1. Dsitributed File Systems
Prepared By:-
Monika Patel.

2. Outline:- What is Distributed File System? (DFS)
Characteristics of DFS
Transparency
Concurrent File Updates
File Replication
Fault Tolerance
Security
IBM’s DCE Distributed File Service

3. Why we need File System?
Persistent Data objects need to be stored on a long term basis for later use. They must be named and saved on a non volatile data Storage device such as magnetic disk.
This data objects are known as a file.
In any operating system File system is a major component and it’s responsible for the organization, storage, retrieval, naming, sharing and protection of files.

4. File systems also take responsibility for the control of access to files, restricting access to files according to users‘ authorizations and the type of access requested
The term "metadata“ is often used to refer to all of the extra information stored by a file system that is needed for the management of files
The file system is responsible for applying access control for files

5. Distributed File System
It’s an implementation of a file system that consist of physically distributed storage sites but provides a traditional centralized file system view for the user.

6. DFS Characteristics:-
Transparency
Access Transparency:-A user can login in at any host in the system with uniform login procedure, independent of the host.
In other word program designed to work on a local file can work on a remote file without modification
Location Independency:-files or groups of files can be
relocated without changing their actual path (or file) names.

7. Transparency cont. (DFS introduction ,lasnost. itcarlow
Transparency cont.(DFS introduction ,lasnost.itcarlow.ie/~barryp/slides/DFSintro.pdf )
Mobility transparency:- A file may be moved, either by systems administrators or automatically.
Performance transparency:- programs should continue to perform satisfactorily, even under heavy load.
Scaling transparency:- file systems can be expanded by incremental growth to deal with a wide range of loads and network sizes.

8. Concurrent File Updates:-Changes to a file by one client should not interfere with the operation of other clients simultaneously accessing or changing the same file.
Transaction based application requires that the application appears to access the files in isolation.
For any DFS it’s desirable to support concurrency control at transaction level.

9. File Replication:-Several copies of a file's contents at different locations enable multiple servers to share the load of providing the service
DFS with replication transparency performs atomic updates on the replicas and client are not aware that there is more then one copy exist.

10. Fault tolerance:-It is essential that the file service continue to operate in the face of client and server failures.
Another important design goal is statelessness the File server can be restarted and the service restored after a failure without any need to recover "previous state“.

11. Security:- To access control lists, there is a need to authenticate client requests so that access control at the server is based on correct user identities.
There is also a need to protect the contents of request and reply messages with digital signatures and encryption of secret data.

12. IBM’s DCE Distributed File Service characteristics (By Agustin Mena III and Carl Burnett)
Client Caching:-Client caching allows the majority of file read/write operations to occur locally on the client, thereby avoiding network traffic and server resource utilization.
Maintaining cache Consistency:-Cache consistency is maintained internally in DFS through the use of tokens. Tokens are guarantees from the file server that a client can perform certain operations on files residing in the client cache.
Security:-DFS is tightly integrated with the DCE Security Service. DCE Access Control Lists (ACLs) protect files in DFS. Communication between DFS clients and servers takes place using an authenticated RPC mechanism

13. Refrences:-
1]Performance Characteristics of the DCE Distributed File Service By Agustin Mena III and Carl Burnett.
[2]
[3] Distributed operating systems & Algorithms,By Randy chow,Theodore johnson.
[4] Distributed File Systems Pierre Boulet Masters Informatique TIIR et IAGL September 28,2006.

14. Thank You.