Distributed File Systems

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Distributed File Systems
By
Pravin D'Souza
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• What is a Distributed File System??
• Allows transparent access to remote files over a
  network.
• Examples
• Network File System (NFS) by Sun Microsystems.
• Remote File Sharing System (RFS) from ATT.
• Andrew File System (AFS) from CMU.
• Centralized File System Vs. Distributed File
  System

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• Properties of Distributed File Systems
• Network transparency.
• Location transparency.
• Location independence.
• User mobility.
• Fault tolerance.
• Scalability.
• File Mobility.

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• Design Considerations
• Different Distributed File Systems can be
  compared according to how they deal with these
  issues
• Name Space.
• Stateful or stateless operation.
• Semantics of sharing.
• Remote access methods.

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• Network File Systems (NFS)
• The architecture is based on the Client-Server
  model.
• Clients and servers communicate via Remote
  Procedure Calls (RPC).
• An NFS server exports one or more filesystems.
• Clients mount such a file system.
• - Hard and Soft mounts.
• e.g mount t nfs nfssrv/usr /usr

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• Design Goals
• NFS should not be restricted to UNIX.
• The protocol should not be dependent on any
  particular hardware.
• Simple recovery mechanisms.
• Remote files should be accessible transparently.
• NFS performance should be comparable to that of
  local disk.
• Implementation must be transport independent.
• Unix file system semantics must be maintained
  for UNIX clients.

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• NFS Components
• The NFS protocol
• The Remote Procedure Call (RPC) protocol.
• The Extended Data Representation (XDR).
• The NFS Server Code.
• The NFS Client Code.
• The Mount protocol.
• Daemon processes.
• The Network Lock Manager (NLM).

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• NFS Implementation
• Control flow When a process on the client makes
  a system call that operates on an NFS file, the
  file-system independent code identifies the vnode
  of the file and invokes the relevant Vnode
  operation.
• File handles The NFS protocol associates an
  object called the file handle with each file or
  directory.
• The mount operation
• Pathname Lookup

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• UNIX Semantics
• Open file permissions.
• Deletion of open files.
• Reads and writes.
• NFS Performance
• Performance Bottlenecks.
• Client-Side Caching.
• Deferral of writes.
• The Retransmissions Cache.

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• Remote File Sharing (RFS)
• Introduced by ATT.
• RFS uses the client-server model.
• The design goal is to provide transparent access
  to remote files by preserving UNIX semantics.
• RFS is a completely stateful architecture.
• RFS uses a reliable, virtual circuit transport
  service such as TCP/IP. A virtual circuit is
  established between a client and server during
  the first mount operation.
• Network independence is achieved.
• There is a name server.

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• RFS Implementations
• Remote mount An RFS server can advertise a
  directory using the advfs system call along with
  some arguments.
• RFS clients and servers The client can access
  an RFS file either through its pathname or
  through a file descriptor.
• Crash Recovery Stateful systems need elaborate
  crash recovery mechanisms.

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Client-Side Caching Client caching is
activated at mount time. The cache is strictly
write through and the data is written to server
immediately after the local cache copy is written
to and thus the consistency is maintained. Cache
Consistency Any modification of a file, either
by user or client, invalidates the cached copy on
all other clients.
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• Andrew File System (AFS)
• This is a distributed file system capable of
  scaling to thousands of users like in a
  university campus.
• Developed by Carnegie-Melon University and IBM.
• AFS is UNIX compatible.
• It provides a uniform, location independent name
  space for shared files.
• AFS is fault tolerant in case a server fails.
• AFS provides security without trusting clients
  or n/w.
• The performance should be comparable to
  time-sharing system.

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The Andrew File System Scalable Architecture
Storage and Name space organization Session
semantics
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AFS Implementation Caching and Consistency The
cache manager implements the vnode operations for
AFS files on clients. Pathname Lookup This is a
CPU intensive operation and AFS handles it
directly on clients. Security AFS considers the
collection of servers as the boundary of
secutriy. It uses the Kerberos authentication
system.
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• The DCE Distributed File System
• In 1989 the Transarc Corporation took over the
  development and production of AFS.
• DFS is similar to AFS in several respects. It
  improves upon AFS in following ways
• It allows a single machine to be both a server
  and client.
• It provides stronger, UNIX like sharing
  semantics and consistency guarantees.
• It allows greater interoperability with other
  file systems.

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Questions??