CSS 430
Final Project: File System
Duedate: See the syllabus

1. Purpose

2. Collaboration

3. What to Use

4. Interface

Each user thread needs to keep track of all files it has opened. For this purpose, it should maintain a table of those open files in its TCB. This table is called a user file descriptor table. It has 32 entries. Whenever a thread opens a file, it must allocate to this file a new table entry, termed a file descriptor. Each file descriptor includes the file access mode and the reference to the corresponding file (structure) table entry. The file access mode indicates "read only", "write only", "read/write", or "append". The file (structure) table is a system-maintained table shared among all user threads, each entry of which maintains the seek pointer and the inode number of a file. Depending on the access mode, the seek pointer is set to the first or the tail of the file, and keeps track of a next position to read from and to write to the file. It is entirely possible for one thread to open the same file many times, thus having several entries in the corresponding TCB's user file descriptor table. Although each of those user file descriptor table entries refer to a different file (structure) table entry with its own seek pointer, all of them eventually points to the same inode.

The file system you will implement must provide the following eight system calls.

1. int SysLib.format( int files );
   formats the disk, (i.e., Disk.java's data contents). The parameter files specifies the maximum number of files to be created, (i.e., the number of inodes to be allocated) in your file system. The return value is 0 on success, otherwise -1.
2. int fd = SysLib.open( String fileName, String mode );
   opens the file specified by the fileName string in the given mode (where "r" = ready only, "w" = write only, "w+" = read/write, "a" = append), and allocates a new file descriptor, fd to this file. The file is created if it does not exist in the mode "w", "w+" or "a". SysLib.open must return a negative number as an error value if the file does not exist in the mode "r". Note that the file descriptors 0, 1, and 2 are reserved as the standard input, output, and error, and therefore a newly opened file must receive a new descriptor numbered in the range between 3 and 31. If the calling thread's user file descriptor table is full, SysLib.open should return an error value. The seek pointer is initialized to zero in the mode "r", "w", and "w+", whereas initialized at the end of the file in the mode "a".
3. int read( int fd, byte buffer[] );
   reads up to buffer.length bytes from the file indicated by fd, starting at the position currently pointed to by the seek pointer. If bytes remaining between the current seek pointer and the end of file are less than buffer.length, SysLib.read reads as many bytes as possible, putting them into the beginning of buffer. It increments the seek pointer by the number of bytes to have been read. The return value is the number of bytes that have been read, or a negative value upon an error.
4. int write( int fd, byte buffer[] );
   writes the contents of buffer to the file indicated by fd, starting at the position indicated by the seek pointer. The operation may overwrite existing data in the file and/or append to the end of the file. SysLib.write increments the seek pointer by the number of bytes to have been written. The return value is the number of bytes that have been written, or a negative value upon an error.
5. int seek( int fd, int offset, int whence );
   Updates the seek pointer corresponding to fd as follows:
   • If whence is SEEK_SET (= 0), the file's seek pointer is set to offset bytes from the beginning of the file
   • If whence is SEEK_CUR (= 1), the file's seek pointer is set to its current value plus the offset. The offset can be positive or negative.
   • If whence is SEEK_END (= 2), the file's seek pointer is set to the size of the file plus the offset. The offset can be positive or negative.
   If the user attempts to set the seek pointer to a negative number you must clamp it to zero. If the user attempts to set the pointer to beyond the file size, you must set the seek pointer to the end of the file. In both cases, you should return success.
6. int close( int fd );
   closes the file corresponding to fd, commits all file transactions on this file, and unregisters fd from the user file descriptor table of the calling thread's TCB. The return value is 0 in success, otherwise -1.
7. int delete( String fileName );
   destroys the file specified by fileName. If the file is currently open, it is not destroyed until the last open on it is closed, but new attempts to open it will fail.
8. int fsize( int fd );
   returns the size in bytes of the file indicated by fd.

5. Implementation

Superblock

   class Superblock {
      public int totalBlocks; // the number of disk blocks
      public int totalInodes; // the number of inodes
      public int freeList;    // the block number of the free list's head
   }

Inodes

   public class Inode {
      private final static int iNodeSize = 32;       // fix to 32 bytes
      private final static int directSize = 11;      // # direct pointers

      public int length;                             // file size in bytes
      public short count;                            // # file-table entries pointing to this
      public short flag;                             // 0 = unused, 1 = used, ...
      public short direct[] = new short[directSize]; // direct pointers
      public short indirect;                         // a indirect pointer

      Inode( ) {                                     // a default constructor
         length = 0;
         count = 0;
         flag = 1;
         for ( int i = 0; i < directSize; i++ )
            direct[i] = -1;
         indirect = -1;
      }

      Inode( short iNumber ) {                       // retrieving inode from disk
         // design it by yourself.
      }

      int toDisk( short iNumber ) {                  // save to disk as the i-th inode
         // design it by yourself.
      }
   }

The system must avoid any inode inconsistency among different user threads. There are two solutions to maintain the inode consistency:

1. Before an inode on memory needs to be updated, check the corresponding inode in the disk, read it from the disk if the disk has been already updated by another thread. Thereafter, you should write back its contents to disk immediately. Note that the inode data to be written back include int length, short count, short flag, short direct[11], and short indirect, thus requiring a space of 32 bytes in total. For this write-back operation, you will need the toDisk method that saves this inode information to the iNumber-th inode in the disk, where iNumber is given as an argument.
2. You should create a Vector< Inode> object that maintains all inode on memory, is shared among all threads, and is exclusively access by each thread.

Root Directory

Upon a boot, the file system instantiates the following Directory class as the root directory through its constructor, reads the file from the disk that can be found through the inode 0 at 32 bytes of the disk block 1, and initializes the Directory instnace with the file contents. On the other hand, prior to a shutdown, the file system must write back the Directory information onto the disk. The methods bytes2directory( ) and directory2bytes will initialize the Directory instance with a byte array read from the disk and converts the Directory instance into a byte array that will be thereafter written back to the disk.

   public class Directory {
      private static int maxChars = 30; // max characters of each file name

      // Directory entries
      private int fsize[];        // each element stores a different file size.
      private char fnames[][];    // each element stores a different file name.

      public Directory( int maxInumber ) { // directory constructor
         fsizes = new int[maxInumber];     // maxInumber = max files
         for ( int i = 0; i < maxInumber; i++ ) 
             fsize[i] = 0;                 // all file size initialized to 0
         fnames = new char[maxInumber][maxChars];
         String root = "/";                // entry(inode) 0 is "/"
         fsize[0] = root.length( );        // fsize[0] is the size of "/".
         root.getChars( 0, fsizes[0], fnames[0], 0 ); // fnames[0] includes "/"
      }

      public int bytes2directory( byte data[] ) {
         // assumes data[] received directory information from disk
         // initializes the Directory instance with this data[]
      }

      public byte[] directory2bytes( ) {
         // converts and return Directory information into a plain byte array
         // this byte array will be written back to disk
         // note: only meaningfull directory information should be converted
         // into bytes.
      }

      public short ialloc( String filename ) {
         // filename is the one of a file to be created.
         // allocates a new inode number for this filename
      }

      public boolean ifree( short iNumber ) {
         // deallocates this inumber (inode number)
         // the corresponding file will be deleted.
      }

      public short namei( String filename ) {
         // returns the inumber corresponding to this filename
      }
   }

File (Structure) Table

1. The user thread allocates a new entry of the user file descriptor table in its TCB. This entry number itself becomes a file descriptor number. The entry maintains a reference to an file (structure) table entry which will be obtained from the file system in the following sequence.
2. The user thread then requests the file system to allocate a new entry of the system-maintained file (structure) table. This entry includes the seek pointer of this file, a reference to the inode corresponding to the file, the inode number, the count to maintain #threads sharing this file (structure) table, and the access mode. The seek pointer is set to the front or the tail of this file depending on the file access mode.
3. The file system locates the corresponding inode and records it in this file (structure) table entry.
4. The user thread finally registers a reference to this file (structure) table entry in its file descriptor table entry of the TCB.

   public class FileTableEntry {          // Each table entry should have
      public int seekPtr;                 //    a file seek pointer
      public final Inode inode;           //    a reference to its inode
      public final short iNumber;         //    this inode number
      public int count;                   //    # threads sharing this entry
      public final String mode;           //    "r", "w", "w+", or "a"
      public FileTableEntry ( Inode i, short inumber, String m ) {
         seekPtr = 0;             // the seek pointer is set to the file top
         inode = i;
         iNumber = inumber;
         count = 1;               // at least on thread is using this entry
         mode = m;                // once access mode is set, it never changes
         if ( mode.compareTo( "a" ) == 0 ) // if mode is append,
            seekPtr = inode.length;        // seekPtr points to the end of file
      }
   }

   public class FileTable {

      private Vector table;         // the actual entity of this file table
      private Directory dir;        // the root directory 

      public FileTable( Directory directory ) { // constructor
         table = new Vector( );     // instantiate a file (structure) table
         dir = directory;           // receive a reference to the Director
      }                             // from the file system

      // major public methods
      public synchronized FileTableEntry falloc( String filename, String mode ) {
         // allocate a new file (structure) table entry for this file name
         // allocate/retrieve and register the corresponding inode using dir
         // increment this inode's count
         // immediately write back this inode to the disk
         // return a reference to this file (structure) table entry
      }

      public synchronized boolean ffree( FileTableEntry e ) {
         // receive a file table entry reference
         // save the corresponding inode to the disk
         // free this file table entry.
         // return true if this file table entry found in my table
      }

      public synchronized boolean fempty( ) {
         return table.isEmpty( );  // return if table is empty 
      }                            // should be called before starting a format
   }

File Descriptor Table

   public class TCB {
      private Thread thread = null;
      private int tid = 0;
      private int pid = 0;
      private boolean terminate = false;

      // User file descriptor table: 
      // each entry pointing to a file (structure) table entry
      public FileTableEntry[] ftEnt = null;

      public TCB( Thread newThread, int myTid, int parentTid ) {
         thread = newThread;
         tid = myTid;
         pid = parentTid;
         terminated = false;

         // The following code is added for the file system
         ftEnt = new FileTableEntry[32];
         for ( int i = 0; i < 32; i++ )
            ftEnt[i] = null;         // all entries initialized to null
         // fd[0], fd[1], and fd[2] are kept null.
      }
   }

6. Statement of Work

1. Can you read the same data from a file as you wrote on it previously.
2. Can you avoid malicious operations and return an appropriate error code? Those operations include file accesses with a negative seek pointer, too many files to be opened at a time per a thread and over the system, etc.
3. Can the file system synchronize all data on memory with disk before a shutdown and reload those data from the disk upon the next boot?

6. What to Turn In