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Advanced File Systems Issues

Advanced File Systems Issues. Adapted from the slides of Andy Wang at FSU. Outline. File systems basics Making file systems faster Making file systems more reliable Making file systems do more Using other forms of persistent storage. File System Basics. File system: a collection of files

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Advanced File Systems Issues

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  1. Advanced File Systems Issues Adapted from the slides of Andy Wang at FSU

  2. Outline • File systems basics • Making file systems faster • Making file systems more reliable • Making file systems do more • Using other forms of persistent storage

  3. File System Basics • File system: a collection of files • An OS may support multiple file systems • Instances of the same type • Different types of file systems • All file systems are typically bound into a single namespace • Often hierarchical

  4. A Hierarchy of File Systems

  5. Some Questions… • Why hierarchical? What are some alternative ways to organize a namespace? • Why not a single file system?

  6. Types of Namespaces • Flat • Hierarchical • Relational • Contextual • Content-based

  7. Example: “Internet FS” • Flat: each URL mapped to one file • Hierarchical: navigation within a site • Relational: keyword search via search engines • Contextual: page rank to improve search results • Content-based: searching for images without knowing their names

  8. Why not a single FS?

  9. Advantages of Independent File Systems • Easier support for multiple hardware devices • More control over disk usage • Fault isolation • Quicker to run consistency checks • Support for multiple types of file systems

  10. Overall Hierarchical Organizations • Constrained • Unconstrained

  11. Constrained Organizations • Independent file systems only located at particular places • Usually at the highest level in the hierarchy (e.g., DOS/Windows and Mac) + Simplicity, simple user model - lack of flexibility

  12. Unconstrained Organizations • Independent file systems can be put anywhere in the hierarchy (e.g., UNIX) + Generality, invisible to user - Complexity, not always what user expects • These organizations require mounting

  13. Mounting File Systems • Each FS is a tree with a single root • Its root is spliced into the overall tree • Typically on top of another file/directory • Or the mount point • Complexities in traversing mount points

  14. Mounting Example tmp root mount(/dev/sd01, /w/x/y/z/tmp)

  15. root After the Mount tmp mount(/dev/sd01, /w/x/y/z/tmp)

  16. Before and After the Mount • Before mounting, if you issue • ls /w/x/y/z/tmp • You see the contents of /w/x/y/z/tmp • After mounting, if you issue • ls /w/x/y/z/tmp • You see the contents of root

  17. Questions • Can we end up with a cyclic graph? • What are some implications? • What are some security concerns?

  18. What is a File? • A collection of data and metadata (often called attributes) • Usually in persistent storage • In UNIX, the metadata of a file is represented by the i_node data structure

  19. i-node • File attributes • Data Name(s) Logical File Representation File

  20. File Attributes • Typical attributes include: • File length • File ownership • File type • Access permissions • Typically stored in special fixed-size area

  21. Extended Attributes • Some systems store more information with attributes (e.g., Mac OS) • Sometimes user-defined attributes • Some such data can be very large • In such cases, treat attributes similar to file data

  22. Storing File Data • Where do you store the data? • Next to the attributes, or elsewhere? • Usually elsewhere • Data is not of single size • Data is changeable • Storing elsewhere allows more flexibility

  23. i-node • File attributes • Data locations • Data blocks Physical File Representation Name(s) File

  24. data block location index block location data block location index block location index block location index block location index block location data block location data block location data block location data block location data block location data block location index block location 12 Ext2 i-node i-node

  25. A Major Design Assumption • File size distribution number of files 22KB – 64 KB file size

  26. Pros/Cons of i_node Design + Faster accesses for small files (also accessed more frequently) + No external fragmentations - Internal fragmentations - Limited maximum file size

  27. Directories • A directory is a special type of file • Instead of normal data, it contains “pointers” to other files • Directories are hooked together to create the hierarchical namespace

  28. data block location index block location index block location index block location data block location data block location file i-node location file1 i-node number file1 file i-node location file2 i-node number file2 file1 file1 Ext2 Directory Representation i-node

  29. Links • Multiple different names for the same file • A Hard link: A second name that points to the same file • A Symbolic link: A special file that directs name translation to take another path

  30. data block location index block location index block location index block location data block location data block location file i-node location file1 i-node number file1 file i-node location file1 i-node number file2 file1 file1 Hard Link Diagram i-node

  31. Implications of Hard Links • Multiple indistinguishable pathnames for the same file • Need to keep link count with file for garbage collection • “Remove” sometimes only removes a name • Rather odd and unexpected semantics

  32. file i-node location data block location data block location index block location index block location data block location index block location file1 i-node number file1 file i-node location file2 i-node number file2 file1 file1 file1 Symbolic Link Diagram file1 i-node

  33. Implications of Symbolic Links • If file at the other end of the link is removed, dangling link • Only one true pathname per file • Just a mechanism to redirect pathname translation • Less system complications

  34. Disk Hardware in Brief One disk head per platter; they typically move together, with one head activated at a time One or more rotating disk platters Disk arm

  35. Disk Hardware in Brief Track Sector Cylinder

  36. Modern Disk Complexities • Zone-bit recording • More sectors near outer tracks • Track skews • Track starting positions are not aligned • Optimize sequential transfers across multiple tracks • Thermal calibrations

  37. Laying Out Files on Disks • Consider a long sequential file • And a disk divided into sectors with 1-KB blocks • Where should you put the bytes?

  38. File Layout Methods • Contiguous allocation • Threaded allocation • Segment-based (variable-sized, extent-based) allocation • Indexed (fixed-sized, extent-based) allocation • Multi-level indexed allocation • Inverted (hashed) allocation

  39. Contiguous Allocation + Fast sequential access + Easy to compute random offsets - External fragmentation

  40. Threaded Allocation • Example: FAT + Easy to grow files - Internal fragmentation - Not good for random accesses - Unreliable

  41. Segment-Based Allocation • A number of contiguous regions of blocks + Combines strengths of contiguous and threaded allocations - Internal fragmentation - Random accesses are not as fast as contiguous allocation

  42. segment list location segment list location end block location begin block location begin block location end block location end block location begin block location begin block location end block location i-node Segment-Based Allocation

  43. data block location data block location data block location data block location Indexed Allocation + Fast random accesses - Internal fragmentation - Complexity in growing/shrinking indices i-node

  44. Multi-level Indexed Allocation • UNIX, ext2 + Easy to grow indices + Fast random accesses - Internal fragmentation - Complexity to reduce indirections for small files

  45. data block location index block location data block location index block location index block location index block location index block location data block location data block location data block location data block location data block location data block location index block location 12 Multi-level Indexed Allocation ext2 i-node

  46. data block location data block location data block location data block location data block location data block location data block location data block location Inverted Allocation • Venti + Reduced storage requirement for archives - Slow random accesses i-node for file A i-node for file B

  47. FS Performance Issues • Disk-based FS performance limited by • Disk seek • Rotational latency • Disk bandwidth

  48. Typical Disk Overheads • ~8.5 msec seek time • ~4.2 msec rotational delay • ~.017 msec to transfer a 1-KB block (based on 58 MB/sec) • To access a random location • ~13 msec to access a 1-KB block • ~ 76KB/sec effective bandwidth

  49. How are disks improving? • Density: 10-25% per year • Capacity: 25% per year • Transfer rate: 20% per year • Seek time: 8% per year • Rotational latency: 5-8% per year • All slower than processor speed increases

  50. The Disk/Processor Gap • Since processor speeds double every two to three years • And disk seek times double every ten years • Processors are waiting longer and longer for data from disk • Important for OS to cover this gap

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