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Implementing a UNIX-like File System in a 1MB Virtual Environment

This project involves creating a UNIX-like file system within a simulated 1MB file. The design document, no more than 2-3 pages long, should detail the implementation steps, including API functions such as open, close, read, write, link, and unlink, as well as directory operations like make and remove. Key components include the superblock, inodes, and data blocks. Students must work independently, with clarifications available through Piazza, and submit their work by January 14 at 5 PM. Testing and a separate test script will be required.

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Implementing a UNIX-like File System in a 1MB Virtual Environment

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  1. Project 6 Unix File System

  2. Administrative • No Design Review • A design document instead • 2-3 pages max • No collaboration with peers • Piazza is for clarifications • Due on January 14 (Dean’s Date) @ 5 • All virtual office hours

  3. Overview • Implement a UNIX-like file system • In a 1MB file! • Determined by FS_SIZE • Pages are 512 bytes, so 2,048 sectors

  4. API • Disk format • File • open, close, read, write, seek • link & unlink (which is how you delete a file) • stat • Directory operations • make, remove • Shell • ls & chdir (cd)

  5. Don’t Worry About • Permissions • Access Control Lists (ACLs) • Concurrency

  6. Disk Layout Boot block & OS stuffs Block Allocation Map Super block Data blocks inodes This project in a 1MB file named “disk” Space between divisions is not representative of actual size

  7. Contains metadata about the disk Examples: • Size • # of inodes • # of data blocks • Where inodes start • Where data blocks start • Magic number Super block

  8. Where do I go to find …? inode array inode metadata direct Only direct for this project indirect inodes

  9. inode Metadata Examples • File or directory? • Link count • Size • Etc.. inodes

  10. fs_init • A “constructor” for the FS code • block_init

  11. fs_mkfs • “Makes” a file system • Writes the super block • Mark inodes and data blocks to be free • Create root directory • Initialize file descriptor table

  12. File Creation & Deletion • fs_open(), fs_link(), fs_unlink() • open: create a new file if it does not exist • link: hard link to a file • create a link to an existing file • hard vs. soft (symbolic) link? • unlink: • delete a file if link count == 0 • delete directory entry • files that are still open

  13. File Access • open: open existing file (allocate file descriptor) • read: read bytes from open file • write: write bytes to open file • lseek: change position in file • close: free file descriptor

  14. fs_lseek() Semantics • Our fs_lseek() only takes two arguments • fd, offset • Unix lseek() takes three • fd, offset, whence • Whence: SEEK_SET, SEEK_CUR, SEEK_END • Our fs_lseek() assumes SEEK_SET • What if lseek() wants to seek past the end of the file?

  15. Directories, part 1 • Like a file: List of files and directories • Name to inode number mapping • Can read it like a file • Use existing file I/O functions to do directory manipulation • Always has at least two entries • “..” parent directory • “.” current directory

  16. Directories, part 2 • mkdir: make a directory • create an entry in parent directory • create two directories: “..” and “.” • rmdir: remove directory if empty • cd: change the current directory • For relative path names only

  17. Example fs_mkdir intfs_mkdir(char *file_name) { if (file_name exists) return ERROR; /* allocate inode */ /* allocate data blocks */ /* set directory entries for “..” & “.” */ /* set inode entries appropriately */ /* update parent */ return SUCCESS }

  18. FAQ

  19. Absolute Path names • Do we need to support absolute path names? • I.e., when I am in “/foo”, do I need to support: chdir /bar/tmp/? • No • You would have to support absolute path names everywhere: open, link, mkdir, etc.

  20. Removing a Directory • Do I need to support recursive directory removal? • I.e., remove all the subdirectories and files contained in a directory I am going to delete? • No • Return an error if directory is not empty

  21. File System Check (fsck) • Useful for debugging • Verifies integrity of: • Superblock magic number • Block allocations • Inode allocations • Block Allocation Map • Directory content • Etc.

  22. Implementing the Project • In Linux • Uses a file to simulate a disk • Code is provided • ./lnxsh • Shell supports • System calls for File System • Commands like “ls”, “cat foo”, “create foo200” • 500-1,000 lines of code

  23. Testing • Apython script to use in testing will be provided • Multiple tests that each • Execute the shell • Open an existing file system (or format a new one) • Write commands to shell (cat foo) • Read output from shell (ABCDEFGHIJKL) • Exit • You should write your own test cases • Submit them with your code

  24. Design Document • Sections on project page • I like diagrams • PDF • Submit together with your implementation • 2-3 pages

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