (e.g., RDONLY, WRONLY, or RDWR) used to open the file
Different processes can open the file in different manners; therefore, the mode must be kept in the per-process table. When the file is opened, the OS checks if that process has the necessary permissions to open the file in that mode. If not, the open call fails. Later, when the process calls read() or write(), the OS checks that the file was opened with the correct mode for this action.
Open file reference count
This is the number of different processes that have opened the file; it corresponds to the number of pointers from entries in the per-process tables to a particular entry in the system table. Therefore, it is kept in the system table. When the count goes to 0, the OS knows that no processes have this file open and the entry in the system table can be deallocated.
Meta-data (also kept on disk) such as file access and modification time, file size, and pointers to data blocks
This information must be the same across all processes. For example, if one process extends the length of a file, all other processes currently accessing that file need to see the new size and the pointers to the newly allocated data blocks. Therefore, this must be kept in the system table.
Current read or write position in file
Each process should be able to read from a position (or location) in the file without determining the position that other processes are reading from. Therefore, this is kept in the per-process table.

b) Modify the diagram to show the final state of all tables after the following system calls have executed. Show the names of the file assigned to each table entry and the pointers from the per-process table to the system table. Include the reference count in the appropriate table(s). Assume that process A and B have the same current working directory and that all system calls complete successfully.

    Process B: fd1 = open("x", mode);
    Process A: fd1 = open("y", mode);
    Process B: fd2 = open("y", mode);
    Process A: close(stdin);
    Process A: fd3 = open("z", mode);

The diagram will look like the following: Open Tables