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Operating Systems
       CMPSCI 377
 Distributed File Systems
            Emery Berger
University of Massachusetts Amherst...
Distributed File Systems
    Numerous drawbacks of local file systems:


        Inconvenient
    

        Administrati...
Complications
    Distributed file systems add complexity


    & many design tradeoffs
        Naming – absolute vs. rel...
Naming & Transparency
    Issues


        How are files named?
    

        Do filenames reveal location?
    

     ...
Location naming
    Location transparency:

    filename does not reveal
    physical storage location
        Normal in ...
Windows: Absolute Names

         lokihomeemery
         machine nameremote pathname
    Advantages:                      ...
NFS: Relative Names

         /nfs/sting/users1/emery


    Advantages:                                Disadvantages:
   ...
NFS: Relative Names

                /courses/cs300/cs377

          Implemented via mount points
      

               ...
NFS Example




  UNIVERSITY OF MASSACHUSETTS AMHERST • Department of Computer Science   9
URLs Viewed as File System
    Uniform Resource Locator names


    increasingly standard way to access data

    protoco...
Distributed File Systems: Issues
    Naming & transparency


    Remote file access & caching


    Server with state or...
Remote File Access & Caching

    Can access files two ways


        Remotely: returns results using RPC
    

        ...
Remote File Caching
    Local disk:

        Reduces access time (compared to remote)
    
        Safe if node fails
  ...
Cache Update Policies
    Write-through: always write to remote disk


        Reliable
    

        Low-performance = ...
Cache Consistency
    Client-initiated consistency:

    client contacts server and checks consistency
        every acce...
Case Study: Network File System

    NFS: standard for distributed UNIX file access


        Designed to run on LANs
   ...
NFS Implementation
Set of RPC operations for remote file access:
           Directory search, reading directory entries
  ...
NFS Implementation




   UNIVERSITY OF MASSACHUSETTS AMHERST • Department of Computer Science   18
The End




  UNIVERSITY OF MASSACHUSETTS AMHERST • Department of Computer Science   19
Global Name Space
    Single name space:


        Examples:
    

              AFS (CMU’s Andrew File System)
        ...
Global Name Space: Pros & Cons
    Advantages:


        Naming – consistent
    

        Ensures all files are same re...
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Operating Systems - Advanced File Systems

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Transcript of "Operating Systems - Advanced File Systems"

  1. 1. Operating Systems CMPSCI 377 Distributed File Systems Emery Berger University of Massachusetts Amherst UNIVERSITY OF MASSACHUSETTS AMHERST • Department of Computer Science
  2. 2. Distributed File Systems Numerous drawbacks of local file systems:  Inconvenient  Administrative overhead  Single point-of-failure  Solution: distributed file systems  FS appears to be local, but data is remote  Two major implementations:  Windows  NFS (Sun’s Network File System)  UNIVERSITY OF MASSACHUSETTS AMHERST • Department of Computer Science 2
  3. 3. Complications Distributed file systems add complexity  & many design tradeoffs Naming – absolute vs. relative (to server)  Remote access vs. caching  Stateless or stateful server  Single image or replication  UNIVERSITY OF MASSACHUSETTS AMHERST • Department of Computer Science 3
  4. 4. Naming & Transparency Issues  How are files named?  Do filenames reveal location?  Do filenames change if file moves?  Do filenames change if user moves?  UNIVERSITY OF MASSACHUSETTS AMHERST • Department of Computer Science 4
  5. 5. Location naming Location transparency:  filename does not reveal physical storage location Normal in Unix  Compare to Windows - C:foobar  Provides location independence:  no change if file’s storage location changes UNIVERSITY OF MASSACHUSETTS AMHERST • Department of Computer Science 5
  6. 6. Windows: Absolute Names lokihomeemery machine nameremote pathname Advantages: Disadvantages:   Easy to find fully User must know   specified filename complete name – local & remote Easy to add & delete  different new names Location dependent No global state   (cannot move file) Scales easily  Makes sharing harder  Not fault-tolerant  UNIVERSITY OF MASSACHUSETTS AMHERST • Department of Computer Science 6
  7. 7. NFS: Relative Names /nfs/sting/users1/emery Advantages: Disadvantages:   Location  Admin  transparent overhead Remote name  can change across reboots UNIVERSITY OF MASSACHUSETTS AMHERST • Department of Computer Science 7
  8. 8. NFS: Relative Names /courses/cs300/cs377 Implemented via mount points  one level of indirection!  Each host: local names ! remote locations  Mount table (/etc/fstab)  <remote pathname @ machine, local pathname>  % cat /etc/fstab elsrv4:/courses /courses nfs intr,hard,rw 0 0 elsrv4:/courses/cs100_200 /courses/cs100_200 nfs intr,hard,rw 0 0 elsrv4:/courses/cs300 /courses/cs300 nfs intr,hard,rw 0 0 UNIVERSITY OF MASSACHUSETTS AMHERST • Department of Computer Science 8
  9. 9. NFS Example UNIVERSITY OF MASSACHUSETTS AMHERST • Department of Computer Science 9
  10. 10. URLs Viewed as File System Uniform Resource Locator names  increasingly standard way to access data protocol://machine/path/to/file Good? Bad?  Looks like Windows… same?  UNIVERSITY OF MASSACHUSETTS AMHERST • Department of Computer Science 10
  11. 11. Distributed File Systems: Issues Naming & transparency  Remote file access & caching  Server with state or without  Replication  UNIVERSITY OF MASSACHUSETTS AMHERST • Department of Computer Science 11
  12. 12. Remote File Access & Caching Can access files two ways  Remotely: returns results using RPC  Locally: transfer part of file = caching  Caching issues:  Performance: Where & when to cache file  blocks? Correctness:  When to propagate updates back to remote file?  What happens when multiple clients cache same file?  UNIVERSITY OF MASSACHUSETTS AMHERST • Department of Computer Science 12
  13. 13. Remote File Caching Local disk:  Reduces access time (compared to remote)  Safe if node fails  Difficult to keep copy consistent with remote file – Requires client to have disk (…) – Local memory:  Quick  Works without disks  Difficult to keep copy consistent with remote file – Smaller cache size – Not fault-tolerant – UNIVERSITY OF MASSACHUSETTS AMHERST • Department of Computer Science 13
  14. 14. Cache Update Policies Write-through: always write to remote disk  Reliable  Low-performance = remote service for all writes – Write-back: write only to cache  Write to disk on evictions, periodic sync  Quick  Reduces network traffic (n writes to same block)  User machine crashes ) data loss – UNIVERSITY OF MASSACHUSETTS AMHERST • Department of Computer Science 14
  15. 15. Cache Consistency Client-initiated consistency:  client contacts server and checks consistency every access  at given intervals  only upon opening a file  Server-initiated consistency:  server detects potential conflicts, invalidates caches Server needs to know:  which clients have cached which parts of which files, plus  which clients are readers & which are writers  UNIVERSITY OF MASSACHUSETTS AMHERST • Department of Computer Science 15
  16. 16. Case Study: Network File System NFS: standard for distributed UNIX file access  Designed to run on LANs  Nodes: both servers & clients  Servers have no state = no info about clients  Uses mount protocol to make global name local  /etc/exports  local names server willing to export  /etc/fstab  global names that local nodes import  global name must be in /etc/exports on server  UNIVERSITY OF MASSACHUSETTS AMHERST • Department of Computer Science 16
  17. 17. NFS Implementation Set of RPC operations for remote file access: Directory search, reading directory entries  Manipulating links & directories  Accessing file attributes  Reading/writing files  UNIVERSITY OF MASSACHUSETTS AMHERST • Department of Computer Science 17
  18. 18. NFS Implementation UNIVERSITY OF MASSACHUSETTS AMHERST • Department of Computer Science 18
  19. 19. The End UNIVERSITY OF MASSACHUSETTS AMHERST • Department of Computer Science 19
  20. 20. Global Name Space Single name space:  Examples:  AFS (CMU’s Andrew File System)  Sprite (Berkeley)  No matter which node you are on,  filenames remain the same Client: gets filename structure from server(s)  When users access files, server sends copies  to workstation, where they are cached UNIVERSITY OF MASSACHUSETTS AMHERST • Department of Computer Science 20
  21. 21. Global Name Space: Pros & Cons Advantages:  Naming – consistent  Ensures all files are same regardless of where you  login Late binding of names ) moving them is easier  Disadvantages:  Difficult for OS to keep files consistent (caching)  Global name space may limit flexibility  Performance issues  UNIVERSITY OF MASSACHUSETTS AMHERST • Department of Computer Science 21
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