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Distributed File System.

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Introduction

Coursework 2         MiniProject Report                JS Swift

Distributed File System

A Distributed File System is an application based on client/server relationship. This application allows clients to access and process objects stored on the server, as if they were stored on their own local hard disk. Whenever a user requests for an object, the server send s a copy of that particular object. The user computer caches the object and sends it back to the server. DFS organizes file and directory services of standalone servers into a global directory. In this way, all servers are interconnected to each other and all files can become available to end-user, truly showing distributed file system. Server has a mechanism installed, whereby many clients may access same data simultaneously. This mechanism also updates files so that the following clients receive most updated version of files and also reduces data conflicts. To protect against data access failures, DFS uses file or database replication. Some of the examples of DFS include:

  • Novell Netware
  • Microsoft’s Distributed File System
  • IBM/Transarc’s Distributed File System
  • NFS (we will discuss this example later)

Distributed File System has many common concepts. We will now discuss some of the concepts of Distributed File System. They include Naming and Transparency, File Replication, Remote File Access, Caching, Fault Tolerance, Security and many more. We will briefly go through some of the main common concepts of Distributed File System before embarking on Network File System. Concepts are as follows:

  • File Replication

File replication is a useful idleness for improving accessibility of files on different machines. This feature also benefits performance as well.

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Middle

  • Portability: NFS is portable. This means that it is independent from machine and operating system. It can easily be ported onto several hardware and operating systems platforms including mainframes.
  • Network Protocol Independence: This is also a very important feature of NFS. It basically means that NFS is flexible to run on various transport protocols instead of residing or sticking to one know protocol. This feature also allows NFS to make maximum use of existing protocols and also open doors for new protocols in the future.
  • Performance: NFS aims to utilize the highest performance standards so that users are able to access remote files just like they access local files. Performance is a key feature of NFS.
  • Security: NFS supports multiple security mechanisms. It gives users or administrators to choose from various available security mechanisms. They have the flexibility to adopt security mechanisms that suits their environment. Again, this feature also opens doors to new security mechanisms that may be available in the near future.
  • All these features reduce costs as they share resources across the global enterprise.

NFS PROTOCOL MECHANISM

NFS depends on two different protocols as a medium for communication between sender and receiver. They are known as RPC and XDR. Below is a description of these two protocols.

  • Remote Procedure Call (RPC)

NFS uses RPC to communicate between client and server. It is a session layer protocol. It implements a client/server link on the hosts that use it. RPC allows hosts to make calls as if they were local but actually they are on remote hosts.

  • External Data Representation (XDR)

XDR provides a facility to translate data between diverse computers and operating systems. It is a presentation layer protocol. XDR is used by RPC to host a reliable data exchange between clients and severs.  

ACHIEVING TRANSPARENCY WITH PROXY PATTERNS

NFS transparency is achieved through RPC and XDR protocols. As mentioned earlier, these protocols are used by NFS to communicate between clients and servers. Below is a diagram showing the client/server relationship.

image00.png

From the figure above, we can see that the client and the server are communicating with each other using stubs. We can also see that client is accompanied with client stub and server is accompanied with server stub. The client just requests a service through service request and after that only the client and the server interact circuitously with each other. The client and the server stubs use the XDR protocol to make a connection and communicate by utilizing the XDR functions. The figure below gives even a better image of the whole process.

image01.png

The interaction between the stubs is clearly shown in the above diagram. NFS uses proxy pattern which shows the remote objects as local objects.

THE FOUR MAIN SERVICES OR PROTOCOLS SUPPORTING NFS

The four main services or protocols driving NFS are nfs, mountd, nsm and nlm. All these protocols are explained below in detail.

  • Nfs

This protocol resides at the base of NFS and it allows manages files, searching, reading and writing, authentication and file statistics.

  • Mountd

This protocol is responsible for mounting exported files systems so that they can be accessed with nfs. Requests such as mount and unmount are received by servers. The server keeps all the information about the exported file systems.

  • Nsm – Network Status Monitor

The main job of this protocol is to monitor network status. It monitors nodes to find out the state of a machine. It also informs functions such as reboot and restart.

  • Nlm – Network Lock Manager
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Conclusion

Data requiring larger disk space can also be kept on one host. Again, clients wishing to access objects will mount the required files onto their local host. For example, programs and files relating to a specific department can be kept and maintained on one host. Likewise, acconting anf finance department can be kept on another host.

A COMPARISON OF NFS AND NTFS

NTFS’s scalability is limited as it can support fewer processors as compared to NFS. A file in NTFS is not simple Byte stream as it is in the NFS. In NTFS, a file is a structured object consisting of attributes.        

In NTFS every file is stored in an array structure called Master File Table (MFT).

In NTFS, each directory uses B+ tree structure to store an index of the filename.

CONCLUSION

In my conclusion, I have to say that NFS is very simple to install and implement. It is a highly portable file system although sometimes it is inconsistent. The only drawback of NFS is the fact that it doesn’t scale to large number of clients.

BIBLIOGRAPHY

Websites:

http://cs.gmu.edu/~menasce/osbook/distfs/sld091.html

http://cs.gmu.edu/~menasce/osbook/distfs/sld096.html

http://osr5doc.ca.caldera.com:457/NetAdminG/fsD.aboutGUI.html

http://www.media.kyoto-u.ac.jp/edu/lec/jnakamu/lecture/y98/miy98/part2/p871bp4.htm

http://www-1.ibm.com/servers/eserver/zseries/zos/nfs/index.html

http://www.cs.wisc.edu/~sschang/OS-Qual/fs/distributed_file_systems.htm

http://www.cse.ucsc.edu/~darrell/classes/111/slides/silbershatz/mod17.1.pdf

Books:

Silberschatz , Abraham. Applied Operating System Concepts / Avi Silberschatz, Peter Galvin, Greg Gagne. New York; Chichester: Wiley , 2000.

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