Distribution File System (DFS)by

Naza Hameed Jan     

  

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

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

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.

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

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.

Structure of distributed file system

• Here there is a need to define 3 aspectsService Server Client

• A DFS is a file whose client server and storage devices are dispersed among the machines of distributed servers

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

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 thereThe time to deliver the request to server as well

as time to get response across the network back to the client.

Mapping

• Mapping between logical and physical components i.e.

U ser deal with logical object like file nameSystems deals with the physical blocks of data

shared on secondary medias.

Multilevel mapping

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

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

Other aspects of transparency

• Divorce of data

• Static location transparency

• Location independence

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.

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.

Location independence

• Separates the naming hierarchy from the stoarge device hierarchy

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

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

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

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

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

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

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

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

Thats it