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Operating Systems Engineering

Operating Systems Engineering. Recitation 5: File Systems. Based on MIT 6.828 (2012, lec9). What Does a File System Do?. Durable Storage Multiplexing Sharing Organization. Why is the FS Interesting?. Critical in many aspects of the system Performance Security Fault tolerance

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Operating Systems Engineering

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  1. Operating Systems Engineering Recitation 5: File Systems Based on MIT 6.828 (2012, lec9)

  2. What Does a File System Do? • Durable Storage • Multiplexing • Sharing • Organization

  3. Why is the FS Interesting? • Critical in many aspects of the system • Performance • Security • Fault tolerance • API design • User interface • You will implement one for JOS

  4. FS - The Unix (& xv6) Choices • Granularity: Files • vs virtual disks, databases • File content: Byte array • vs 80-byte records, numbers, boolean values • Organization: Name hierarchy • vs flat names, GUIDs • Synchronization: None • vs locks, transaction rollbacks

  5. File System API • fd = open("x/y", O_CREATE); • write(fd, "abc", 3); • link("x/y", "x/z"); • unlink("x/y");

  6. FS Abstraction is Quite Useful • Pipes, Sockets • Devices • /dev/console, /dev/urandom • Linux’s /proc • Apps can regard them all the same

  7. Implications of Unix API • FD (file descriptor) needs to remain valid after name change or even deletion • A file can have many (symmetrical) names • Thus - a file is independent of its names • It’s contained in an “inode” • inodes must keep counters • For links in the FS • For open FDs

  8. A bit about xv6 • FS software layers: • System calls • Name ops or file descriptors • inode ops • inode cache (for active inodes) • Transactions • Buffer cache • Disk driver

  9. xv6 – on-disk layout • Viewing disk as a 512-byte sector array: • 0: unused • 1: super block (size, ninodes, root, …) • 2+: array of inodes • X: block in-use bitmap (0=free, 1=inuse) • Y: file (or dir) content blocks • Z: transaction log

  10. xv6 –inodes • On-disk layout: • Type (free, file, directory, device) • Nlink • Size • addrs[12+1] • Direct and indirect blocks • Each inode has an i-number • inode location: sector 2 + 64*inum

  11. xv6 - directories • Directories are much like files • Except user can’t directly write the content • Content is an array of dirents • Dirent: • i-number • 14-byte file name • Dirent is free if the i-number is zero

  12. xv6 – The code • Q: How does xv6 create a file? • create (sheet 56) calls: • ialloc(sheet 46) • iupdate (sheet 46) • dirlink (sheet 50) calls: • readi (sheet 49) • writei(sheet 49)

  13. xv6 – The code • Q: What about concurrent calls to ialloc? • will they get the same inode? • bread (sheet 41) calls: • bget (sheet 40) • brelse (sheet 41)

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