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A distributed file system is a resource management
component of a distributed operating system. It implements a
common file system that can be shared by all the autonomous
computers in the system.
DISTRIBUTED FILE SYSTEM
Two important goals :
1. Network transparency – to access files distributed over a
network. Ideally, users do not have to be aware of the
location of files to access them.
2. High Availability - to provide high availability. Users should
have the same easy access to files, irrespective of their
physical location.

ARCHITECTURE
In a distributed file system, files can be stored at any
machine and the computation can be performed at any
machine.
 The two most important services present in a distributed file
system are name server and cache manager.
 A name server is a process that maps names specified by
clients to stored objects such as files and directories.
 The mapping (also referred to as name resolution)
 A cache manager is a process that implements file caching.
 In file caching, a copy of data stored at a remote file server is
brought to the client’s machine when referenced by the
client.

 Cache managers can be present at both clients and file servers.
 Cache managers at the servers cache files in the main memory
to reduce delays due to disk latency.
 If multiple clients are allowed to cache a file and modify it,
the copies can become inconsistent.
 To avoid this inconsistency problem, cache managers at both
servers and clients coordinate to perform data storage and
retrieval operations.

Architecture of a Distributed File System

 A request by a process to access a data block is presented to
the local cache (client cache) of the machine (client) on
which the process is running .
 If the block is not in the cache, then the local disk, if
present, is checked for the presence of the data block.
 If the block is present, then the request is satisfied and the
block is loaded into the client cache.
 If the block is not stored locally, then the request is passed
on to the appropriate file server
 The server checks its own cache for the presence of the data
block before issuing a disk I/O request.
 The data block is transferred to the client cache in any case
and loaded to the server cache if it was missing in the server
cache.

MECHANISMS FOR BUILDING DISTRIBUTED
FILE SYSTEM
Mounting
A mount mechanism allows binding together of different
filename spaces to form a single hierarchically structured name
space.
Two approaches to maintain the mount information:
 Mount information can be maintained at clients, in which
case each client has to individually mount every required file
system. This approach is employed in the Sun network file
system. Since each client can mount a file system at any node
in the name space tree, every client need not necessarily see
an identical filename space.

 Mount information can be maintained at servers, in which
case it is possible that every client sees an identical filename
space. If files are moved to a different server, then mount
information need only be updated at the servers. In the first
approach, every client needs to update its mount table.

 Caching is commonly employed in distributed files systems to
reduce delays in the accessing of data.
 In file caching, a copy of data stored at a remote file server is
brought to the client when referenced by the client.
 The temporal locality of reference refers to the fact that a file
recently accessed is likely to be accessed again in the near
future.
 Data can either be cached in the main memory (server cache) at
the servers to reduce disk access latency.
 Caching improves files system performance by reducing the
delay in accessing data.
CACHING

 An alternative approach is to treat cached data as hints.
 In this case, cached data are not expected to be completely
accurate.
 However, valid cache entries improve performance
substantially without incurring the cost of maintaining
cost consistency.
 The class of applications that can utilize hints are those
which can recover after discovering that the cached data
are invalid.
HINTS

 Transferring data in bulk reduces the protocol processing overhead
at both servers and clients.
 In bulk data transfer, multiple consecutive data blocks are
transferred from servers to clients instead of just the block
referenced by clients.
 While file caching amortizes the high cost of accessing remote
servers over several local references to the same information
 Bulk transfer amortizes the protocol processing overhead and disk
seek time over many consecutive blocks of a file.
 Bulk transfers reduce file access overhead through obtaining a
multiple number of blocks with a single seek; by formatting and
transmitting a multiple number of large packets in a single context
switch; and by reducing the number of acknowledgements that
need to be sent.
BULK DATA TRANSFER

 Encryption is used for enforcing security in distributed
systems.
 The work of Needham and Schroeder is the basis for most of
the current security mechanisms in distributed systems.
 In their scheme, two entities wishing to communicate with
each other establish a key for conversation with the help of
an authentication server.
 It is important to note that the conversation key is
determined by the authentication server, but is never spent
in plain (unencrypted) text to either of the entities.
ENCRYPTION

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