CSS430 Final Project Solution

In this project, you will build a Unix-like file system on our ThreadOS. User thread programs will be now relieved from painful direct access to disk blocks and given a vision of stream-oriented files.
2. Collaboration
3. What to Use
Use ThreadOS' original features, (i.e., the original classes) except Scheduler.java and Kernel.java. For Scheduler.java, replace it with Scheduler_fil.java, (i.e. use Scheduler_fil.java). For Kernel.java, use the one you got in the assignment 4, (i.e., Kernel_org.java). To be simple, all you need is re-download all readable files from the ThreadOS directory again.
4. Interface
The "/" root directory is only the one predefined by this file system and permanently available for user threads to store their files. No other directories are provided by the system and created by users dynamically. Therefore, all files are maintained in the "/" root directory. The file system must provide user threads with the following system calls that allow them to format, to open, to read from, to write to, to update the seek pointer of, to close, to delete, and to get the size of their files.

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.
5. int seek( int fd, int offset, int whence );
Updates the seek pointer corresponding to fd as follows:
o If whence is SEEK_SET (= 0), the file's seek pointer is set to offset bytes from the beginning of
the file
o 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. o 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.
Unless specified otherwise, each of the above system calls returns -1 negative when detecting an error.
5. Implementation
Superblock
The disk block 0 is called a superblock and used to describe (1) the number of disk blocks, (2) the number of inodes, and (3) the block number of the head block of the free list. This is the OS-managed block. No other information must be recorded in and no user threads must be able to get access to the 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
After the superblock, (i.e., from the second block) are inode blocks. Each inode describes one file. Our inode is a simplified version of the Unix inode (as explained in our textbook.) It includes 12 pointers of the index block. The first 11 of these pointers point to direct blocks. The last pointer points to an indirect block. In addition, each inode must include (1) the length of the corresponding file, (2) the number of file (structure) table entries that point to this inode, and (3) the flag to indicate if it is unused (= 0), used(= 1), or in some other status (= 2, 3, 4, ...). 16 inodes can be stored in one block.
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.
} }
As shown above, you will need the 2nd constructor that retrieves an existing inode from the disk onto the memory. Given an inode number, termed inumber, this constructor reads the corresponding disk block, locates the corresponding inode information in that block, and initializes a new inode with this information.
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 iNumberth 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
The "/" root directory maintains each file in a different directory entry that contains its file name (in maximum 30 characters = in max. 60 bytes in Java) and the corresponding inode number. The directory receives the maximum number of inodes to be created, (i.e., thus the max. number of files to be created) and keeps track of which inode numbers are in use. Since the directory itself is considered as a file, its contents are maintained by an inode, specifically saying inode 0. This can be located in the first 32 bytes of the disk block 1.

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 instance 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
The file system maintains the file (structure) table shared among all user threads. When a user thread opens a file, it follows the sequence listed below:
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 a file (structure) table entry which will be obtained from the file system.
2. The user thread 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.
The file (structure) table entry should be:
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
} }
The file (structure) table is defined as follows:
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
Each user thread maintains a user file descriptor table in its own TCB. Every time it opens a file, it allocates a new entry table including a reference to the corresponding file (structure) table entry. Whenever a thread spawns a new child thread, it passes a copy of its TCB to this child which thus has a copy of its parent's user file descriptor table. This in turn means the both the parent and the child refer to the same file (structure) table entries and eventually share the same files.


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.
} }
Note that the file descriptors 0, 1, and 2 are reserved for the standard input, output, and error, and thus those entires must remain null.
6. Statement of Work
Task 1: Design and implement the file system in ThreadOS as specified above.
Each file must have a heading comment mentioning who coded it.
1. Any assumptions and limitations
2. Detailed internal designs of your inode, file table, and file system.
3. Consideration on performance estimation, current functionality, and possible extensions of your file system. Task 4: Write a confidential evaluation of your teammate’s contribution to the project. This evaluation must be submitted independently by each student. If you worked on everything by yourself, write why you have chosen this solo option.
7. What to Turn in
Materials Points
1 Task 1: Your Implementation (all the source code you implemented or modified)
a. Completed Inode coding: +5pts,
b. Completed Directory coding: +5pts,
c. Completed File (structure) table coding: +5pts,
d. Completed TCB user file descriptor table coding (already provided): +5pts, and
e. Sufficient coding of file system: +13pts
f. Code organization: +20pts
• Well organized: 20pts,
• Poor comments or poor code organization: 15pts, or
• No comments and horrible code: 10pts 53
2 Task 2: Your Test5.java execution results
Correct behavior of format...........................................2
Correct behavior of open.............................................2
Correct behavior of writing a few bytes (appx.16 bytes)..............2
Correct behavior of close............................................2
Correct behavior of reading a few bytes..............................2
Correct behavior of appending a few bytes (appx.32 bytes)............1
Correct behavior of seeking in a small file (appx.48 bytes)..........1
Correct behavior of reading/writing a small file (appx.48 bytes)...0.5

Correct behavior of writing a lot of bytes (appx. 512 * 13 bytes)..0.5
Correct behavior of reading a lot of bytes.........................0.5
Correct behavior of appending a few bytes to a large file..........0.5
Correct behavior of seeking in a large file (appx. 512 * 13 + 48)..0.5
Correct behavior of reading/writing a large file with w+...........0.5 17

Correct behavior of delete.........................................0.5
Correct behavior of creating over 40 files ........................0.5
Correct behavior of a parent and a child reading the same file.....0.5
Correct behavior of reading the same file with two independent fds.0.5
3 Task 3: Your report
a. Specification including your assumptions and limitations: +8pts
b. Descriptions on internal design including inode, file table, etc: +8pts, and
c. Consideration on execution performance and functionality: +9pts 25
4 Task 4: A confidential evaluation of your teammate (to be submitted independently). If you worked on everything by yourself, write why you have chosen this solo option. 5
Total 100pts.