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Towards Automatic File Systems

CS 736 Spring 2008. Towards Automatic File Systems. By Kanchan Damle Course Instructor Prof. Remzi Arpaci-Dusseau. Lot of Questions Implementation. Basic File System Design. FUSE Some Measures. Performance of File System. Conclusions Questions?. AGENDA.

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Towards Automatic File Systems

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  1. CS 736 Spring 2008 Towards Automatic File Systems By Kanchan Damle Course Instructor Prof. Remzi Arpaci-Dusseau

  2. Lot of Questions Implementation. Basic File System Design. FUSE Some Measures. Performance of File System. Conclusions Questions? AGENDA

  3. The usual questions for any new idea !! • What? • User :Specifies • Implementer: Will be Done! • Why? • User knows his needs better • Flexibility • Optimum Resource Usage • How? • Let the user specify • Just redirect the system calls

  4. Implementation Details • Say user wants : “Simple FS” (I picked another ,“S”- SFS) • 1 superblock -no redundancy like FFS • Inode and Data Bitmaps -no problems of free-list . • Limit to 100 files. • User not sure of how he will access the data..

  5. Other Details • Max Number of Open Files: 25 • Max File Size : 15K • Some reserved blocks. • Maximum Filename length = 16 • Maximum Pathname length = 256 • Root Exists !(Inode #2) • Persistent Storage on File: • Raw device • File on existing file system.

  6. Implementation Details METADATA STRUCTURES 0 1 2 3 4 5-39 INODE-BITMAP DATA-BLOCK BITMAP SECTORS 5-39 INODES SUPER BLOCK ***Data blocks follow 40 onwards

  7. Implementation Details Why FUSE? • User level FS implementation done • But how can we have system level calls directed to implemented FS? • Need ? -perhaps Uniformity, hassle to remember the new calls.. • Solution : Indirection! • FUSE can help

  8. FUSE unveiled! • What is FUSE? • A framework: Redirects FS calls to (User level ) Simple FS API • Has two modules + Mount Utility • User Library : Interfaces with the Simple FS API • Kernel Module : Binds to VFS, Calls FUSE Library

  9. Basic Call Flow..

  10. Interface Details [0] Blocking read call on kernel module [1] Request from VFS [2] Unblocks the thread and sends request [3] Fuse Library invokes respective Simple FS Module [4] Module returns with requested data. [5] Transfer data to kernel module [6] Transfer data to VFS Simple FS User Space [4] [3] FUSE Library Blocking Read Call [2] [5] [0] [1] Kernel Module [6] Kernel Space

  11. Internal Details • VFS System Call Map • unlink() -> sys_unlink • Fuse kernel call transfer • sys_unlink -> fuse_read_device • Request Processing • User Level FS call invoked • Blocking Calls : Queues Used • Pending, processing, etc

  12. Benefits : Flexibility Not tied to any particular OS Avoid Kernel recompilation Non-privileged mounts possible Simplicity : Coding in user space is simpler Cost : More overhead: More user and kernel switching Cost-Benefit …

  13. Performance Measures .. Simple FS Time in msec Run Number

  14. Performance Measures .. Simple FS

  15. Conclusions • Good performance • Buffered writes, fewer writes • No worry about positioning time • Uniform access time • Flexibility • User gets what he requires • System calls redirected hence easy to use. • Don’t remember erratic name and be uniform • But, • Periodic FS_Sync required • What if file deleted? • Yet not secure for sensitive data

  16. Lessons Learnt • Reading does not imply doing. • Simple things can be bigger hurdles! • Planning work helps • Give it “time” and “thought” • Believe that you can do it!

  17. Thank you… Questions?

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