about Operating system

1. Distribution File System (DFS)
by
Naza Hameed Jan

2. Contents
 File
 Introduction to file system
 Facilities
 Distribution File system
 Structure of distributed file system
 Important features
 Mapping
 Transparency
 Remote file access
 State full versus stateless
 File replication

3. What actually file is?
• A file is a sequence of bits, bytes, line , or
records the meaning of which is defined by the
one who creates it.
• Many different types of information can be
stored on files.
• It can be text,audio,video,graphics,images,source
programs, object programs, executable programs
etc

4. File system
• In a computer a file system is the way in
which files are named and where they are placed
logically for storage and retrieval.
• Most visible aspect of any of the operating
system with which user interact the most.

5. Facilities
• Mechanism for permanent storage of information
in various secondary medias.
• It provides a uniform view of all these storage
medias to the user and operating system

6. Distributed file system
• A distributed system is a collection of loosely
coupled machines interconnected by a
communication network.
• Distribution File System (DFS) is a simplified
way to access files that are dispersed
geographically throughout an organization.

7. Structure of distributed file system
• Here there is a need to define 3 aspects
Service
Server
Client
• A DFS is a file whose client server and storage
devices are dispersed among the machines of
distributed servers

8. What is service, server and client?
• A service is a software entity running on
different machines providing particular type of
function to a client.
• A server is a service software running on singles
machines.
• A client is a service that can invoke a service
using a set of information

9. Important features
• The most important performance of a DFS is the
amount of time needed to satisfy service
requests.
• In distributed the overhead concept is there
The time to deliver the request to server as well
as time to get response across the network back
to the client.

10. Mapping
• Mapping between logical and physical
components i.e.
U ser deal with logical object like file name
Systems deals with the physical blocks of data
shared on secondary medias.

11. Multilevel mapping
• Abstract view of a file that hides the details of
how and where on the disk the file is actually
stored.

12. Transparency
• A transparent DFS hides the location where in
the network the file is stored
Location transparency:- file name does not
reveal the file’s physical storage location
Location independence:- – file name does not
need to be changed when the file’s physical
storage location changes

13. Other aspects of transparency
• Divorce of data
• Static location transparency
• Location independence

14. Divorce of data
• Provides better abstraction of files.
• A file name should denote the file significant
attributes which are its contents rather then it
attribute.

15. Static location transparency
• It provides a convenient way to share a data.
• User can share a file by simply naming the files
in a location transparent manners though the files
have.

16. Location independence
• Separates the naming hierarchy from the stoarge
device hierarchy

17. Remote file access
• Remote-service mechanism is one transfer
approach
• Reduce network traffic by retaining recently
accessed disk blocks in a cache, so that repeated
accesses to the same information can be handled
locally
• If needed data not already cached, a copy of data is
brought from the server to the user
• Accesses are performed on the cached copy

18. Stateful file service
• Client opens a file
• Server fetches information about the file from
its disk, stores it in its memory, and gives the
client a connection identifier unique to the
client and the open file.
• Identifier is used for subsequent accesses until
the session ends.
• Server must reclaim the main-memory space
used by clients who are no longer active

19. Cont….
• Positive aspects is that
• Increased performance
• Stateful server knows if a file was opened for
sequential access and can thus read ahead the
next blocks

20. Stateless file service
• Avoids state information by making each request
self-contained
• Each request identifies the file and position in the
file.
• No need to establish and terminate a connection
by open and close operations

21. File replication
• Replicas of the same file reside on failure-
independent machines
• Improves availability and can shorten service
time
• Naming scheme maps a replicated file name to a
particular replica
• Existence of replicas should be invisible to
higher levels
• Replicas must be distinguished from one
another by different lower-level names

22. Cont…..
• Updates should be applied to all the replicas
where need i.e. replicas of a file denote the same
logical entity, and thus an update to any replica
must be reflected on all other replicas
• Existence of replicas should be invisible to
higher levels

23. DFS Namespaces
• This allows administrators to group shared folders
located on different servers into one or more logically
structured namespaces.
•Each namespace appears to users as a single shared folder
with a series of subfolders. The subfolders typically point
to shared folders that are located on various servers in
multiple geographical sites throughout the organization

24. DFS Replication
This is a MultiMaster replication engine used to
synchronize files between servers for both local and WAN
network connections. DFS Replication supports replication
scheduling, bandwidth throttling, and uses RDC to update
only the portions of files that have changed since the last
replication.
• Folders that are stored on both files server will be
available to users in both offices
• Network bandwidth usage between the two offices will
be minimized.
• Users will be able to access all files in the event that the
WAN link fails

25. Thats it