Frequently Asked Questions for Lab 4

File Systems

• The file system resides on a disk that is 128KB in size.
• There is only one root directory. No subdirectories are allowed.
• The file system supports a maximum of 16 files.
• The maximum size of a file is 8 blocks; each block is 1KB in size.
• Each file has a unique name, the file name can be no longer than 8 characters.

The layout of your 128 KB disk is as follows

• The first 1KB block is called the super block. It stores the free block list and index nodes (inode) for each file.
• The remaining 127 1KB blocks store data blocks of the files on your file system.
• The exact structure of the super block is as follows.
  • The first 128 bytes stores the free block list. Each entry in this list indicates whether the corresponding block is free or in use (if the i-th byte is 0, it indicates that the block is free, else it is in use). Intially all blocks except the super block are free.
  • Immediately following the free block list are the 16 index nodes, one for each file that is allowed in the file system. Initially, all inode are free. Each inode stores the following information:

    char name[8]; //file name
    int size;     // file size (in number of blocks)
    int blockPointers[8]; // direct block pointers
    int used;             // 0 => inode is free; 1 => in use
    

    Note that each inode is 56 bytes in size; Since you have 16 of these, the total size of occupied by the inodes is 896 bytes. The free/used block information (mentioned above) is 128 byes. So the total space used in the super block is 1024 bytes.

You need to implement the following operations for your file system.

• create(char name[8], int size): create a new file with this name and with these many blocks. (We shall assume that the file size is specified at file creation time and the file does not grow or shrink from this point on)

• delete(char name[8]): delete the file with this name

• read(char name[8], int blockNum, char buf[1024]): read the specified block from this file into the specified buffer; blockNum can range from 0 to 7.

• write(char name[8], int blockNum, char buf[1024]): write the data in the buffer to the specified block in this file.

• ls(void): list the names of all files in the file system and their sizes.

Getting Started

• We will use a 128KB file to act as the "disk" for your file system. I have provided a program to create this file for you. Run this program by typing the command
  

   create_fs disk0

  This will create a file with the name

  disk0

  in your current directory. The program also "formats" your file system---this is done by initializing all blocks in the super block to be free and marking all 16 inodes to be free.

  If you work on this assignment on your home PC, you can find the source code for the create_fs program [ java version C version . ]

• Here is template code for your file system.
• Remember that your file system must be persistent. If you shutdown your program and then restart it at a later time, all files on your file system must be intact.

• Input file

  You program should take input from a input file and perform actions specified in the file, while printing out the result of each action. The format of the input file is as follows.

  
  diskName  // name of the file that emulates the disk
  C fileName Size //create a file of this size
  D fileNAme     // delete this file
  L              // list all files on disk and their sizes
  R fileName blockNum // read this block from this file
  W fileName blockNum // write to this block in the file (use a dummy
  1KB buffer)
  
  

  An sample input file looks like this:

  disk0
  C file1 3
  W file1 0
  W file1 1
  C lab.java 7
  L
  C file2 4
  R file1 1
  D lab.java
  L
  

  A sample input file is available. Be sure to print out what your program does after reading each line from this file. It'd also be helpful if you printed out the disk addresses of any block that you allocate, deallocate, read or write.

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  Extra credit question

  (15 pts) Write a defragmenter for the above file system. The defragmenter is invoked (by the user) whenever free space is fragmented and when files blocks are randomly scattered on the disk. Your defragmenter should attempt to place each file contiguously on disk and move all free space to the end of the disk. Just as in compaction (lab 3), any valid defragmentation algorithm will get you full credit on this question. The only restriction is that your defragmenter should not use a memory buffer of more than 17 KB (a trivial defragmentation algorithm is to read the entire disk into memory, rearrange blocks and write everything back. Things get more interesting if you assume that you have limited memory space (17KB)---in this case, you will need to repeatedly read a small number of files or disk blocks, rearrange them and then write them back.). Sample input files that create a fragmented disk and the defragmented version of this disk are available. You need to run these scripts on an empty disk to obtain the framented and defragmented versions of the disk.

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  How to Turn in Lab 4

  All of the following files must be submitted on SPARK as a zip file to get full credit for this assignment.
  1. Your zip file should contain a copy of all source files.
  2. Your zip file should contain a copy of a README file identifying your lab partner (if you have one) and containing an outline of what you did for the assignment. It should also explain and motivate your design choices. Keep it short and to the point, while explaining explain your design decisions, data structures and algorithms.
     If your implementation does not work, you should also document the problems in the README, preferably with your explanation of why it does not work and how you would solve it if you had more time. Of course, you should also comment your code. We can't give you credit for something we don't understand!
  3. Fnally, your zip file should contain a copy showing sample output from your programs.
  4. Important:All files for the first and second parts of this lab should be distinct from one another. Feel free to keep them in separate sub-directories. This will allow us to grade each part separately.
  5. Individual Group Assessment (for students working in groups only)

  A percent of your lab grade will come from your participation in this project as a member of your group.

  What you need to turn in (each person individually):

  Include in your zip file a copy of your assessment of the division of labor in the group in the format shown below.  For a 2 person group, if you give yourself a 50% and your partner gives you a 50%, you will get the full credit for group participation.  If you give your partner a 40% and your partner gives himself or herself a 40%, he or she will get fewer points for group participation.  And so on...

  6. Note: We will strictly enforce policies on cheating. Remember that we routinely run similarity checking programs on your solutions to detect cheating. Please make sure you turn in your own work.

     You should be very careful about using code snippets you find on the Internet. In general your code should be your own. It is OK to read tutorials on the web and use these concepts in your assignment. Blind use of code from web is strictly disallowed. Feel free to check with us if you have questions on this policy. And be sure to document any Internet sources/ tutorials you have used to complete the assignment in your README file.

  7. Late Policy: Please refer to the course syllabus for late policy on labs assignments. This late policy will be strictly enforced. Please start early so that you can submit the assignment on time.

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     Prashant Shenoy
     Last modified: Thu Apr 22 09:39:01 EDT 2010